Heterogeneity in Aerosol Liquid Water Content and pH Levels: Distinct Impacts on Secondary Inorganic Aerosol Formation Between Lanzhou and Beijing, China

Abstract Aerosol liquid water content (ALWC) and pH significantly influence secondary inorganic aerosol (SIA) formation. However, the spatiotemporal variations in ALWC and aerosol pH, along with their impacts on SIA formation, remain poorly understood. Based on a‐year‐long field observations of aerosol chemical components, ALWC and aerosol pH for Lanzhou and Beijing were calculated using ISORROPIA II. Aerosol liquid water content in Lanzhou was highest in winter and lowest in summer (17.2 vs. 3.30 μg m −3 ), opposite to that of Beijing (10.6 vs. 22.9 μg m −3 ). Aerosol pH in both cities was highest in winter and lowest in summer (4.68 vs. 2.96 in Lanzhou, 5.34 vs. 2.88 in Beijing). Machine learning identified TNH 3 (NH 4 + + NH 3 ), SO 4 2− , NO 3 − , relative humidity (RH), and temperature as crucial factors influencing ALWC and aerosol pH in both cities, while Mg 2+ and Cl − were unique factors influencing pH in Lanzhou during summer and winter, respectively. During pollution periods, the effect of ALWC on enhancing heterogeneous and aqueous‐phase reactions for SIA formation was more pronounced in Beijing than in Lanzhou, as elevated ALWC levels provided more reaction medium and facilitated solid‐to‐liquid phase transitions. Additionally, elevated pH notably enhanced aqueous‐phase sulfate production via the O 3 oxidation pathway in winter in both cities and via the H 2 O 2 oxidation pathway in summer in Beijing. This study highlights the heterogeneity of ALWC and pH, along with their distinct impacts on SIA formation, which should be considered in atmospheric models to improve predictions of secondary aerosol formation and better assess the associated environmental and climatic effects.