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Assessing fossil CO2 emissions in Europe: how reliably do ICOS 14CO2 and CO observations constrain atmospheric inversions?
Accurate quantification of country-scale fossil fuel CO2 (ffCO2) emissions is essential for monitoring efforts to mitigate climate change. This thesis employs the regional isotope budget approach (RIBA) to calculate ffCO2 observations based on flask, CO, and integral samples, and assesses their constraint on top-down fossil emission estimates within a Bayesian inversion framework. Flasks provide a limited number of precise hour-long observations, CO-based estimates are less precise but recorded every minute, and integral samples yield reliable two-week averages. The coverage of currently available 14CO2 data from the Integrated Carbon Observation System (ICOS) infrastructure mainly includes Germany and nearby regions. When comparing to the emission inventories by the Global Carbon Project (GCP) and the Emissions Database for Global Atmospheric Research (EDGAR), corresponding winter emissions estimated from the atmospheric data differ by 0–4% from GCP but by 10–14% from EDGAR, reducing the discrepancy between the GCP and EDGAR inventories from 21% to about half. Incorporating ffCO2 observations decreases the GCP inventory uncertainty by 30% in the Germany+ domain. Uncertainty analysis indicates that biases in 14CO2 background are critical for the RIBA. A background bias of 2‰ results in estimated flux variations of 20%. Since integral-based samples have lower requirements for hourly-specific model transport, they are found to be the most suitable proxy for constraining trends and seasonal fossil fuel emissions. They effectively track trends and align with realistic expectations derived from bottom-up inventories. Flask-based estimates perform equally well if sampled more frequently than weekly, but demand more analytical resources. CO-based estimates are found to be less suitable due to their additional dependence on CO background estimates and unmodeled air chemistry. The two main recommendations for ICOS to monitor country-scale fossil emissions and their trends are: (a) diversifying European 14CO2 background observations to reduce bias risk, and (b) prioritizing integral 14CO2 sampling.
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