A CO<sub>2</sub>–Δ<sup>14</sup>CO<sub>2</sub> inversion setup for estimating European fossil CO<sub>2</sub> emissions
<p>Independent estimation and verification of fossil <span class="inline-formula">CO<sub>2</sub></span> emissions on a regional and national scale are crucial for evaluating the fossil <span class="inline-formula">CO<sub>2</sub><...
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| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2025-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/25/397/2025/acp-25-397-2025.pdf |
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| Summary: | <p>Independent estimation and verification of fossil <span class="inline-formula">CO<sub>2</sub></span> emissions on a regional and national scale are crucial for evaluating the fossil <span class="inline-formula">CO<sub>2</sub></span> emissions and reductions reported by countries as part of their nationally determined contributions (NDCs). Top-down methods, such as the assimilation of in situ and satellite observations of different tracers (e.g., <span class="inline-formula">CO<sub>2</sub></span>, <span class="inline-formula">CO</span>, <span class="inline-formula">Δ<sup>14</sup>CO<sub>2</sub></span>, <span class="inline-formula">XCO<sub>2</sub></span>), have been increasingly used for this purpose. In this paper, we use the Lund University Modular Inversion Algorithm (LUMIA) to estimate fossil <span class="inline-formula">CO<sub>2</sub></span> emissions and natural fluxes by simultaneously inverting in situ synthetic observations of <span class="inline-formula">CO<sub>2</sub></span> and <span class="inline-formula">Δ<sup>14</sup>CO<sub>2</sub></span> over Europe. We evaluate the inversion system by conducting a series of observing system simulation experiments (OSSEs). We find that in regions with a dense sampling network, such as western/central Europe, adding <span class="inline-formula">Δ<sup>14</sup>CO<sub>2</sub></span> observations in an experiment where the prior fossil <span class="inline-formula">CO<sub>2</sub></span> and biosphere fluxes are set to zero allows LUMIA to recover the time series of both categories. This reduces the prior-to-truth root mean square error (RMSE) from 1.26 to 0.12 TgC d<span class="inline-formula"><sup>−1</sup></span> in fossil <span class="inline-formula">CO<sub>2</sub></span> and from 0.97 to 0.17 TgC d<span class="inline-formula"><sup>−1</sup></span> in biosphere fluxes, reflecting the true total <span class="inline-formula">CO<sub>2</sub></span> budget by 91 %. In a second set of experiments using realistic prior fluxes, we find that in addition to retrieving the time series of the optimized fluxes, we are able to recover the true regional fossil <span class="inline-formula">CO<sub>2</sub></span> budget in western/central Europe by 95 % and in Germany by 97 %. In all experiments, regions with low sampling coverage, such as southern Europe and the British Isles, show poorly resolved posterior fossil <span class="inline-formula">CO<sub>2</sub></span> emissions. Although the posterior biosphere fluxes in these regions follow the seasonal patterns of the true fluxes, a significant bias remains, making it impossible to close the total <span class="inline-formula">CO<sub>2</sub></span> budget. We find that the prior uncertainty of fossil <span class="inline-formula">CO<sub>2</sub></span> emissions does not significantly impact the posterior estimates, showing similar results in regions with good sampling coverage like western/central Europe and northern Europe. Finally, having a good prior estimate of the terrestrial isotopic disequilibrium is important to avoid introducing additional noise into the posterior fossil <span class="inline-formula">CO<sub>2</sub></span> fluxes.</p> |
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| ISSN: | 1680-7316 1680-7324 |