Rising alkali-to-acid ratios in the atmosphere may correspond to increased aerosol acidity

Aerosol acidity (or pH) is one central parameter in determining the health, climate and ecological effects of aerosols. While it is traditionally assumed that the long-term aerosol pH levels are determined by the relative abundances of atmospheric alkaline to acidic substances (referred to as R_C/A hereinafter), we observed contrasting pH - R_C/A trends at different sites globally, i.e., rising alkali-to-acid ratios in the atmosphere may unexpectedly lead to increased aerosol acidity. Here, we examined this apparently counter-intuitive phenomenon using the multiphase buffer theory. We show that the aerosol water content (AWC) set a pH “baseline” as the peak buffer pH, while the R_C/A and particle-phase chemical compositions determine the deviation of pH from this baseline within the buffer ranges. Therefore, contrasting long-term pH trends may emerge when R_C/A increases while AWC or nitrate fraction decreases, or vice versa. Our results provided a theoretical framework for a quantitatively understanding the response of aerosol pH to variations in SO₂, NO_x versus NH₃ and dust emissions, offering broad applications in studies on aerosol pH and the associated environmental and health effects.