Decadal trends and drivers of global aerosol acidity: insights from model simulations and observational data

Aerosol acidity is an essential property of atmospheric particles that affects not only atmospheric processes such as cloud formation, oxidation capacity, climate, and gas-particle phase partitioning, but also the Earth system, such as nutrient availability in terrestrial and marine ecosystems, and human health. The global distribution of aerosol acidity exhibits distinct spatial and temporal patterns, driven by variability in aerosol chemical composition, aerosol abundance, and local meteorological parameters. Due to the implementation of related clean air policies, a substantial reduction in aerosol abundance and a significant shift in chemical composition have been observed in recent times (Tsimpidi et al., 2024). However, the response of aerosol acidity remains modest and depends on the combined effect of aerosol changes and meteorology (Karydis et al., 2021). The contribution of each driving factor is debated, and the decadal trend of aerosol acidity is not well understood. In this study, we present a decadal simulation of global aerosol acidity using the EMAC atmospheric chemistry-climate model. The simulation is evaluated with results derived from field measurements over the continents of North America, Europe, and Asia. A one-at-a-time approach is employed to quantify the contributions of key driving factors, including temperature, relative humidity, and the availability of sulfate, total (gas and aerosol) nitrate, ammonium, and chloride, and nonvolatile cations  (sum of Na⁺, Ca²⁺, K⁺, and Mg²⁺) to annual and seasonal trends in aerosol acidity. Compared to field measurements, our simulation accurately reproduces temperature and relative humidity and shows good agreement of aerosol acidity with field measurements in Europe and the Pearl River Delta. We find that the underestimation of acidic ions, particularly sulfate, is the main reason for the low bias in simulated aerosol acidity in North America, and the underestimation of alkaline nonvolatile cations leads to high bias in aerosol acidity in the North China Plain. These findings highlight the nuanced interplay between chemical composition and meteorological factors in shaping global aerosol acidity trends and emphasize the importance of regional analyses in understanding long-term changes.

 

References

Karydis, V.A., Tsimpidi, A.P., Pozzer, A., Lelieveld, J., 2021. How alkaline compounds control atmospheric aerosol particle acidity. Atmospheric Chemistry and Physics 21, 14983-15001.

Tsimpidi, A.P., Scholz, S.M.C., Milousis, A., Mihalopoulos, N., Karydis, V.A., 2024. Aerosol Composition Trends during 2000-2020: In depth insights from model predictions and multiple worldwide observation datasets. EGUsphere 2024, 1-66.