Top-Down Estimates of Methane Emissions in Romania Using Multiple TROPOMI CH₄ Products in ICON-ART CIF Inversions

Methane (CH₄) is a potent greenhouse gas, and its emission mitigation plays a crucial role in efforts to combat climate change. The oil and gas (O&G) sector in Romania is a major CH₄ emitter, and the ROMEO campaign in 2019 found O&G emissions to be much higher than previously assumed.

We estimate Romania’s CH₄ emissions for 2019 using three TROPOMI CH₄ products - the operational SRON retrieval, the blended (TROPOMI+GOSAT) product from Harvard University, and the WFM-DOAS retrieval from University of Bremen - combined with the ICON-ART model and an Ensemble Square Root Filter in the Community Inversion Framework (CIF). Additionally, we perform an inversion using the operational TROPOMI retrieval for 2021.

Inversions for 2019 reveal noticeable spatial and temporal inconsistencies across the satellite products, indicating that a posteriori distributions at fine scales should be interpreted with caution. Despite these differences, the country-total emissions agree within the expected range, suggesting robustness at aggregated scales. Applying the system to the operational TROPOMI product for 2021 shows a reduction of 20% in corresponding posterior emissions compared to 2019, with stronger reductions of 30% over a region dominated by oil and gas infrastructure. This decrease is consistent with previous independent findings, namely results from a 2021 aircraft campaign over Romanian O&G infrastructure reporting a 20%-60% reduction in methane emissions.

Our study highlights the limitations of current TROPOMI CH₄ products for estimating regional emission patterns and emphasizes the need for further investigation into the significant discrepancies between them. Nevertheless, trends derived consistently from a single product appear robust and align with independent findings, making them valuable for assessing long-term emission changes in regions with sparse in-situ monitoring.