Rising Alkali-to-Acid Ratios in the Atmosphere\nMay Correspond to Increased Aerosol Acidity

Aerosol acidity (or pH) is one central parameter in determining\nthe health, climate, and ecological effects of aerosols. While it\nis traditionally assumed that the long-term aerosol pH levels are\ndetermined by the relative abundances of atmospheric alkaline to acidic\nsubstances (referred to as <i>R</i><sub>C/A</sub> hereinafter),\nwe observed contrasting pH<i>R</i><sub>C/A</sub> trends at different sites globally, i.e., rising alkali-to-acid\nratios in the atmosphere may unexpectedly lead to increased aerosol\nacidity. Here, we examined this apparently counterintuitive phenomenon\nusing the multiphase buffer theory. We show that the aerosol water\ncontent (AWC) set a pH “baseline” as the peak buffer\npH, while the <i>R</i><sub>C/A</sub> and particle-phase\nchemical compositions determine the deviation of pH from this baseline\nwithin the buffer ranges. Therefore, contrasting long-term pH trends\nmay emerge when <i>R</i><sub>C/A</sub> increases while the\nAWC or nitrate fraction decreases, or vice versa. Our results provided\na theoretical framework for a quantitative understanding of the response\nof aerosol pH to variations in SO<sub>2</sub>, NO<sub>x</sub> versus\nNH<sub>3</sub>, and dust emissions, offering broad applications in\nstudies on aerosol pH and the associated environmental and health\neffects.