Key Factors Determining the Formation of Sulfate Aerosols Through Multiphase Chemistry—A Kinetic Modeling Study Based on Beijing Conditions

Abstract

Severe haze in Beijing is characterized by the rapid formation of sulfate via the multiphase oxidation of SO2. While many factors, including aerosol oxidants and atmospheric variables, were discovered and investigated, their relative importance remains unclear. Herein, based on the field observation data obtained in Beijing, China, we developed a kinetic model to explore the key factors that determine the multiphase formation of sulfate. Sensitivity tests give the kinetics of each oxidation pathway varying with pH and temperature, based on which the total sulfate formation rate at room temperature (298 K) is calculated to be generally greater than that at standard temperature (273 K), especially during nighttime. Interfacial oxidants are responsible for sulfate formation within a wide pH range, and transition metal ions become more efficient with increased temperature. The multiphase chemistry is additionally affected by aerosol liquid water content (ALWC), particle radius (Rp), and ionic strength (IS). Within the usual aerosol acidity, the kinetic discrepancy induced by different ALWC levels is more significant at the lower temperature, in contrast to the temperature dependence related to Rp, and the effect of IS depends highly on pH. Machine learning reveals the potential importance of temperature, acidity, and Rp. Temperature and acidity are impactful for the formation of both aqueous and interfacial sulfates, whereas Rp only affects the interfacial processes. The discrepancy between nighttime and daytime is considered throughout this study. Overall, this study reveals the key factors for multiphase sulfate formation and is recommended for kinetic evaluation in future laboratory research.