How do Cl concentrations matter for the simulation of CH<sub>4</sub> and <i>δ</i><sup>13</sup>C(CH<sub>4</sub>) and estimation of the CH<sub>4</sub> budget through atmospheric inversions?
<p>Atmospheric methane (<span class="inline-formula">CH<sub>4</sub></span>) concentrations have been rising since 2007 due to an imbalance between <span class="inline-formula">CH<sub>4</sub></span> sources and sinks. The <s...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/15489/2022/acp-22-15489-2022.pdf |
| Summary: | <p>Atmospheric methane (<span class="inline-formula">CH<sub>4</sub></span>) concentrations have been rising since 2007 due to an imbalance between <span class="inline-formula">CH<sub>4</sub></span> sources and sinks. The <span class="inline-formula">CH<sub>4</sub></span> budget is generally estimated through top-down approaches using chemistry transport models (CTMs) and <span class="inline-formula">CH<sub>4</sub></span> observations as constraints. The atmospheric isotopic <span class="inline-formula">CH<sub>4</sub></span> composition, <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span>, can also provide additional constraints and helps to discriminate between emission categories. Nevertheless, to be able to use the information contained in these observations, the models must correctly account for processes influencing <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span>. The oxidation by chlorine (Cl) likely contributes less than 5 % to the total oxidation of atmospheric <span class="inline-formula">CH<sub>4</sub></span>. However, the large kinetic isotope effect of the Cl sink produces a large fractionation of <span class="inline-formula"><sup>13</sup>C</span>, compared with <span class="inline-formula"><sup>12</sup>C</span> in atmospheric <span class="inline-formula">CH<sub>4</sub></span>, and thus may strongly influence <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span>. When integrating the Cl sink in their setup to constrain the <span class="inline-formula">CH<sub>4</sub></span> budget, which is not yet standard, atmospheric inversions prescribe different Cl fields, therefore leading to discrepancies between flux estimates. To quantify the influence of the Cl concentrations on <span class="inline-formula">CH<sub>4</sub></span>, <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span>, and <span class="inline-formula">CH<sub>4</sub></span> budget estimates, we perform sensitivity simulations using four different Cl fields. We also test removing the tropospheric and the entire Cl sink. We find that the Cl fields tested here are responsible for between 0.3 % and 8.5 % of the total chemical <span class="inline-formula">CH<sub>4</sub></span> sink in the troposphere and between 1.0 % and 1.6 % in the stratosphere. Prescribing these different Cl amounts in atmospheric inversions can lead to differences of up to 53.8 <span class="inline-formula">Tg CH<sub>4</sub> yr<sup>−1</sup></span> in global <span class="inline-formula">CH<sub>4</sub></span> emissions and of up to 4.7 ‰ in the globally averaged isotopic signature of the <span class="inline-formula">CH<sub>4</sub></span> source <span class="inline-formula"><i>δ</i><sup>13</sup></span>C(CH<span class="inline-formula"><sub>4</sub></span>)<span class="inline-formula"><sub>source</sub></span>), although these differences are much smaller if only recent Cl fields are used. More specifically, each increase by 1000 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">molec</mi><mo>.</mo><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="60pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="c1110748befa05a6229e7a2fa83e5471"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15489-2022-ie00001.svg" width="60pt" height="13pt" src="acp-22-15489-2022-ie00001.png"/></svg:svg></span></span> in the mean tropospheric Cl concentration would result in an adjustment by <span class="inline-formula">+</span>11.7 <span class="inline-formula">Tg CH<sub>4</sub> yr<sup>−1</sup></span>, for global <span class="inline-formula">CH<sub>4</sub></span> emissions, and <span class="inline-formula">−</span>1.0 ‰, for the globally averaged <span class="inline-formula"><i>δ</i><sup>13</sup></span>C(CH<span class="inline-formula"><sub>4</sub></span>)<span class="inline-formula"><sub>source</sub></span>. Our study also shows that the <span class="inline-formula">CH<sub>4</sub></span> seasonal cycle amplitude is modified by less than 1 %–2 %, but the <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span> seasonal cycle amplitude can be significantly modified by up to 10 %–20 %, depending on the latitude. In an atmospheric inversion performed with isotopic constraints, this influence can result in significant differences in the posterior source mixture. For example, the contribution from wetland emissions to the total emissions can be modified by about 0.8 % to adjust the globally averaged <span class="inline-formula"><i>δ</i><sup>13</sup></span>C(CH<span class="inline-formula"><sub>4</sub></span>)<span class="inline-formula"><sub>source</sub></span>, corresponding to a 15 <span class="inline-formula">Tg CH<sub>4</sub> yr<sup>−1</sup></span> change. This adjustment is small compared to the current wetland source uncertainty, albeit far from negligible. Finally, tested Cl concentrations have a large influence on the simulated <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span> vertical profiles above 30 km and a very small impact on the simulated <span class="inline-formula">CH<sub>4</sub></span> vertical profiles. Overall, our model captures the observed <span class="inline-formula">CH<sub>4</sub></span> and <span class="inline-formula"><i>δ</i><sup>13</sup>C(CH<sub>4</sub>)</span> vertical profiles well, especially in the troposphere, and it is difficult to prefer one Cl field over another based uniquely on the available observations of the vertical profiles.</p> |
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| ISSN: | 1680-7316 1680-7324 |