XCO₂ observations compared to km-scale ICON-ART simulations indicate an underestimation of Thessaloniki’s emissions in the ODIAC inventory

Abstract. An accurate inventory of CO₂ emissions is important for the implementation of effective reduction measures and thus for climate change mitigation. Most current inventories are based on reported activities and rely little or not at all on atmospheric data. However, these inventories have large uncertainties, especially for smaller scales such as urban areas. For example, for the city of Thessaloniki, Greece, the EDGAR inventory reports 3.1 Mt, which differs by 72 % from the emission estimate of the ODIAC inventory (1.8 Mt) for the same area for the year 2019. With a measurement campaign in the framework of the Collaborative Carbon Column Observing Network (COCCON), we collected observations for three months in October 2021 and summer 2022 in Thessaloniki. A total of 30 days of column averaged molar fractions of CO₂ (XCO₂) were recorded. We combine these data with km-scale simulations from the numerical weather prediction model ICON-ART. The ODIAC inventory was used for simulating the emission of CO₂. We optimized the simulated atmospheric time series of XCO₂ to best match the observed data by scaling the prior emissions using a least-squares approach. With different configurations, we found a consistent up-scaling of the prior emissions, with total emissions ranging from 2.9 to 4.4 Mt in the urban area of Thessaloniki. This estimate is significantly higher than the emissions reported in ODIAC. The result demonstrates the potential of including ground-based column measurements of CO₂ in the construction of emission estimates to reduce uncertainties at the urban scale.