Chemodiversity Mechanisms of Aerosol Acidity: Effects of Seasonal Pattern and Chemical Species

Abstract Aerosol acidity has become a prominent focus due to its unclear role in regulating atmospheric chemistry and toxicity, as well as the ongoing debate regarding the influence of meteorological factors and aerosol chemical compositions. In this study, we investigated the characteristics of PM 2.5 acidity and the underlying chemical mechanisms. The results showed that PM 2.5 acidity exhibited weakly acidic patterns, with a bimodal pattern on seasonal scales, primarily due to high loadings of ammonium ions and frequent sandstorms. Sensitivity tests indicated that temperature had a more pronounced effect on aerosol acidity than relative humidity, owing to its significant influence on ammonium partitioning. Furthermore, PM 2.5 acidity responded to changes in aerosol chemical composition, with sulfate and total ammonium (TNH x ) being the most significant contributors, followed by total nitrate (TNO 3 ) and total chloride (TCl). Sulfate modified aerosol acidity by preferentially promoting the formation of sulfate and bisulfate, which in turn regulated the partitioning of semi‐volatile species. TNO 3 contributed to the dynamic equilibrium between sulfate and bisulfate by interacting with the partitioning of semi‐volatile species. Initially, TNH x triggered a sharp increase in aerosol acidity by preferentially utilizing sulfate; however, subsequent neutralization and partitioning between TNHx and of semi‐volatile acidic species limited further evolutions in acidity. The comprehensive analysis of key chemical species suggested that reducing PM 2.5 pH can only be achieved through effective ammonia control. This study not only addresses theoretical gaps in the numerical simulations of aerosol acidity in ammonia‐rich environments but also enhances our understanding of the global differential distribution characteristics.