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New contributions of measurements in Europe to the global inventory of the stable isotopic composition of methane

Bibliographic Details
Title: New contributions of measurements in Europe to the global inventory of the stable isotopic composition of methane
Authors: M. Menoud, C. van der Veen, D. Lowry, J. M. Fernandez, S. Bakkaloglu, J. L. France, R. E. Fisher, H. Maazallahi, M. Stanisavljević, J. Nęcki, K. Vinkovic, P. Łakomiec, J. Rinne, P. Korbeń, M. Schmidt, S. Defratyka, C. Yver-Kwok, T. Andersen, H. Chen, T. Röckmann
Source: Earth System Science Data, Vol 14, Pp 4365-4386 (2022)
Publisher Information: Copernicus Publications, 2022.
Publication Year: 2022
Collection: LCC:Environmental sciences
LCC:Geology
Subject Terms: Environmental sciences, GE1-350, Geology, QE1-996.5
More Details: Recent climate change mitigation strategies rely on the reduction of methane (CH4) emissions. Carbon and hydrogen isotope ratio (δ13CCH4 and δ2HCH4) measurements can be used to distinguish sources and thus to understand the CH4 budget better. The CH4 emission estimates by models are sensitive to the isotopic signatures assigned to each source category, so it is important to provide representative estimates of the different CH4 source isotopic signatures worldwide. We present new measurements of isotope signatures of various, mainly anthropogenic, CH4 sources in Europe, which represent a substantial contribution to the global dataset of source isotopic measurements from the literature, especially for δ2HCH4. They improve the definition of δ13CCH4 from waste sources, and demonstrate the use of δ2HCH4 for fossil fuel source attribution. We combined our new measurements with the last published database of CH4 isotopic signatures and with additional literature, and present a new global database. We found that microbial sources are generally well characterised. The large variability in fossil fuel isotopic compositions requires particular care in the choice of weighting criteria for the calculation of a representative global value. The global dataset could be further improved by measurements from African, South American, and Asian countries, and more measurements from pyrogenic sources. We improved the source characterisation of CH4 emissions using stable isotopes and associated uncertainty, to be used in top-down studies. We emphasise that an appropriate use of the database requires the analysis of specific parameters in relation to source type and the region of interest. The final version of the European CH4 isotope database coupled with a global inventory of fossil and non-fossil δ13CCH4 and δ2HCH4 source signature measurements is available at https://doi.org/10.24416/UU01-YP43IN (Menoud et al., 2022a).
Document Type: article
File Description: electronic resource
Language: English
ISSN: 1866-3508
1866-3516
Relation: https://essd.copernicus.org/articles/14/4365/2022/essd-14-4365-2022.pdf; https://doaj.org/toc/1866-3508; https://doaj.org/toc/1866-3516
DOI: 10.5194/essd-14-4365-2022
Access URL: https://doaj.org/article/9ae50a7b6c2f4f2d93dd6d0fcd376441
Accession Number: edsdoj.9ae50a7b6c2f4f2d93dd6d0fcd376441
Database: Directory of Open Access Journals
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  Value: <anid>AN0159611867;[auei]01sep.22;2022Oct13.08:00;v2.2.500</anid> <title id="AN0159611867-1">New contributions of measurements in Europe to the global inventory of the stable isotopic composition of methane </title> <sbt id="AN0159611867-2">1 Introduction</sbt> <p>Recent climate change mitigation strategies rely on the reduction of methane (CH 4) emissions. Carbon and hydrogen isotope ratio (δ13 C CH4 and δ2 H CH4) measurements can be used to distinguish sources and thus to understand the CH 4 budget better. The CH 4 emission estimates by models are sensitive to the isotopic signatures assigned to each source category, so it is important to provide representative estimates of the different CH 4 source isotopic signatures worldwide. We present new measurements of isotope signatures of various, mainly anthropogenic, CH 4 sources in Europe, which represent a substantial contribution to the global dataset of source isotopic measurements from the literature, especially for δ2 H CH4. They improve the definition of δ13 C CH4 from waste sources, and demonstrate the use of δ2 H CH4 for fossil fuel source attribution. We combined our new measurements with the last published database of CH 4 isotopic signatures and with additional literature, and present a new global database. We found that microbial sources are generally well characterised. The large variability in fossil fuel isotopic compositions requires particular care in the choice of weighting criteria for the calculation of a representative global value. The global dataset could be further improved by measurements from African, South American, and Asian countries, and more measurements from pyrogenic sources. We improved the source characterisation of CH 4 emissions using stable isotopes and associated uncertainty, to be used in top-down studies. We emphasise that an appropriate use of the database requires the analysis of specific parameters in relation to source type and the region of interest. The final version of the European CH 4 isotope database coupled with a global inventory of fossil and non-fossil δ13 C CH4 and δ2 H CH4 source signature measurements is available at 10.24416/UU01-YP43IN.</p> <p>The current change of the Earth's climate is mainly caused by the emissions of greenhouse gases from anthropogenic activities [[<reflink idref="bib65" id="ref1">65</reflink>]]. Methane (CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> ) is a strong greenhouse gas, with a global warming potential 32 times that of CO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">2</mn></msub></math> </ephtml> over 100 years [<reflink idref="bib41" id="ref2">41</reflink>]. The increase in CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> concentration has contributed to an average warming of 0.5  <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mo>∘</mo></msup></math> </ephtml> C in 2010–2019 compared to 1850–1900, which is slightly smaller than the contribution of CO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">2</mn></msub></math> </ephtml> [<reflink idref="bib67" id="ref3">67</reflink>]. The global CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> mole fraction ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="italic">χ</mi></math> </ephtml> (CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> )) in the atmosphere has drastically increased since 1984, when direct regular measurements started, changing from 1645 to 1850 ppb in 2017 [<reflink idref="bib108" id="ref4">108</reflink>]. Compared to pre-industrial times (before 1750), the global <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="italic">χ</mi></math> </ephtml> (CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> ) has increased by 160 %, from 720 to 1850 ppb [<reflink idref="bib66" id="ref5">66</reflink>].</p> <p>In the past 30 years, we have not observed a steady growth of atmospheric CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> mole fraction. Instead the increase in <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="italic">χ</mi></math> </ephtml> (CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> ) levelled off between 2000 and 2007, and has been increasing again since then, from 2014 at the highest rate since the 1980s [<reflink idref="bib108" id="ref6">108</reflink>]. This renewed increase presents a significant threat to reaching the goals of the Paris agreement, and mitigation policies are now also targeting CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions [[<reflink idref="bib131" id="ref7">131</reflink>], [<reflink idref="bib91" id="ref8">91</reflink>], [<reflink idref="bib109" id="ref9">109</reflink>]]. Efficient strategies require good knowledge of the different kinds of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources, their location, and relative contributions. While emission estimates are reported at a country level using statistical methods, atmospheric inversions, based on observations, can be used to verify the inventories [[<reflink idref="bib62" id="ref10">62</reflink>], [<reflink idref="bib162" id="ref11">162</reflink>], [<reflink idref="bib59" id="ref12">59</reflink>], [<reflink idref="bib86" id="ref13">86</reflink>]]. But the results from two approaches, respectively called bottom-up and top-down, are not in full agreement, reflecting a lack in our understanding of the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> cycle [[<reflink idref="bib40" id="ref14">40</reflink>], [<reflink idref="bib125" id="ref15">125</reflink>], [<reflink idref="bib134" id="ref16">134</reflink>]].</p> <p>Measurements of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopologues provide additional constraints on the relative contribution of the various source categories, because CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic composition depends on the formation processes [[<reflink idref="bib127" id="ref17">127</reflink>], [<reflink idref="bib159" id="ref18">159</reflink>], [<reflink idref="bib115" id="ref19">115</reflink>]]. Time series of ambient CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic ratios are already used to derive emission scenarios in global models [<reflink idref="bib14" id="ref20">14</reflink>], [<reflink idref="bib126" id="ref21">126</reflink>], [<reflink idref="bib146" id="ref22">146</reflink>], [<reflink idref="bib143" id="ref23">143</reflink>], [<reflink idref="bib48" id="ref24">48</reflink>], [<reflink idref="bib76" id="ref25">76</reflink>], and at the regional scale [[<reflink idref="bib122" id="ref26">122</reflink>], [<reflink idref="bib137" id="ref27">137</reflink>], [<reflink idref="bib96" id="ref28">96</reflink>], [<reflink idref="bib150" id="ref29">150</reflink>]]. In addition, isotope measurements have proven to be very successful for source attribution in cities [[<reflink idref="bib113" id="ref30">113</reflink>], [<reflink idref="bib166" id="ref31">166</reflink>], [<reflink idref="bib87" id="ref32">87</reflink>], [<reflink idref="bib161" id="ref33">161</reflink>], [<reflink idref="bib36" id="ref34">36</reflink>], [<reflink idref="bib42" id="ref35">42</reflink>]], and larger regions [[<reflink idref="bib142" id="ref36">142</reflink>], [<reflink idref="bib45" id="ref37">45</reflink>], [<reflink idref="bib9" id="ref38">9</reflink>], [<reflink idref="bib157" id="ref39">157</reflink>], [<reflink idref="bib46" id="ref40">46</reflink>], [<reflink idref="bib85" id="ref41">85</reflink>]]. The uncertainties in the resulting emission rates of the different source categories depend on our knowledge of the different isotopic source signatures, and understanding of their variability [[<reflink idref="bib119" id="ref42">119</reflink>], [<reflink idref="bib128" id="ref43">128</reflink>], [<reflink idref="bib93" id="ref44">93</reflink>], [<reflink idref="bib141" id="ref45">141</reflink>]].</p> <p>Direct measurements of the isotopic signature of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources allow us to precisely characterise the type of emission, and a lot of data are available in the literature. Several review articles on CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic source signatures were previously published [[<reflink idref="bib118" id="ref46">118</reflink>], [<reflink idref="bib29" id="ref47">29</reflink>], [<reflink idref="bib16" id="ref48">16</reflink>]]. The most recent one presented by [<reflink idref="bib129" id="ref49">129</reflink>], and recently updated in [<reflink idref="bib130" id="ref50">130</reflink>], gathered values from 13 489 locations (10 778 fossil fuel, 2711 non-fossil) from 347 published references. The 2017 study focused on (fugitive) fossil fuel sources, and allowed to re-evaluate the global <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> value assigned to this emission category towards more depleted values [<reflink idref="bib128" id="ref51">128</reflink>]. A disadvantage of this database is that it is rather US-centred, and that the dataset is strongest for fossil fuel sources, but less robust for non-fossil sources. Therefore the database can be completed by more studies, especially concerning non-fossil sources.</p> <p>The MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> project (MEthane goes MObile – MEasurements and MOdelling) was a H2020 MSCA European Training Network with the goal to use innovative mobile measurement and modelling tools to improve the quantification of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions in Europe [<reflink idref="bib156" id="ref52">156</reflink>]. An important component of MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> was the isotopic characterisation of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources. Two laboratories involved in MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> , at Utrecht University, the Netherlands and at the Royal Holloway University of London, UK, carried out a large number of high-precision measurements with isotope ratio mass spectrometry (IRMS). Another method, using cavity ring-down spectroscopy (CRDS) was developed for the mobile measurements of ambient CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic composition. Several research groups were involved in field work with mobile measurements that targeted specific sources or environments in several European countries. Using this network, air samples from numerous CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources could be measured for isotopic composition. The resulting isotopic source signatures were gathered in a publicly available database, with the first version made accessible on 1 October 2020 [<reflink idref="bib94" id="ref53">94</reflink>], and described in a publicly available report [<reflink idref="bib95" id="ref54">95</reflink>]. The European data were used in several publications over the past two years by [[<reflink idref="bib96" id="ref55">96</reflink>], [<reflink idref="bib99" id="ref56">99</reflink>]], [<reflink idref="bib87" id="ref57">87</reflink>], [<reflink idref="bib36" id="ref58">36</reflink>], [<reflink idref="bib6" id="ref59">6</reflink>], [<reflink idref="bib42" id="ref60">42</reflink>], [<reflink idref="bib7" id="ref61">7</reflink>]. These studies emphasised the benefits from regional estimates of source CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic composition. The last update of the MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> isotopic data was compiled into the European methane isotope database (EMID).</p> <p>The present study provides an in-depth analysis of the EMID, a comparison with the global data, and the implications for the global understanding of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> source isotopic composition. To this purpose, we compiled all the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic source signatures from MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> with the latest version of the [[<reflink idref="bib129" id="ref62">129</reflink>]] global database. We also searched the literature for more measured CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> source signatures to add to the dataset.</p> <hd id="AN0159611867-3">2 Methods</hd> <p></p> <hd id="AN0159611867-4">2.1 Measurements within the MEMO 2 project</hd> <p></p> <hd id="AN0159611867-5">2.1.1 Sampling</hd> <p>The data were collected by the research teams of eight universities and research institutes: Utrecht University (UU), the Royal Holloway University of London (RHUL), the Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Heidelberg University (UHEI), AGH University of Science and Technology (AGH), Lund University (LU), the University of Groningen (UG), and the Netherlands Organisation for Applied Scientific Research (TNO). They participated in several campaigns in the Netherlands, the United Kingdom, France, Germany, Poland, Sweden, Romania, and Turkey. Several other teams collaborated in two intensive campaigns: the CoMet campaign in the Upper Silesian Coal Basin (USCB) in Poland [[<reflink idref="bib43" id="ref63">43</reflink>], [<reflink idref="bib50" id="ref64">50</reflink>]], and the ROMEO campaign in Romania[<reflink idref="bib120" id="ref65">120</reflink>]. The samples were collected mostly between 2017 and 2020, but three locations in the UK were sampled in February 2015, September, and October 2016.</p> <p>Different sampling methods were used:</p> <p></p> <ulist> <item> Mobile sampling on road vehicles, using a fast (0.1 to 10 Hz) analyser on board to detect CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> enhancements (G2301, G2201-i, and G4302, Picarro Inc., USA; MGGA-918 and UGGA, Los Gatos Research, ABB, USA; LI-7810 Trace Gas analyser, LI-COR, USA; Dual Laser Trace Gas Monitor, Aerodyne Research, USA). Different setups were used by different teams with one or two of these instrument on board, but the sampling procedure was the same. The samples were taken using a small electric pump connected to an inlet outside of the vehicle. The sample receptacles were bags of 1 to 3 L (Supel™-Inert Multi-Layer Foil bags, Sigma-Aldrich Co. LLC, USA; Tedlar or FlexFoil sample bags, SKC Inc., USA). Surveys were made around known sources of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> , where we sampled the elevated mole fractions and background CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> on the same day. If it was not practical to approach a source with the vehicle during mobile surveys, samples were taken on foot.</item> <p></p> <item> Mobile sampling onboard of an aircraft, during the ROMEO campaign. A CRDS instrument (G4302, Picarro Inc., USA) was installed in the aircraft, and samples were taken from the outflow of the instrument into bags of 2 L (Supel™-Inert Multi-Layer Foil bags, Sigma-Aldrich Co. LLC, USA) when an increase in CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> mole fractions was observed. The method is described in detail in [<reflink idref="bib99" id="ref66">99</reflink>].</item> <p></p> <item> Mobile sampling on foot, without analyser. The samples were taken at regularly spread locations around a known CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> source, to make sure we collected air with CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> from the emission plume and background. In this case, the sample receptacles were bags of 2 to 3 L (Supel™-Inert Multi-Layer Foil bags, Sigma-Aldrich Co. LLC, USA; Tedlar sample bags, SKC Inc., USA), filled with a portable hand pump.</item> <p></p> <item> Soil chambers on wetlands in north Sweden and coal waste disposal areas in Poland. In wetlands, we installed transparent Plexiglas chambers on top of stainless steel collars that were pushed 20 cm into the peat. Samples from the chambers were taken during closure times, when <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="italic">χ</mi></math> </ephtml> (CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> ) increased, generally after 10 to 25 min. The soil chambers in Poland were made of plastic buckets covered with aluminium foil that were pushed about 5 cm in the ground and left for 30 min. In both cases, air was pumped into 2 L sample bags (Supel™-Inert Multi-Layer Foil, Sigma-Aldrich Co. LLC, USA) for further analysis in the lab.</item> <p></p> <item> From an unmanned aerial vehicle (UAV), carrying an AirCore (coiled tubing) system to collect air samples [<reflink idref="bib4" id="ref67">4</reflink>]. The air samples were continuously pulled into the AirCore while flying transects across the plume of a CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emission source, and were transferred to a 0.5 or 1 L bag sample after landing (Supel™-Inert Multi-Layer Foil, Sigma-Aldrich Co. LLC, USA) for further analysis in the laboratory.</item> </ulist> <hd id="AN0159611867-6">2.1.2 Measurements of isotopic composition</hd> <p>The mass spectrometry measurements were performed at two laboratories: the IMAU (Institute for Marine and Atmospheric research Utrecht) at UU, and at the Department of Earth Sciences at RHUL. Both laboratories use a CF–IRMS (continuous flow isotopic ratio mass spectrometry) system to measure <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C, and also <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H at IMAU. The system at IMAU was described by [<reflink idref="bib122" id="ref68">122</reflink>] and the one at RHUL by [<reflink idref="bib44" id="ref69">44</reflink>]. The reproducibility both groups can achieve is of 0.05 ‰ to 0.1 ‰ for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> . At IMAU, <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H measurements have a reproducibility lower than 2 ‰. For consistency of the results, the two laboratories measured a set of five cylinders that contained air with CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> of different isotopic composition. The resulting differences in <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> for each cylinder ranged between 0.02 ‰ and 0.04 ‰. They were within the analytical error reported by the two laboratories, so that the isotopic results obtained within the MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> project are consistent across the laboratories. The inter-comparison exercise is presented in detail in a MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> deliverable report, and publicly available [<reflink idref="bib83" id="ref70">83</reflink>].</p> <p>The UHEI and LSCE groups performed isotopic measurements using CRDS instruments (G2201-i, Picarro inc., USA). Their measurement and calibration methods are described in [<reflink idref="bib60" id="ref71">60</reflink>] and [<reflink idref="bib36" id="ref72">36</reflink>].</p> <p>In the database, the method of isotopic measurements is specified by the "measurement type" parameter, as either "IRMS" or "CRDS". The laboratory where the measurements were performed is specified in the column "measurement lab".</p> <hd id="AN0159611867-7">2.1.3 Reported variables</hd> <p>The analytical parameters reported in the database are <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> , which are defined as</p> <p> <ephtml> <math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi mathvariant="italic">δ</mi><mi>X</mi><mo>=</mo><mfenced close=")" open="("><mrow><mstyle displaystyle="true"><mfrac style="display"><mrow><msub><mi>R</mi><mi mathvariant="normal">sample</mi></msub></mrow><mrow><msub><mi>R</mi><mi mathvariant="normal">standard</mi></msub></mrow></mfrac></mstyle><mo>-</mo><mn mathvariant="normal">1</mn></mrow></mfenced><mo>,</mo></mrow></math> </ephtml> </p> <p>with <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>R</mi><mo>=</mo><mstyle displaystyle="false"><mfrac style="text"><mrow><msup><mi /><mn mathvariant="normal">13</mn></msup><mi>C</mi></mrow><mrow><msup><mi /><mn mathvariant="normal">12</mn></msup><mi>C</mi></mrow></mfrac></mstyle></mrow></math> </ephtml> for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>X</mi><msup><mo>=</mo><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C or <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>R</mi><mo>=</mo><mstyle displaystyle="false"><mfrac style="text"><mrow><msup><mi /><mn mathvariant="normal">2</mn></msup><mi>H</mi></mrow><mrow><msup><mi /><mn mathvariant="normal">1</mn></msup><mi>H</mi></mrow></mfrac></mstyle></mrow></math> </ephtml> for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>X</mi><msup><mo>=</mo><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H.</p> <p> <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="italic">δ</mi></math> </ephtml> values are reported in per mille (‰), relative to the international standard materials Vienna Peedee Belemnite (VPDB) for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C, and Vienna Standard Mean Ocean Water (VSMOW) for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H.</p> <hd id="AN0159611867-8">2.1.4 Calculation of isotopic signatures</hd> <p>The measurement results of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> are for ambient air, and not the sources themselves. There are different methods to derive the isotopic source signatures from the sampled CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> enhancement signatures; the Keeling plot and Miller–Tans methods are commonly used mass balance approaches. The Keeling plot method is based on the assumption that the background is stable during the sampling period [[<reflink idref="bib69" id="ref73">69</reflink>], [<reflink idref="bib112" id="ref74">112</reflink>]]. The Miller–Tans method is also applicable when the condition of a stable background is not fulfilled [<reflink idref="bib103" id="ref75">103</reflink>]. Because background samples were taken on each survey day and in the same region, the condition of stable background was usually fulfilled. [<reflink idref="bib35" id="ref76">35</reflink>] showed that in this case, both methods lead to similar results within their uncertainty.</p> <p>Both methods involve a linear regression model to fit the observed data. Different models were used: ordinary least squares (OLS) minimising the difference in the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi>y</mi></math> </ephtml>  axis coordinate, bivariate correlated errors and intrinsic scatter (BCES) [<reflink idref="bib1" id="ref77">1</reflink>], and orthogonal distance regression (ODR) [<reflink idref="bib13" id="ref78">13</reflink>]. [<reflink idref="bib167" id="ref79">167</reflink>] compared different regression methods to be applied in Keeling plots. The ODR method can induce a bias towards lower values, in the case the data points cover a relatively small range on the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi>x</mi></math> </ephtml>  axis. Therefore, the OLS and BCES methods were usually preferred to calculate the source signatures for this study.</p> <p>All the mass balance and regression methods are statistically valid. We did not work towards a uniform procedure, to not modify the data that were processed by each lab. The different approaches are specified for each entry of the database by the parameters "mass balance approach" and "regression method".</p> <hd id="AN0159611867-9">2.2 Revision of the global database of CH 4 isotope ratios</hd> <p></p> <hd id="AN0159611867-10">2.2.1 Structure of the database to include previous and new measurements</hd> <p>We used the same parameters as in the database of [[<reflink idref="bib129" id="ref80">129</reflink>]] for non-fossil data. That is because our objectives concern only values for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H of emitted CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> , and do not include measurements of other gases or isotope signatures that [<reflink idref="bib129" id="ref81">129</reflink>] reported in the fossil fuel database. The variables of interest are listed in Table  and include the site description (country, region, group, category, and sub-category), and the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> . There are two types of values:</p> <p></p> <ulist> <item> Single measurement values, as from the characterisation of one emission event. Most fossil fuel data from [<reflink idref="bib130" id="ref82">130</reflink>] are single measurements, and the entries in the EMID.</item> <p></p> <item> Average values from repeated measurements at the same location or over time. The values found in the literature are usually averages of multiple measurements.</item> </ulist> <p>A direct comparison between these two types of values would be unbalanced and lead to the over-representation of single measurements. Therefore, to combine the different kinds of data and perform statistical analyses, we aggregated the sources reported in the EMID by region and sub-category, and in the fossil fuel database of [<reflink idref="bib130" id="ref83">130</reflink>] per production basin. Throughout the article, the aggregated values are referred to as data <emph>locations</emph>, to distinguish them from <emph>measurements</emph> values which refer to the single events.</p> <p>The source categories and sub-categories from [[<reflink idref="bib129" id="ref84">129</reflink>]] were kept as they were, but when the new entries from MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> measurements and published literature required it, we added additional source categories or sub-categories. The categories are grouped into the three main CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> formation pathways: modern microbial, pyrogenic, and fossil fuels. The "modern microbial" CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> is formed by microorganisms in surface ecosystems or in animals through enteric fermentation, and is referred to simply as "microbial" throughout the paper. Microbial CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> formation in the subsurface related to petroleum systems belongs to the "fossil fuels" category. Compared to [[<reflink idref="bib129" id="ref85">129</reflink>]], we extended the "biomass burning" category to "pyrogenic" to include emissions from other combustion sources, such as traffic or industry. All categories and sub-categories are listed in Table .</p> <p>Table 1 Variables reported in the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic signature database published with this article, which combines three datasets of different origins.</p> <p> <ephtml> <table><thead><tr><td>Parameter</td><td>Description</td><td>Present in dataset </td></tr><tr><td /><td /><td>EMID</td><td><xref ref-type="bibr" rid="bibr130">130</xref>,</td><td>Literature</td></tr><tr><td /><td /><td /><td>fossil fuel locations</td><td /></tr></thead><tbody><tr><td>CONTINENT</td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>COUNTRY</td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>STATE_REGION</td><td>administrative region or state</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>BASIN</td><td>Fossil fuel area</td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /></tr><tr><td>GROUP_TYPE</td><td>category level 3</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>GROUP</td><td>category level 2</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>CATEGORY</td><td>category level 1</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>SUB-CATEGORY</td><td>category level 0</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>SNAP</td><td>category in SNAP<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup xmlns=""><mi /><mo>∗</mo></msup></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /></tr><tr><td>LONG</td><td>longitude</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /><td /></tr><tr><td>LAT</td><td>latitude</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /><td /></tr><tr><td>d13C_CH4_MEAN</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p>, in ‰ VPDB</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>d13C_CH4_ERR</td><td>error in the calculated <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /><td /></tr><tr><td>d13C_CH4_UNCERTAINTY</td><td>uncertainty in the reported <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td /><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>d13C_CH4_SD</td><td>standard deviation of <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /></tr><tr><td>d13C_CH4_SE</td><td>standard error of the mean <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /></tr><tr><td>d13C_CH4_N</td><td>number of <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p> values</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>d2H_CH4_MEAN</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p>, in ‰ VSMOW</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>d2H_CH4_ERR</td><td>error in the calculated <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /><td /></tr><tr><td>d2H_CH4_UNCERTAINTY</td><td>uncertainty in the reported <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td /><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>d2H_CH4_SD</td><td>standard deviation of <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /></tr><tr><td>d2H_CH4_SE</td><td>standard error of the mean <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /></tr><tr><td>d2H_CH4_N</td><td>number of <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p> values</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>TYPE_UNCERTAINTY</td><td>type of uncertainty reported</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>COMMENTS</td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr><tr><td>REFERENCE</td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">×</mo></math></p></td></tr></tbody></table> </ephtml> </p> <p> <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mo>∗</mo></msup></math> </ephtml>  Selected Nomenclature for Air Pollution, https://en.eustat.eus/documentos/elem%5f13173/definicion.html (last access: 24 March 2022)</p> <p>Table 2 Number of measurements ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> / <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="bold-italic">δ</mi><mn mathvariant="bold">2</mn></msup></mrow></math> </ephtml><bold>H</bold><ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="bold">CH</mi><mn mathvariant="bold">4</mn></msub></mrow></msub></math> </ephtml> ) per source category in the updated CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic signature database.</p> <p> <ephtml> <table><thead><tr><td /><td /><td /><td /><td><xref ref-type="bibr" rid="bibr129">129</xref></td><td>Additional literature</td><td>MEMO<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup xmlns=""><mi /><mn mathvariant="normal">2</mn></msup></math></p></td></tr></thead><tbody><tr><td>microbial</td><td>agriculture</td><td>ruminants</td><td>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mn mathvariant="normal">3</mn></msub></math></p>/C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mn mathvariant="normal">4</mn></msub></math></p></td><td>227/<bold>86</bold></td><td>45/<bold>12</bold></td><td>30/<bold>11</bold></td></tr><tr><td>rice paddies</td><td>flooded, flooded seasonally</td><td>360/<bold>139</bold></td><td>15/<bold>0</bold></td><td /></tr><tr><td>piggery</td><td /><td /><td>10/<bold>10</bold></td><td /></tr><tr><td>waste</td><td>landfill</td><td /><td>161/<bold>25</bold></td><td>91/<bold>24</bold></td><td>54/<bold>22</bold></td></tr><tr><td>sewage</td><td>wastewater, manhole</td><td>2/<bold>2</bold></td><td>27/<bold>6</bold></td><td>83/<bold>64</bold></td></tr><tr><td>biogas</td><td>manure, C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mn mathvariant="normal">4</mn></msub></math></p>/C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mn mathvariant="normal">3</mn></msub></math></p></td><td>15/<bold>15</bold></td><td>21/<bold>2</bold></td><td>39 / <bold>8</bold></td></tr><tr><td>manure</td><td>cattle</td><td /><td>9/<bold>0</bold></td><td>22/<bold>0</bold></td></tr><tr><td>compost</td><td /><td /><td /><td>4/<bold>0</bold></td></tr><tr><td>abattoir</td><td>cattle</td><td /><td>18/<bold>9</bold></td><td /></tr><tr><td>wetlands</td><td>temperate</td><td>marsh, bog, swamp, lake, estuary, pond, delta, fen, lagoon, reeds, flooded forest, wet prairie, river, mangrove</td><td>246/<bold>124</bold></td><td>150/<bold>8</bold></td><td>6/<bold>6</bold></td></tr><tr><td>tropical</td><td>floodplain, lake, swamp, marsh, river, riverine reeds, mixed</td><td>177/<bold>22</bold></td><td>60/<bold>34</bold></td><td /></tr><tr><td>polar (incl. boreal)</td><td>bog, marsh, swamp, tundra, lake, estuary, fen, wet tundra, (thawed) permafrost, mire, forest</td><td>558/<bold>14</bold></td><td>72/<bold>2</bold></td><td>15/<bold>15</bold></td></tr><tr><td>other</td><td>termites</td><td /><td>29/<bold>1</bold></td><td>7/<bold>0</bold></td><td /></tr><tr><td>fossil fuels</td><td>exploitation</td><td>conventional</td><td>gas leak, gas installation, oil field, mixed, natural gas, oil refinery</td><td>6517/<bold>2152</bold></td><td>102/<bold>10</bold></td><td>377/<bold>219</bold></td></tr><tr><td>coal</td><td>active coal mine, inactive coal mine, coal seam gas</td><td>2108/<bold>796</bold></td><td>113/<bold>71</bold></td><td>71/<bold>40</bold></td></tr><tr><td>shale</td><td /><td>447/<bold>290</bold></td><td /><td /></tr><tr><td>seeps</td><td>oceans</td><td>marine seep</td><td /><td /><td>4/<bold>4</bold></td></tr><tr><td>coal seam gas</td><td /><td /><td>39/<bold>31</bold></td><td /></tr><tr><td>volcanoes</td><td /><td /><td>0/<bold>8</bold></td><td /></tr><tr><td>pyrogenic</td><td>biomass burning</td><td>grass, pasture, brush, woodland, wood, forest, crop</td><td>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mn mathvariant="normal">3</mn></msub></math></p>/C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mn mathvariant="normal">4</mn></msub></math></p></td><td>109/<bold>4</bold></td><td>1/<bold>1</bold></td><td /></tr><tr><td>fossil fuel burning</td><td>conventional</td><td>car, traffic, residential heating</td><td /><td>44/<bold>27</bold></td><td>4/<bold>1</bold></td></tr></tbody></table> </ephtml> </p> <hd id="AN0159611867-11">2.2.2 Data from previously published literature</hd> <p>We found an additional number of 48 sources [[<reflink idref="bib70" id="ref86">70</reflink>], [<reflink idref="bib21" id="ref87">21</reflink>], [<reflink idref="bib23" id="ref88">23</reflink>], [<reflink idref="bib77" id="ref89">77</reflink>], [<reflink idref="bib158" id="ref90">158</reflink>], [<reflink idref="bib53" id="ref91">53</reflink>], [<reflink idref="bib79" id="ref92">79</reflink>], [<reflink idref="bib139" id="ref93">139</reflink>], [<reflink idref="bib57" id="ref94">57</reflink>], [<reflink idref="bib11" id="ref95">11</reflink>], [<reflink idref="bib58" id="ref96">58</reflink>], [<reflink idref="bib22" id="ref97">22</reflink>], [<reflink idref="bib140" id="ref98">140</reflink>], [<reflink idref="bib12" id="ref99">12</reflink>], [<reflink idref="bib80" id="ref100">80</reflink>], [<reflink idref="bib114" id="ref101">114</reflink>], [<reflink idref="bib24" id="ref102">24</reflink>], [<reflink idref="bib27" id="ref103">27</reflink>], [<reflink idref="bib132" id="ref104">132</reflink>], [<reflink idref="bib82" id="ref105">82</reflink>], [<reflink idref="bib25" id="ref106">25</reflink>], [<reflink idref="bib106" id="ref107">106</reflink>], [<reflink idref="bib15" id="ref108">15</reflink>], [<reflink idref="bib138" id="ref109">138</reflink>], [<reflink idref="bib61" id="ref110">61</reflink>], [<reflink idref="bib49" id="ref111">49</reflink>], [<reflink idref="bib145" id="ref112">145</reflink>], [<reflink idref="bib148" id="ref113">148</reflink>], [<reflink idref="bib9" id="ref114">9</reflink>], [<reflink idref="bib144" id="ref115">144</reflink>], [<reflink idref="bib54" id="ref116">54</reflink>], [<reflink idref="bib113" id="ref117">113</reflink>], [<reflink idref="bib8" id="ref118">8</reflink>], [<reflink idref="bib33" id="ref119">33</reflink>], [<reflink idref="bib68" id="ref120">68</reflink>], [<reflink idref="bib89" id="ref121">89</reflink>], [<reflink idref="bib116" id="ref122">116</reflink>], [<reflink idref="bib164" id="ref123">164</reflink>], [<reflink idref="bib111" id="ref124">111</reflink>], [<reflink idref="bib165" id="ref125">165</reflink>], [<reflink idref="bib81" id="ref126">81</reflink>], [<reflink idref="bib110" id="ref127">110</reflink>], [<reflink idref="bib60" id="ref128">60</reflink>], [<reflink idref="bib84" id="ref129">84</reflink>], [<reflink idref="bib161" id="ref130">161</reflink>], [<reflink idref="bib47" id="ref131">47</reflink>], [<reflink idref="bib85" id="ref132">85</reflink>], [<reflink idref="bib2" id="ref133">2</reflink>]] in the literature to complete the referred data listed in [<reflink idref="bib130" id="ref134">130</reflink>]. Because we aim at reflecting the actual CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> surface emissions to the atmosphere, we excluded studies that reported results from laboratory experiments, and of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> dissolved in water (i.e. in oceans, wetlands, and inland waters). We note that the search for data was biased because of the use of English language. The references we added concern published peer-reviewed articles, and to a lesser extent thesis and conference papers. We did not perform additional data quality assessment. The studies were performed from 1982 to 2021 in various laboratories in the world. The study locations do not overlap with the ones of the EMID or the literature gathered in [<reflink idref="bib130" id="ref135">130</reflink>], and we do not provide an analysis of potential temporal changes in the isotopic composition of the same source.</p> <hd id="AN0159611867-12">3 Results and discussion</hd> <p>The data on isotopic source signatures from the measurement campaigns carried out within the MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> project (2017–2020) were compiled into the EMID. The final version of this database is combined with the global database and additional literature, and is available at 10.24416/UU01-YP43IN[<reflink idref="bib98" id="ref136">98</reflink>].</p> <hd id="AN0159611867-13">3.1 The European methane isotope database (EMID)</hd> <p>The isotopic signatures obtained within the MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> project concern 734 locations over eight countries, with <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H source signatures being measured at 54 % of the sites (Table ). Measurements of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H are less numerous than of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C because only the measurement system at IMAU was able to measure this isotope signature. Depending on the availability of the measurement system, the sampling location, and the timing of the campaign, it was not possible to systematically measure all samples at IMAU. Figure  shows the geographical distribution of the sampled sites in the different countries, according to the type of source. The number of sources we sampled does not represent the emission magnitudes.</p> <p>During mobile surveys, we mostly targeted anthropogenic emissions from the exploitation and use of fossil fuels and waste processing facilities (Fig. ). These are the most obvious anthropogenic CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources in densely populated regions, and we acknowledge a deliberate sampling bias towards urbanised areas. No biomass burning emissions were characterised during the MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> project. The EMID partially addresses the geographical bias pointed out by [<reflink idref="bib129" id="ref137">129</reflink>]: it particularly includes a large number of measurements made in Romania, where almost no data were available before.</p> <p>Table 3 Number of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic source signatures derived from sample measurements in the EMID.</p> <p> <ephtml> <table><thead><tr><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub xmlns=""><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math></p></td></tr></thead><tbody><tr><td>The Netherlands</td><td>50</td><td>27</td></tr><tr><td>United Kingdom</td><td>240</td><td>54</td></tr><tr><td>Poland</td><td>98</td><td>73</td></tr><tr><td>Germany</td><td>73</td><td>23</td></tr><tr><td>France</td><td>46</td><td>23</td></tr><tr><td>Sweden</td><td>21</td><td>21</td></tr><tr><td>Romania</td><td>184</td><td>174</td></tr><tr><td>Turkey</td><td>2</td><td>0</td></tr></tbody></table> </ephtml> </p> <p>Graph: Figure 1 Geographical distribution of isotopic signature measurements (δ13C and/or δ2H of CH4) carried out within the MEMO2 project (2017 to 2020), depending on the type of source. (a) All locations. (b) Only in the UK, the Netherlands, and Germany.</p> <p>We characterised 376 locations by both <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H values, and we compared the results to ranges reported in the literature in Fig. . The fossil fuel sources partly overlap with the range of thermogenic CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> , but also spread towards lower <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C or higher <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H. This is due to the presence of natural gas of microbial origin in the coal reservoirs of Silesia, in Poland [[<reflink idref="bib73" id="ref138">73</reflink>], [<reflink idref="bib97" id="ref139">97</reflink>]], and in Romania [[<reflink idref="bib5" id="ref140">5</reflink>], [<reflink idref="bib42" id="ref141">42</reflink>], [<reflink idref="bib99" id="ref142">99</reflink>]]. We concluded that this microbial CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> originates from the CO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">2</mn></msub></math> </ephtml> reduction pathway as defined by [<reflink idref="bib100" id="ref143">100</reflink>], with relatively depleted <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo><</mo><mo>-</mo><mn mathvariant="normal">60</mn></mrow></math> </ephtml>  ‰) and relatively enriched <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>></mo><mo>-</mo><mn mathvariant="normal">250</mn></mrow></math> </ephtml>  ‰). The <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H measurements were in these cases particularly useful to distinguish fossil fuels from microbial sources [[<reflink idref="bib97" id="ref144">97</reflink>], [<reflink idref="bib42" id="ref145">42</reflink>], [<reflink idref="bib99" id="ref146">99</reflink>]].</p> <p>With an average <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">53.6</mn><mo>±</mo><mn mathvariant="normal">0.4</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">202</mn></mrow></math> </ephtml> ), the waste-related source signatures in the EMID generally have higher <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C values compared to typical microbial fermentation CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> (between <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">90</mn></mrow></math> </ephtml>  ‰ and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">50</mn></mrow></math> </ephtml>  ‰; [<reflink idref="bib100" id="ref147">100</reflink>]). Waste sources measured in previous studies are less enriched, with an average of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">56.0</mn><mo>±</mo><mn mathvariant="normal">1.0</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">56</mn></mrow></math> </ephtml> ) in [<reflink idref="bib129" id="ref148">129</reflink>]. The average value in the EMID is strongly influenced by particularly enriched isotopic compositions in CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emitted from sewage water (range between <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">72.7</mn></mrow></math> </ephtml> and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">36.5</mn></mrow></math> </ephtml> , average of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">50.5</mn><mo>±</mo><mn mathvariant="normal">0.7</mn></mrow></math> </ephtml> , <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">88</mn></mrow></math> </ephtml> ) and to a smaller extent from biogas plants (range between <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">64.4</mn></mrow></math> </ephtml>  ‰ and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">45.5</mn></mrow></math> </ephtml>  ‰, <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">54</mn></mrow></math> </ephtml> ). A new study also reported surprisingly enriched <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> (and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H) around a wastewater treatment plant in Australia: <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C  <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">47.6</mn><mo>±</mo><mn mathvariant="normal">2</mn></mrow></math> </ephtml>  ‰ [<reflink idref="bib85" id="ref149">85</reflink>]. Other recent studies in different regions of the world have also reported significantly higher <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> from sewage plants compared to landfills [[<reflink idref="bib60" id="ref150">60</reflink>], [<reflink idref="bib161" id="ref151">161</reflink>], [<reflink idref="bib2" id="ref152">2</reflink>]]. The <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emitted from sewage treatment plants depends on process parameters: oxic conditions lead to more enriched signatures than anaerobic treatment [<reflink idref="bib145" id="ref153">145</reflink>]. Regarding biogas facilities, [<reflink idref="bib7" id="ref154">7</reflink>] emphasised the link between the type of substrate and the emitted CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic signatures: facilities that operate with C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plant substrates emit CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> with higher <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C values in comparison with C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> plant substrates. Changes in waste management practices towards less disposal and more biogas production can likely explain the higher range of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C values found in recent studies [<reflink idref="bib6" id="ref155">6</reflink>]. Another driver for more or less enriched <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> emissions from waste sources is isotopic fractionation when CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> reacts or diffuses. Diffusion and oxidation in the soil layers when CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> migrates from the deeper layers are secondary processes that cause isotopic fractionation [[<reflink idref="bib10" id="ref156">10</reflink>], [<reflink idref="bib34" id="ref157">34</reflink>], [<reflink idref="bib31" id="ref158">31</reflink>], [<reflink idref="bib52" id="ref159">52</reflink>], [<reflink idref="bib110" id="ref160">110</reflink>], [<reflink idref="bib6" id="ref161">6</reflink>]], which increases the range of possible isotopic signatures of the emitted CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> .</p> <p>Graph: Figure 2 Dual isotope plot presenting measurement results from the EMID (circles); the literature data for the same source categories, taken from [[<reflink idref="bib129" id="ref162">129</reflink>]], and completed with the mean and standard deviation values from additional publications (squares with error bars); shaded areas represent the different methanogenesis pathways from [<reflink idref="bib100" id="ref163">100</reflink>]: MF: microbial fermentation, MC: microbial CO2 reduction, T: thermogenic, A: abiotic.</p> <p>Graph: Figure 3 Measurement results of δ13C (top) and δ2H (bottom) in CH4 from the EMID. (a) CH4 fugitive emissions from the exploitation of fossil fuels (gas leaks, oil and gas extraction, and processing sites). (b) CH4 emissions from modern microbial fermentation sources (ruminants, landfills, sewage treatment plants, and biogas plants).</p> <p>The maps in Fig.  emphasise the similarities between <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C source signatures from modern microbial and fossil fuel sources in Poland and Romania. The average <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> of fugitive emissions from fossil fuel extraction sites in Poland and Romania was <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">48.5</mn><mo>±</mo><mn mathvariant="normal">0.6</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">235</mn></mrow></math> </ephtml> ), and of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">55.3</mn><mo>±</mo><mn mathvariant="normal">1.2</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">42</mn></mrow></math> </ephtml> ) for gas leaks and gas fields in Romania. From gas leaks in only the UK and the Netherlands, the average <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> was <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">38.9</mn><mo>±</mo><mn mathvariant="normal">0.3</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">154</mn></mrow></math> </ephtml> ), and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">40.4</mn><mo>±</mo><mn mathvariant="normal">0.3</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">217</mn></mrow></math> </ephtml> ) when including France and Germany, which reflect differences in the natural gas formation pathway compared to Poland and Romania. This distinction is also visible in the histograms of the EMID in Fig. a. In western Europe, <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C allows for a good separation between microbial and fossil fuel sources, which is well-established in the literature [[<reflink idref="bib79" id="ref164">79</reflink>], [<reflink idref="bib82" id="ref165">82</reflink>], [<reflink idref="bib122" id="ref166">122</reflink>], [<reflink idref="bib166" id="ref167">166</reflink>], [<reflink idref="bib84" id="ref168">84</reflink>]]. Yet we show that only <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C data are not sufficient to distinguish microbial and fossil fuel CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> from all European regions. Fortunately, the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> source signatures allow for a clear distinction between fossil fuel and modern microbial emissions of anthropogenic origin (Figs.  and a).</p> <p>Previous isotopic measurements in Europe generally focused on western European countries [[<reflink idref="bib79" id="ref169">79</reflink>], [<reflink idref="bib10" id="ref170">10</reflink>], [<reflink idref="bib82" id="ref171">82</reflink>], [<reflink idref="bib122" id="ref172">122</reflink>], [<reflink idref="bib166" id="ref173">166</reflink>], [<reflink idref="bib19" id="ref174">19</reflink>], [<reflink idref="bib46" id="ref175">46</reflink>], [<reflink idref="bib84" id="ref176">84</reflink>], [<reflink idref="bib161" id="ref177">161</reflink>], [<reflink idref="bib36" id="ref178">36</reflink>]]. This geographical bias should be addressed by focusing on western Balkan countries (Croatia, Bosnia, and Serbia) because of coal extraction activities (EDGAR inventory), and densely populated areas in southern European countries such as Italy.</p> <hd id="AN0159611867-14">3.2 New global database</hd> <p></p> <hd id="AN0159611867-15">3.2.1 Overview and representativeness</hd> <p>The extended global database including all literature data and the aggregated MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> data consists of 13 313 and 4337 measurements of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H, respectively, from 64 countries. The map in Fig.  shows the partitioning of the measurement data per country, and Table  the number of records per CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> source. Table  contains statistics on the data from the EMID only, and the overall database including the EMID.</p> <p>Graph: Figure 4 Number of isotopic signature measurements, (a) δ13C and (b) δ2H of CH4, carried out in different countries. Top maps show the total numbers reported in the new global database. Bottom maps show the percentage of additional data brought by the EMID and the new published literature compared to [[<reflink idref="bib129" id="ref179">129</reflink>]].</p> <p>The number of measurements made in fossil fuel reservoirs and compiled in the database by [<reflink idref="bib130" id="ref180">130</reflink>] is comparatively larger than from studies of other CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emission sources (Table ), and the number of measurements is not evenly spread geographically: significantly more measurements were made in North American and European countries, Australia, Brazil, and Japan. In Russia and China, there were relatively more measurements as well, but only for fossil fuel sources. Despite including the first few measurements reported from Africa and the Middle East [[<reflink idref="bib47" id="ref181">47</reflink>], [<reflink idref="bib2" id="ref182">2</reflink>]], the data distribution remains unbalanced. Nevertheless, specific isotope signatures dependencies can be further analysed for the different source categories.</p> <p> <emph>Fossil fuels.</emph> Fugitive emissions from fossil fuel reservoirs are highly variable not only on a large scale, but also from one basin to another, or even within the same basin [[<reflink idref="bib129" id="ref183">129</reflink>], [<reflink idref="bib100" id="ref184">100</reflink>], [<reflink idref="bib3" id="ref185">3</reflink>], [<reflink idref="bib101" id="ref186">101</reflink>], [<reflink idref="bib76" id="ref187">76</reflink>]]. Therefore, CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic composition from one basin cannot simply be upscaled to a country scale. Any new isotopic measurement from a production basin with large fugitive CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions brings relevant information. The recent measurements made in Romania, included in the EMID, illustrate well this heterogeneity [<reflink idref="bib99" id="ref188">99</reflink>].</p> <p>[<reflink idref="bib129" id="ref189">129</reflink>] pointed out the lack of data for a list of conventional oil and gas and coal production countries, in Africa, the Middle East, central and southern Asia, and South America. Previous estimates of global CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic signatures from the exploitation of fossil fuels weighted the source signatures from one basin by its fuel production [<reflink idref="bib128" id="ref190">128</reflink>]. Recent work suggests that fuel production is not a reliable proxy to estimate CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> fugitive emissions [[<reflink idref="bib162" id="ref191">162</reflink>], [<reflink idref="bib3" id="ref192">3</reflink>], [<reflink idref="bib124" id="ref193">124</reflink>], [<reflink idref="bib28" id="ref194">28</reflink>], [<reflink idref="bib88" id="ref195">88</reflink>]]. Thus, the most relevant sampling locations would be ideally related to estimated emission rates from top-down measurements, instead of production or bottom-up emission estimates. Unfortunately, these data are lacking in many cases. Recently, particularly large CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions were detected in central Asia [<reflink idref="bib151" id="ref196">151</reflink>], or measured in Mexico [<reflink idref="bib163" id="ref197">163</reflink>]. Besides the new measurements in Romania, the EMID and additional literature we added to the global isotope database does not address the geographical representation issue.</p> <p> <emph>Modern microbial.</emph> The isotopic signatures of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> from modern microbial sources (mainly wetlands, ruminants, waste degradation, rice paddies, and termites) are largely dependent on environmental parameters such as the type of substrate and other ecosystem conditions. Figures  and show that our new data confirm the trends previously observed: the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C sensitivity to C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> or C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plants in ruminant diet [[<reflink idref="bib123" id="ref198">123</reflink>], [<reflink idref="bib79" id="ref199">79</reflink>], [<reflink idref="bib72" id="ref200">72</reflink>], [<reflink idref="bib17" id="ref201">17</reflink>]], to wetland latitudes ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C depletion in polar regions because of less oxidation and the absence of C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plants) [[<reflink idref="bib46" id="ref202">46</reflink>], [<reflink idref="bib17" id="ref203">17</reflink>], [<reflink idref="bib51" id="ref204">51</reflink>]], and the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H dependency on <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></math> </ephtml> of precipitation, and ultimately on the latitude (established for freshwater emissions) [[<reflink idref="bib155" id="ref205">155</reflink>], [<reflink idref="bib26" id="ref206">26</reflink>], [<reflink idref="bib37" id="ref207">37</reflink>], [<reflink idref="bib135" id="ref208">135</reflink>]]. Based on the correlation with the plant metabolism (C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> or C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> ), <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> from wetlands could be mapped on a global scale [<reflink idref="bib51" id="ref209">51</reflink>]. [<reflink idref="bib37" id="ref210">37</reflink>] also suggested a spatial extrapolation of wetland <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> using <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></math> </ephtml> data, which can be interesting for under-sampled locations, for example in the Southern Hemisphere. However, a certain variability will always remain because of the influence of other parameters such as the dominant methanogenic pathway (acetate fermentation or CO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">2</mn></msub></math> </ephtml> reduction) [[<reflink idref="bib154" id="ref211">154</reflink>], [<reflink idref="bib34" id="ref212">34</reflink>], [<reflink idref="bib31" id="ref213">31</reflink>], [<reflink idref="bib92" id="ref214">92</reflink>], [<reflink idref="bib64" id="ref215">64</reflink>], [<reflink idref="bib20" id="ref216">20</reflink>], [<reflink idref="bib37" id="ref217">37</reflink>]], or the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C composition of the organic matter substrate [[<reflink idref="bib32" id="ref218">32</reflink>], [<reflink idref="bib51" id="ref219">51</reflink>]].</p> <p> <emph>Biomass burning.</emph> Similarly to microbial degradation, the product of biomass burning is influenced by the plant constituents. CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> produced from the burning of C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> or C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plants can be distinguished based on the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> values [<reflink idref="bib24" id="ref220">24</reflink>], [<reflink idref="bib17" id="ref221">17</reflink>]. Higher <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C signatures are measured when the burned plants are mostly C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plants, and the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> is lower for C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> plants. This trend is clearly visible in the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotope dataset, and is shown in Fig. . The <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> values are expected to depend on the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H of local precipitations [[<reflink idref="bib133" id="ref222">133</reflink>], [<reflink idref="bib121" id="ref223">121</reflink>]], but more measurements are needed to support this hypothesis.</p> <p>Table 4 Statistical information on the results for the main CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> source categories of the EMID and the update of the global database including the EMID and additional literature data. SD: standard deviation, SE: standard error of the mean.</p> <p> <ephtml> <table><thead><tr><td>Variable</td><td>Statistic</td><td>Fossil fuel </td><td>Modern microbial </td><td>Pyrogenic </td></tr><tr><td>Conventional</td><td>Coal</td><td>Shale</td><td>All</td><td>Wetlands</td><td>Rice paddies</td><td>Ruminants</td><td>Waste</td><td>All</td><td>Biomass burning</td><td>Fuel combustion</td></tr></thead><tbody><tr><td>EMID <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">n</mi></math></p> events</td><td>381</td><td>71</td><td /><td>457</td><td>21</td><td /><td>30</td><td>202</td><td>253</td><td /><td>4</td></tr><tr><td>mean</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">43.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">48.7</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">45.0</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">73.6</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.0</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">53.6</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">56.4</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">34.6</mn></mrow></math></p></td></tr><tr><td>median</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">42.0</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">48.9</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">43.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">72.7</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">62.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">53.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">55.6</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">38.0</mn></mrow></math></p></td></tr><tr><td>min</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">71.2</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">65.4</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">82.1</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">96.1</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">73.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">72.7</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">96.1</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">42.7</mn></mrow></math></p></td></tr><tr><td>max</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">19.6</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">18.3</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">18.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">55.1</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">56.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">36.5</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">36.5</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">19.6</mn></mrow></math></p></td></tr><tr><td>SD</td><td>8.19</td><td>7.84</td><td /><td>8.93</td><td>10.4</td><td /><td>3.87</td><td>5.90</td><td>8.60</td><td /><td>10.3</td></tr><tr><td>SE</td><td>0.42</td><td>0.93</td><td /><td>0.42</td><td>2.27</td><td /><td>0.71</td><td>0.42</td><td>0.54</td><td /><td>5.15</td></tr><tr><td>global <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">n</mi></math></p> locations</td><td>238</td><td>66</td><td>5</td><td>313</td><td>108</td><td>24</td><td>43</td><td>102</td><td>285</td><td>30</td><td>10</td></tr><tr><td>mean</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">44.5</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">50.7</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">43.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">45.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">59.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.0</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">54.6</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">59.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">26.1</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">22.7</mn></mrow></math></p></td></tr><tr><td>median</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">42.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">50.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">42.2</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">44.6</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.1</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">59.5</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">54.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">59.0</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">27.2</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">20.3</mn></mrow></math></p></td></tr><tr><td>min</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">77.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">72.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">49.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">77.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">88.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">67.2</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">74.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">73.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">88.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">33.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">39.6</mn></mrow></math></p></td></tr><tr><td>max</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">18.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">25.6</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">39.5</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">18.9</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">44.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">50.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">50.3</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">45.1</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">44.4</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">12.5</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">9.00</mn></mrow></math></p></td></tr><tr><td>SD</td><td>8.44</td><td>10.4</td><td>3.84</td><td>9.16</td><td>8.17</td><td>4.53</td><td>5.31</td><td>4.90</td><td>7.61</td><td>5.24</td><td>11.2</td></tr><tr><td>SE</td><td>0.55</td><td>1.28</td><td>1.72</td><td>0.52</td><td>0.79</td><td>0.92</td><td>0.81</td><td>0.49</td><td>0.45</td><td>0.96</td><td>3.55</td></tr><tr><td>EMID <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">n</mi></math></p> events</td><td>220</td><td>40</td><td /><td>268</td><td>21</td><td /><td>11</td><td>94</td><td>126</td><td /><td>1</td></tr><tr><td>mean</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">181</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">185</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">182</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">325</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">310</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">305</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">309</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">129</mn></mrow></math></p></td></tr><tr><td>median</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">184</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">184</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">185</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">337</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">304</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">303</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">307</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">129</mn></mrow></math></p></td></tr><tr><td>min</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">355</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">271</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">355</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">379</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">359</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">466</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">466</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">129</mn></mrow></math></p></td></tr><tr><td>max</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">85.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.8</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">63.8</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">258</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">259</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">93.2</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">93.2</mn></mrow></math></p></td><td /><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">129</mn></mrow></math></p></td></tr><tr><td>SD</td><td>39.5</td><td>30.7</td><td /><td>39.1</td><td>41.2</td><td /><td>25.6</td><td>54.8</td><td>51.1</td><td /><td /></tr><tr><td>SE</td><td>2.7</td><td>4.9</td><td /><td>2.4</td><td>9.0</td><td /><td>7.7</td><td>5.7</td><td>4.6</td><td /><td /></tr><tr><td>global <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mi xmlns="">n</mi></math></p> locations</td><td>118</td><td>37</td><td>4</td><td>164</td><td>32</td><td>4</td><td>13</td><td>41</td><td>92</td><td>5</td><td>6</td></tr><tr><td>mean</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">183</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">210</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">147</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">189</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">319</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">323</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">310</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">292</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">306</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">226</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">136</mn></mrow></math></p></td></tr><tr><td>median</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">179</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">208</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">140</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">187</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">309</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">328</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">308</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">301</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">308</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">210</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">126</mn></mrow></math></p></td></tr><tr><td>min</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">263</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">310</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">191</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">349</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">472</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">336</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">404</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">344</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">472</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">285</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">192</mn></mrow></math></p></td></tr><tr><td>max</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">101</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">162</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">116</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">101</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">246</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">301</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">224</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">113</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">113</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">195</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">81.0</mn></mrow></math></p></td></tr><tr><td>SD</td><td>32.4</td><td>27.4</td><td>32.1</td><td>35.8</td><td>53.2</td><td>15.6</td><td>45.0</td><td>45.7</td><td>48.3</td><td>35.8</td><td>39.4</td></tr><tr><td>SE</td><td>3.0</td><td>4.5</td><td>16.0</td><td>2.8</td><td>9.4</td><td>7.8</td><td>12.5</td><td>7.1</td><td>5.0</td><td>16.0</td><td>16.1</td></tr></tbody></table> </ephtml> </p> <hd id="AN0159611867-16">3.2.2 Global data and the EMID</hd> <p>Statistical information on the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic signatures in the complete extended database are presented in Table . Figure  shows the distribution frequency of isotope signatures for the source categories that represent the largest reported emissions [<reflink idref="bib125" id="ref224">125</reflink>]. The categories agriculture, waste, wetlands, and partly other natural are all of modern microbial origin, mostly from acetate fermentation [<reflink idref="bib100" id="ref225">100</reflink>]. The different categories within microbial processes generally overlap (Fig. ). Some differences can however be observed, such as the wetlands mean <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> being lower in the EMID than globally ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">73.6</mn><mo>±</mo><mn mathvariant="normal">2.27</mn></mrow></math> </ephtml>  ‰ compared to <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">63.3</mn><mo>±</mo><mn mathvariant="normal">0.79</mn></mrow></math> </ephtml>  ‰), because the European samples were taken at relatively high latitudes (Sect. ). Table  also shows that waste sources present more enriched <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> values than other modern microbial sources. This difference is particularly visible in the EMID, where a relatively large number of sites from waste-related sources were sampled. As mentioned in Sect. , additional parameters control the isotopic signature of the emitted CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> , such as the type of substrate, the presence of oxygen, or secondary (e.g. oxidation) processes. The minimum waste <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C signature of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">73.9</mn></mrow></math> </ephtml>  ‰ is comparable to the low values of other microbial sources, which supports the hypothesis of a larger influence of secondary processes in waste degradation relative to other microbial CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> formation. We recommend to separate the waste category from the other microbial sources to minimise the uncertainty in the assigned isotopic signature, at least for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C. The range of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H signatures from waste sources is larger than of the other modern microbial sources, but the average <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H from the different microbial sources are similar. One can see that <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H is not systematically correlated with <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C, and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H can also vary with other parameters such as the isotopic composition of water in the substrate. The <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H signatures for waste are based on less measurements compared to <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C (42 % of all measured waste sources included <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H signatures). The relation between <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> from wetlands and the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></math> </ephtml> from precipitation has been established previously [[<reflink idref="bib155" id="ref226">155</reflink>], [<reflink idref="bib26" id="ref227">26</reflink>], [<reflink idref="bib37" id="ref228">37</reflink>]]. We also know that the fractionation factors derived for CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> microbial oxidation are much larger for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H than for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C [[<reflink idref="bib30" id="ref229">30</reflink>], [<reflink idref="bib10" id="ref230">10</reflink>], [<reflink idref="bib26" id="ref231">26</reflink>]]. Nevertheless, further <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H measurements are required to better define the isotopic dependencies to secondary processes.</p> <p>In [[<reflink idref="bib129" id="ref232">129</reflink>]], the pyrogenic category only contained biomass burning data, and the binary distribution clearly illustrates the difference between C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> and C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plants in terms of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> signatures: the averages in the global database are <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">28.4</mn><mo>±</mo><mn mathvariant="normal">0.65</mn></mrow></math> </ephtml>  ‰ and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">18.0</mn><mo>±</mo><mn mathvariant="normal">1.9</mn></mrow></math> </ephtml>  ‰, for C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> and C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plants, respectively. The additional biomass burning data we added from published literature confirm the dependency of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> on the plant metabolism (Fig. ). We also added pyrogenic data from fuel combustion (burning of fossil fuel) from both our measurements and the literature. The resulting distribution of the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C data is smoother than in [<reflink idref="bib129" id="ref233">129</reflink>] (Fig. ), because the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> from fossil fuel burning does not show a clear distinction between C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">3</mn></msub></math> </ephtml> /C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> plant metabolisms. <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> isotopic signatures from pyrogenic sources cover a wide range of values, and overlap with the ones of fossil fuels. <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H signatures allow to clearly distinguish between biomass and fuel combustion (Table ), but this is based on a very low number of measurements. Further analysis including data on <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></math> </ephtml> could help to parameterise the biomass burning <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> in more detail [<reflink idref="bib152" id="ref234">152</reflink>], similar to the above mentioned relation between <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></math> </ephtml> [[<reflink idref="bib155" id="ref235">155</reflink>], [<reflink idref="bib26" id="ref236">26</reflink>], [<reflink idref="bib121" id="ref237">121</reflink>], [<reflink idref="bib37" id="ref238">37</reflink>]].</p> <p>Fugitive CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions from fossil fuel source locations present a wide range of isotopic signatures: <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C from <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">77.4</mn></mrow></math> </ephtml>  ‰ to <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">18.9</mn></mrow></math> </ephtml>  ‰, and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H from <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">349</mn></mrow></math> </ephtml>  ‰ to <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">101</mn></mrow></math> </ephtml>  ‰ (Table ). The average signatures of all fugitive CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions from the exploitation of fossil fuels (excluding seeps) in the EMID were <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow class="chem"><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup><mi mathvariant="normal">C</mi></mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">44.6</mn><mo>±</mo><mn mathvariant="normal">0.4</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">452</mn></mrow></math> </ephtml> ) and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow class="chem"><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup><mi mathvariant="normal">H</mi><mo>=</mo><mo>-</mo><mn mathvariant="normal">182</mn><mo>±</mo><mn mathvariant="normal">2.4</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">259</mn></mrow></math> </ephtml> ), which compares well with the global average of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi mathvariant="normal">−</mi><mn mathvariant="normal">44.8</mn><mo>±</mo><mn mathvariant="normal">0.1</mn></mrow></math> </ephtml>  ‰ ( <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>n</mi><mo>=</mo><mn mathvariant="normal">8128</mn></mrow></math> </ephtml> ) calculated in [<reflink idref="bib129" id="ref239">129</reflink>]. Regarding the present updated global database, the weighted averages were <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C  <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">46.6</mn><mo>±</mo><mn mathvariant="normal">1.8</mn></mrow></math> </ephtml>  ‰ and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H  <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">192</mn><mo>±</mo><mn mathvariant="normal">7.5</mn></mrow></math> </ephtml>  ‰, weighted by the relative emissions from conventional and coal fuels production worldwide. The average values from the different databases are lower than <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H values used in most global models (Table ), and to the value of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>-</mo><mn mathvariant="normal">44.0</mn><mo>±</mo><mn mathvariant="normal">0.7</mn></mrow></math> </ephtml>  ‰ suggested by [<reflink idref="bib128" id="ref240">128</reflink>]. The global means in Table  do not necessarily represent the global isotopic signature of fossil fuel emissions, because this should be weighted by the magnitude of emissions in the different basins, which was taken into account (using production as indicator) in the calculation by [<reflink idref="bib128" id="ref241">128</reflink>]. However, our averages are indications of the general CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic signatures from all measurements until now. Because of the high heterogeneity of the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> values from fossil fuel-related activities, and the temporal variations in the production from the different regions [[<reflink idref="bib134" id="ref242">134</reflink>], [<reflink idref="bib149" id="ref243">149</reflink>], [<reflink idref="bib76" id="ref244">76</reflink>]], it is important to assume a relatively large uncertainty when estimating in the global signature of fossil fuel emissions in atmospheric models.</p> <p>In Sect. , we have shown the use of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> to distinguish fossil fuel emissions in western Europe, and the need for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> measurements in central and eastern Europe. In the global database, most fossil fuels records (83.5 %) have <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> values <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo>></mo><mo>-</mo><mn mathvariant="normal">250</mn></mrow></math> </ephtml>  ‰. The few values of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mo><</mo><mo>-</mo><mn mathvariant="normal">300</mn></mrow></math> </ephtml>  ‰, indicating microbial fermentation as gas origin, were found in some coal formations in the United States and Canada. Figure  still allows us to generally conclude that <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H measurements are more suitable to distinguish fossil fuel vs. biogenic CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources at the global scale than <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C only, which further emphasises the need for more <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> measurements.</p> <p>The extraction of shale gas is growing worldwide [[<reflink idref="bib38" id="ref245">38</reflink>]], and the associated CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> emissions [[<reflink idref="bib63" id="ref246">63</reflink>], [<reflink idref="bib102" id="ref247">102</reflink>]]. However, shale gas commercial production does not increase in Europe [<reflink idref="bib38" id="ref248">38</reflink>], and so the emphasis of this study is limited to oil, gas, and coal fuels.</p> <p>Graph: Figure 5 Distribution of δ13C (a, c) and δ2H (b, d) in CH4 for different source categories. (a) Single measurements reported in the EMID (absolute numbers). "fossil fuels -E" shows fossil fuels data from Poland and Romania, and "fossil fuels -W" from the UK, the Netherlands, Germany, and France. (b) Measured locations in all datasets [<reflink idref="bib130" id="ref249">130</reflink>], with EMID locations and additional literature) (normalised probability density). "Agriculture" represents ruminants and rice paddies emissions.</p> <p>Table 5 CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic source signatures assigned to the fossil fuel-related emissions in past global-scale models (not an exhaustive list).</p> <p> <ephtml> <table><thead><tr><td>Reference</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math></p>C VPDB [‰]</td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math></p>H VSMOW [‰]</td></tr></thead><tbody><tr><td><xref ref-type="bibr" rid="bibr56">56</xref>, <xref ref-type="bibr" rid="bibr147">147</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">38</mn></mrow></math></p>/<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><msup><mn mathvariant="normal">37</mn><mi mathvariant="normal">a</mi></msup></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">175</mn></mrow></math></p></td></tr><tr><td><xref ref-type="bibr" rid="bibr107">107</xref>, <xref ref-type="bibr" rid="bibr104">104</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">40</mn></mrow></math></p>/<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><msup><mn mathvariant="normal">35</mn><mi mathvariant="normal">a</mi></msup></mrow></math></p></td><td /></tr><tr><td><xref ref-type="bibr" rid="bibr119">119</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><msup><mn mathvariant="normal">40</mn><mi mathvariant="normal">b</mi></msup></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">175</mn></mrow></math></p></td></tr><tr><td><xref ref-type="bibr" rid="bibr117">117</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">41.7</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">175</mn></mrow></math></p></td></tr><tr><td><xref ref-type="bibr" rid="bibr126">126</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">37</mn></mrow></math></p></td><td /></tr><tr><td><xref ref-type="bibr" rid="bibr128">128</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">44</mn></mrow></math></p></td><td /></tr><tr><td><xref ref-type="bibr" rid="bibr93">93</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">42.6</mn></mrow></math></p></td><td /></tr><tr><td><xref ref-type="bibr" rid="bibr48">48</xref></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">45.2</mn></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">209</mn></mrow></math></p></td></tr><tr><td>This database, mean <p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mo xmlns="">±</mo></math></p> sem<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup xmlns=""><mi /><mi mathvariant="normal">c</mi></msup></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">44.5</mn><mo>±</mo><mn mathvariant="normal">0.5</mn></mrow></math></p>/<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">50.7</mn><mo>±</mo><msup><mn mathvariant="normal">1.3</mn><mi mathvariant="normal">a</mi></msup></mrow></math></p></td><td><p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">182</mn><mo>±</mo><mn mathvariant="normal">3.0</mn></mrow></math></p>/<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow xmlns=""><mo>-</mo><mn mathvariant="normal">210</mn><mo>±</mo><msup><mn mathvariant="normal">4.5</mn><mi mathvariant="normal">a</mi></msup></mrow></math></p></td></tr></tbody></table> </ephtml> </p> <p> <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mi mathvariant="normal">a</mi></msup></math> </ephtml>  for natural gas/coal; <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mi mathvariant="normal">b</mi></msup></math> </ephtml>  also in [<reflink idref="bib78" id="ref250">78</reflink>], [<reflink idref="bib62" id="ref251">62</reflink>], [<reflink idref="bib14" id="ref252">14</reflink>], [<reflink idref="bib55" id="ref253">55</reflink>], [<reflink idref="bib143" id="ref254">143</reflink>]; <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mi mathvariant="normal">c</mi></msup></math> </ephtml>  standard error of the mean.</p> <hd id="AN0159611867-17">4 Data availability</hd> <p>The database is made freely available to the scientific community in the belief that it provides the most complete picture of the stable isotopic composition of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> sources. The free availability of these data does not constitute permission for publication of the data. For research projects, if the data used are essential to the work to be published, or if the conclusion or results largely depend on the data, co-authorship should be considered. Full contact details and information on how to cite the data are given in the accompanying database. The database is currently stored in a publicly available repository: 10.24416/UU01-YP43IN[<reflink idref="bib98" id="ref255">98</reflink>].</p> <hd id="AN0159611867-18">5 Conclusions</hd> <p>This study presents an updated dataset of isotopic source signatures of CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> from recent atmospheric measurements, while including additional data from published literature which were not previously included. The new data are a contribution from the EMID, that results from the sampling activities performed within the MEMO <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi /><mn mathvariant="normal">2</mn></msup></math> </ephtml> project. It represents a substantial contribution to the global dataset for fugitive fossil fuels and waste sources, mainly sampled in urban areas.</p> <p>We have highlighted two main improvements in our understanding of the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> isotopic composition: (i) a more robust range of values for modern microbial sources, and a better characterisation of the <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C enrichment in CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> from waste sources; and (ii) fossil fuel-related sources could have more depleted values than previous estimates used in global models. In this respect, our data confirm the analysis made by [<reflink idref="bib128" id="ref256">128</reflink>].</p> <p>Finally, the new European data contain comparatively more <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H measurements. In the case of fossil fuel emissions, the use of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> is of particular interest. In general, utilising both <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C and <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H for CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> improves our ability to clearly separate fossil fuel and microbial sources, compared to <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">13</mn></msup></mrow></math> </ephtml> C alone. The use of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H as additional constraint could help to answer open questions regarding the CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> global budget. To better understand the drivers of <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H variability (except for <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H of precipitation), more measurements are required, especially of pyrogenic and waste sources.</p> <p>The present dataset can be used for CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> source attribution, studies at local and regional scales, and to derive global source signatures for input to global methane cycle modelling studies. The larger dataset will also help to estimate the uncertainties to take into account when using isotopic data in top-down studies, and with prior knowledge of the specificities of the studied region, the use of isotopic data in top-down studies is a powerful tool to evaluate the bottom-up emission inventories [[<reflink idref="bib3" id="ref257">3</reflink>], [<reflink idref="bib40" id="ref258">40</reflink>], [<reflink idref="bib124" id="ref259">124</reflink>], [<reflink idref="bib134" id="ref260">134</reflink>]]. A future improvement of this database would be to include more measurements on the African, Asian, and South American continents, where experimental studies are lacking. Because of its potential for source characterisation, new studies should also focus on <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi mathvariant="italic">δ</mi><mn mathvariant="normal">2</mn></msup></mrow></math> </ephtml> H <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mrow><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></math> </ephtml> measurements. The maintenance of a CH <ephtml> <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi /><mn mathvariant="normal">4</mn></msub></math> </ephtml> stable isotope database relies on a certain transparency of different groups around the world on their work. Therefore we strongly encourage the scientific community to pursue the efforts to make scientific data open-access more systematically.</p> <hd id="AN0159611867-19">Appendix A Supplementary material</hd> <p>Graph: Figure A1 Measured δ13CCH4 signatures from ruminants in the literature studies [[<reflink idref="bib2" id="ref261">2</reflink>], [<reflink idref="bib17" id="ref262">17</reflink>], [<reflink idref="bib71" id="ref263">71</reflink>], [<reflink idref="bib79" id="ref264">79</reflink>], [<reflink idref="bib85" id="ref265">85</reflink>], [<reflink idref="bib123" id="ref266">123</reflink>], [<reflink idref="bib144" id="ref267">144</reflink>], [<reflink idref="bib153" id="ref268">153</reflink>]] according to the feed: a majority of C3 plants (blue) or C4 plants (red). Bar heights represent the number of measurements and black lines standard deviations.</p> <p>Graph: Figure A2 Measured δ13CCH4 and δ2HCH4 signatures from wetlands sites as reported in the literature [[<reflink idref="bib9" id="ref269">9</reflink>], [<reflink idref="bib18" id="ref270">18</reflink>], [<reflink idref="bib33" id="ref271">33</reflink>], [<reflink idref="bib57" id="ref272">57</reflink>], [<reflink idref="bib75" id="ref273">75</reflink>], [<reflink idref="bib77" id="ref274">77</reflink>], [<reflink idref="bib79" id="ref275">79</reflink>], [<reflink idref="bib90" id="ref276">90</reflink>], [<reflink idref="bib105" id="ref277">105</reflink>], [<reflink idref="bib132" id="ref278">132</reflink>], [<reflink idref="bib138" id="ref279">138</reflink>], [<reflink idref="bib148" id="ref280">148</reflink>], [<reflink idref="bib153" id="ref281">153</reflink>], [<reflink idref="bib158" id="ref282">158</reflink>], [<reflink idref="bib160" id="ref283">160</reflink>]] (solid circled) and EMID (open circles) database, colour-coded by the latitude zones. Error bars show the standard deviations.</p> <p>Graph: Figure A3 Measured δ13CCH4 signatures from biomass burning in literature studies [[<reflink idref="bib136" id="ref284">136</reflink>], [<reflink idref="bib153" id="ref285">153</reflink>], [<reflink idref="bib79" id="ref286">79</reflink>], [<reflink idref="bib24" id="ref287">24</reflink>], [<reflink idref="bib133" id="ref288">133</reflink>], [<reflink idref="bib45" id="ref289">45</reflink>], [<reflink idref="bib148" id="ref290">148</reflink>], [<reflink idref="bib17" id="ref291">17</reflink>]] according to the type of vegetation: a majority of C3 plants (blue) or C4 plants (red). Bar heights represent the number of measurements and black lines standard deviations.</p> <hd id="AN0159611867-20">Acknowledgements</hd> <p>We thank all the staff from different organisations involved in the MEMO2, CoMet, and ROMEO projects who participated in the sample collection.We acknowledge the technical staff at UU and RHUL for the maintenance of the IRMS systems. This work was supported by the ITN project "Methane goes Mobile – Measurements and Modelling" (MEMO2; , last access: 3 November 2021).</p> <ref id="AN0159611867-21"> <title> Footnotes </title> <blist> <bibl id="bib1" idref="ref77" type="bt">1</bibl> <bibtext> Marie Skłodowska-Curie Actions, Horizon 2020 Innovative Training Networks founded under the grant agreement no. 722479: https://cordis.europa.eu/project/id/722479 (last access: 21 September 2022)</bibtext> </blist> <blist> <bibl id="bib2" idref="ref133" type="bt">2</bibl> <bibtext> Carbon dioxide and Methane mission, May–June 2018</bibtext> </blist> <blist> <bibl id="bib3" idref="ref185" type="bt">3</bibl> <bibtext> ROmanian Methane Emissions from Oil & gas, October 2019</bibtext> </blist> <blist> <bibl id="bib4" idref="ref67" type="bt">4</bibl> <bibtext> European Commission, Joint Research Centre (EC–JRC)/Netherlands Environmental Assessment Agency (PBL), May 2021. Emissions Database for Global Atmospheric Research (EDGAR).</bibtext> </blist> <blist> <bibl id="bib5" idref="ref140" type="bt">5</bibl> <bibtext> Relative weights of 0.66 for conventional fuels (oil and natural gas) and 0.34 for coal. Emission data from [125].</bibtext> </blist> <blist> <bibl id="bib6" idref="ref59" type="bt">6</bibl> <bibtext> Author contributions MM, CvdV, DL, JMF, SB, JLF, and REF performed the isotopic measurements. MM, TR, DL, JLF, SB, JMF, REF, HM, MS, JN, KV, PŁ, PK, MS, SD, and TA took part in the collection of samples. MM gathered and analysed the data and prepared the figures; TR and DL contributed to the interpretation of the data. 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  Data: New contributions of measurements in Europe to the global inventory of the stable isotopic composition of methane
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  Data: <searchLink fieldCode="AR" term="%22M%2E+Menoud%22">M. Menoud</searchLink><br /><searchLink fieldCode="AR" term="%22C%2E+van+der+Veen%22">C. van der Veen</searchLink><br /><searchLink fieldCode="AR" term="%22D%2E+Lowry%22">D. Lowry</searchLink><br /><searchLink fieldCode="AR" term="%22J%2E+M%2E+Fernandez%22">J. M. Fernandez</searchLink><br /><searchLink fieldCode="AR" term="%22S%2E+Bakkaloglu%22">S. Bakkaloglu</searchLink><br /><searchLink fieldCode="AR" term="%22J%2E+L%2E+France%22">J. L. France</searchLink><br /><searchLink fieldCode="AR" term="%22R%2E+E%2E+Fisher%22">R. E. Fisher</searchLink><br /><searchLink fieldCode="AR" term="%22H%2E+Maazallahi%22">H. Maazallahi</searchLink><br /><searchLink fieldCode="AR" term="%22M%2E+Stanisavljević%22">M. Stanisavljević</searchLink><br /><searchLink fieldCode="AR" term="%22J%2E+Nęcki%22">J. Nęcki</searchLink><br /><searchLink fieldCode="AR" term="%22K%2E+Vinkovic%22">K. Vinkovic</searchLink><br /><searchLink fieldCode="AR" term="%22P%2E+Łakomiec%22">P. Łakomiec</searchLink><br /><searchLink fieldCode="AR" term="%22J%2E+Rinne%22">J. Rinne</searchLink><br /><searchLink fieldCode="AR" term="%22P%2E+Korbeń%22">P. Korbeń</searchLink><br /><searchLink fieldCode="AR" term="%22M%2E+Schmidt%22">M. Schmidt</searchLink><br /><searchLink fieldCode="AR" term="%22S%2E+Defratyka%22">S. Defratyka</searchLink><br /><searchLink fieldCode="AR" term="%22C%2E+Yver-Kwok%22">C. Yver-Kwok</searchLink><br /><searchLink fieldCode="AR" term="%22T%2E+Andersen%22">T. Andersen</searchLink><br /><searchLink fieldCode="AR" term="%22H%2E+Chen%22">H. Chen</searchLink><br /><searchLink fieldCode="AR" term="%22T%2E+Röckmann%22">T. Röckmann</searchLink>
– Name: TitleSource
  Label: Source
  Group: Src
  Data: Earth System Science Data, Vol 14, Pp 4365-4386 (2022)
– Name: Publisher
  Label: Publisher Information
  Group: PubInfo
  Data: Copernicus Publications, 2022.
– Name: DatePubCY
  Label: Publication Year
  Group: Date
  Data: 2022
– Name: Subset
  Label: Collection
  Group: HoldingsInfo
  Data: LCC:Environmental sciences<br />LCC:Geology
– Name: Subject
  Label: Subject Terms
  Group: Su
  Data: <searchLink fieldCode="DE" term="%22Environmental+sciences%22">Environmental sciences</searchLink><br /><searchLink fieldCode="DE" term="%22GE1-350%22">GE1-350</searchLink><br /><searchLink fieldCode="DE" term="%22Geology%22">Geology</searchLink><br /><searchLink fieldCode="DE" term="%22QE1-996%2E5%22">QE1-996.5</searchLink>
– Name: Abstract
  Label: Description
  Group: Ab
  Data: Recent climate change mitigation strategies rely on the reduction of methane (CH4) emissions. Carbon and hydrogen isotope ratio (δ13CCH4 and δ2HCH4) measurements can be used to distinguish sources and thus to understand the CH4 budget better. The CH4 emission estimates by models are sensitive to the isotopic signatures assigned to each source category, so it is important to provide representative estimates of the different CH4 source isotopic signatures worldwide. We present new measurements of isotope signatures of various, mainly anthropogenic, CH4 sources in Europe, which represent a substantial contribution to the global dataset of source isotopic measurements from the literature, especially for δ2HCH4. They improve the definition of δ13CCH4 from waste sources, and demonstrate the use of δ2HCH4 for fossil fuel source attribution. We combined our new measurements with the last published database of CH4 isotopic signatures and with additional literature, and present a new global database. We found that microbial sources are generally well characterised. The large variability in fossil fuel isotopic compositions requires particular care in the choice of weighting criteria for the calculation of a representative global value. The global dataset could be further improved by measurements from African, South American, and Asian countries, and more measurements from pyrogenic sources. We improved the source characterisation of CH4 emissions using stable isotopes and associated uncertainty, to be used in top-down studies. We emphasise that an appropriate use of the database requires the analysis of specific parameters in relation to source type and the region of interest. The final version of the European CH4 isotope database coupled with a global inventory of fossil and non-fossil δ13CCH4 and δ2HCH4 source signature measurements is available at https://doi.org/10.24416/UU01-YP43IN (Menoud et al., 2022a).
– Name: TypeDocument
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  Data: article
– Name: Format
  Label: File Description
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  Data: electronic resource
– Name: Language
  Label: Language
  Group: Lang
  Data: English
– Name: ISSN
  Label: ISSN
  Group: ISSN
  Data: 1866-3508<br />1866-3516
– Name: NoteTitleSource
  Label: Relation
  Group: SrcInfo
  Data: https://essd.copernicus.org/articles/14/4365/2022/essd-14-4365-2022.pdf; https://doaj.org/toc/1866-3508; https://doaj.org/toc/1866-3516
– Name: DOI
  Label: DOI
  Group: ID
  Data: 10.5194/essd-14-4365-2022
– Name: URL
  Label: Access URL
  Group: URL
  Data: <link linkTarget="URL" linkTerm="https://doaj.org/article/9ae50a7b6c2f4f2d93dd6d0fcd376441" linkWindow="_blank">https://doaj.org/article/9ae50a7b6c2f4f2d93dd6d0fcd376441</link>
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  Data: edsdoj.9ae50a7b6c2f4f2d93dd6d0fcd376441
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      – Text: English
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      – SubjectFull: Environmental sciences
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      – SubjectFull: GE1-350
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      – SubjectFull: Geology
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      – TitleFull: New contributions of measurements in Europe to the global inventory of the stable isotopic composition of methane
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