| 05 Sep 2025
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. First, the tropospheric sulfur budget simulated by EMAC is verified to be close, that is, all sulfur sources and sinks are balanced, ensuring 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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Geoscientific Model Development.
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General comments
This manuscript is a thorough evaluation of the EMAC v2.55 sulfur simulations and makes a useful contribution by (i) closing a model‑internal sulfur budget (ii) documenting how the model compares with satellite data in 2019 to evaluate how it responds to volcanic emissions and (iii) evaluating against long‑term measurements, 2010–2019
The paper is well organized and generally clear. However, it is unnecessarily long, and the presentation of results is at times too detailed, making it difficult to extract the main messages and scientific significance. The description of the model setup partly repeats work published elsewhere, and it is not entirely clear what is new compared to earlier model versions (e.g., Jöckel et al., 2016). For the interactive gas–particle chemistry, more detail would be beneficial, as the current description is incomplete for interpreting SO₂ lifetime and deposition. I.e. the statement that “the simulation did not involve an interactive aerosol submodel” needs clarification. Does this mean that interactions with ammonia are excluded? If so, this should be explicitly stated, as ammonia strongly influences sulfur oxidation pathways, cloud pH, and the partitioning and deposition of sulfur.
Specific comments
Technical corrections/spelling errors