Estimating emissions of methane consistent with atmospheric measurements of methane and <i>δ</i><sup>13</sup>C of methane
<p>We have constructed an atmospheric inversion framework based on TM5-4DVAR to jointly assimilate measurements of methane and <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> of methane in order to estimate source-specific methane emissi...
| Main Authors: | , , , , , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2022-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/15351/2022/acp-22-15351-2022.pdf |
| Summary: | <p>We have constructed an atmospheric inversion framework based on TM5-4DVAR to jointly assimilate measurements of methane and <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> of methane in order to estimate source-specific methane emissions. Here we present global emission estimates from this framework for the period 1999–2016. We assimilate a newly constructed, multi-agency database of <span class="inline-formula">CH<sub>4</sub></span> and <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> measurements. We find that traditional <span class="inline-formula">CH<sub>4</sub></span>-only atmospheric inversions are unlikely to estimate emissions consistent with atmospheric <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> data, and assimilating <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> data is necessary to derive emissions consistent with both measurements. Our framework attributes ca. 85 % of the post-2007 growth in atmospheric methane to microbial sources, with about half of that coming from the tropics between 23.5<span class="inline-formula"><sup>∘</sup></span> N and 23.5<span class="inline-formula"><sup>∘</sup></span> S. This contradicts the attribution of the recent growth in the methane budget of the Global Carbon Project (GCP). We find that the GCP attribution is only consistent with our top-down estimate in the absence of <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> data. We find that at global and continental scales, <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> data can separate microbial from fossil methane emissions much better than <span class="inline-formula">CH<sub>4</sub></span> data alone, and at smaller scales this ability is limited by the current <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> measurement coverage. Finally, we find that the largest uncertainty in using <span class="inline-formula"><i>δ</i><sup>13</sup>C</span> data to separate different methane source types comes from our knowledge of atmospheric chemistry, specifically the distribution of tropospheric chlorine and the isotopic discrimination of the methane sink.</p> |
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| ISSN: | 1680-7316 1680-7324 |