Evaluation of atmospheric sulfur dioxide simulated with the EMAC (version 2.55) Chemistry–Climate Model using satellite and ground-based observations

Abstract. Sulfur dioxide (SO2) is a key atmospheric pollutant, primarily emitted through human activities such as fossil fuel combustion. In atmospheric models, accurate representation of SO2 emission sources, transport, and removal processes are essential for evaluating air quality and radiative forcing. In this study, we present, for the first time, a comprehensive examination of atmospheric SO2 simulated by the ECHAM/MESSy Atmospheric Chemistry (EMAC) model, here operated under the Chemistry–Climate Model Initiative (CCMI-2022) protocol. First, the tropospheric sulfur budget simulated by EMAC is verified to be closed. This closure means that all sulfur sources and sinks are balanced and no artificial gain or loss occurs over time due to numerical or conceptual errors. This budget closure is a prerequisite for any further analysis. Second, the results of EMAC simulations are compared with observations from three ground-based networks (the Clean Air Status and Trends Network (CASTnet), the European Monitoring and Evaluation Program (EMEP), and the Acid Deposition Monitoring Network in East Asia (EANET)), mainly over polluted regions, and with vertical column densities retrieved from a TROPOspheric Monitoring Instrument (TROPOMI) on board the Copernicus Sentinel-5 Precursor mission (Sentinel-5P) satellite. The EMAC simulated SO2 concentrations near the Earth's surface for the year 2019 are, depending on the region, between 1.4 and 1.8 times larger than observed. This discrepancy aligns well with the differences between simulated and retrieved satellite-based measurements of SO2 vertical column densities over the same regions. It indicates that the prescribed SO2 emissions used for the EMAC simulations might be overestimated. Over a longer time period (2000–2019), the EMAC simulation reproduces the measured declining trends of SO2 concentrations and deposited sulfur fluxes in the USA and Europe, but fails to simulate the observed trends in East Asia. This is most likely attributable to the prescribed SO2 emission inventories. Furthermore, sensitivity simulations are performed to assess the emitted amount of SO2 following the Raikoke and Ulawun volcanic eruptions in 2019. The results show a very good agreement of the simulated temporal evolution of the amount of atmospheric SO2 after the eruptions with that retrieved from satellite-based observations.