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<p>The carbon isotopic signature (<span class="inline-formula"><i>δ</i>¹³CH₄</span>) of several methane sources in Germany (around Heidelberg and in North Rhine-Westphalia) were characterised. Mobile measurements of the plume of <span class="inline-formula">CH₄</span> sources are carried out using an analyser based on cavity ring-down spectroscopy (CRDS). To achieve precise results a CRDS analyser, which measures methane (<span class="inline-formula">CH₄</span>), carbon dioxide (<span class="inline-formula">CO₂</span>) and their <span class="inline-formula">¹³C</span>-to-<span class="inline-formula">¹²C</span> ratios, was characterised especially with regard to cross sensitivities of composition differences of the gas matrix in air samples or calibration tanks. The two most important gases which affect <span class="inline-formula"><i>δ</i>¹³CH₄</span> are water vapour (<span class="inline-formula">H₂O</span>) and ethane (<span class="inline-formula">C₂H₆</span>). To avoid the cross sensitivity with <span class="inline-formula">H₂O</span>, the air is dried with a Nafion dryer during mobile measurements. <span class="inline-formula">C₂H₆</span> is typically abundant in natural gases and thus in methane plumes or samples originating from natural gas. A <span class="inline-formula">C₂H₆</span> correction and calibration are essential to obtain accurate <span class="inline-formula"><i>δ</i>¹³CH₄</span> results, which can deviate by up to 3<span class="inline-formula"> ‰</span> depending on whether a <span class="inline-formula">C₂H₆</span> correction is applied.</p> <p>The isotopic signature is determined with the Miller–Tans approach and the York fitting method. During 21 field campaigns the mean <span class="inline-formula"><i>δ</i>¹³CH₄</span> signatures of three dairy farms (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">63.9</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace width="0.125em" linebreak="nobreak"/><mn mathvariant="normal">0.9</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="27fd5f11090c722c5495c5a9d9d19db4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00001.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00001.png"/></svg:svg></span></span>), a biogas plant (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">62.4</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace linebreak="nobreak" width="0.125em"/><mn mathvariant="normal">1.2</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="b2ddbe2dd3a2809e51b55aedd8579198"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00002.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00002.png"/></svg:svg></span></span>), a landfill (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">58.7</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace width="0.125em" linebreak="nobreak"/><mn mathvariant="normal">3.3</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="50b39e440729ad23ad6b0178294ff829"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00003.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00003.png"/></svg:svg></span></span>), a wastewater treatment plant (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">52.5</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace linebreak="nobreak" width="0.125em"/><mn mathvariant="normal">1.4</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="85bf95251d077709df5838a2f4b7c507"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00004.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00004.png"/></svg:svg></span></span>), an active deep coal mine (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">56.0</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace linebreak="nobreak" width="0.125em"/><mn mathvariant="normal">2.3</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="91102ea32efe46eaf90e129c09b005c5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00005.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00005.png"/></svg:svg></span></span>) and two natural gas storage and gas compressor stations (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">46.1</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace width="0.125em" linebreak="nobreak"/><mn mathvariant="normal">0.8</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="33bf1d0d43d7bdc04ff3a92ebb305e15"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00006.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00006.png"/></svg:svg></span></span>) were recorded.</p> <p>In addition, between December 2016 and November 2018 gas samples from the Heidelberg natural gas distribution network were measured with a mean <span class="inline-formula"><i>δ</i>¹³CH₄</span> value of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">43.3</mn><mspace width="0.125em" linebreak="nobreak"/><mo>±</mo><mspace linebreak="nobreak" width="0.125em"/><mn mathvariant="normal">0.8</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">‰</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="382005c8abfd1751ba430f661ee39d45"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1123-2019-ie00007.svg" width="73pt" height="10pt" src="amt-12-1123-2019-ie00007.png"/></svg:svg></span></span>. Contrary to previous measurements between 1991 and 1996 by <span class="cit" id="xref_text.1"><a href="#bib1.bibx17">Levin et al.</a> (<a href="#bib1.bibx17">1999</a>)</span>, no strong seasonal cycle is shown.</p>