Multiphase Buffering: A Mechanistic Regulator of Aerosol Sulfate Formation and Its Dominant Pathways

Sulfate formation in the aerosol aqueous phase represents a pH-sensitive atmospheric chemical process, with the formation pathways significantly influenced by the fluctuations in aerosol acidity. Buffer capacity, stemming from conjugate acid-base pairs, can resist pH changes in aerosol multiphase systems under external perturbations. However, the regulating role of multiphase buffering in pH-dependent aqueous sulfate formation mechanisms remains unexplored. Here, we propose that multiphase buffering can stabilize aerosol pH and further regulate dominant sulfate formation pathways. In this work, we delve into the instantaneous buffer capacity β and sulfate formation pathways based on field observation and theoretical calculation and further introduce the total buffer capacity α in the aerosol multiphase system to quantify the buffer-constrained pH change after the external acid/base variation during the entire buffering process. The NH4+/NH3 agent (average β 30.8 mol kg-1) shows a superior buffering effect in stabilizing aerosol pH and regulating sulfate formation pathway transition compared with the HNO3/NO3- agent (average β 15.1 mol kg-1). Geos-Chem simulation and machine learning results also validate the buffer capacity as a pivotal factor in sulfate formation. In addition to reactants, the buffer agents and acid/base can also be factors of concern for the sulfate formation mechanism. The diverse sensitivities to acid/base variation and the region-specific responses to pH change can provide insight into regulating acid and base emission measures, modulating regional aerosol acidity, and understanding pH-related atmospheric chemical processes.