Anthropogenic–Biogenic Interactions: Influence of SO ₂ on Early-Stage Secondary Organic Aerosol Formation from α-Pinene Ozonolysis

This study investigates the impact of SO2 on early stage secondary organic aerosol (SOA) formation from α-pinene ozonolysis under dry, high α-pinene/O3 conditions by combining flow tube reactor (FTR) experiments with kinetic modeling. The presence of trace SO2 significantly enhances SOA formation, increasing both particle number and mass concentrations in proportion to the initial SO2 concentration while maintaining a consistent mode size. Master Chemical Mechanism (MCM) simulations reveal that SO2 reacts with α-pinene-derived stabilized Criegee intermediates to form H2SO4, which promotes binary nucleation with oxidized organic vapors. A simplified two-phase aerosol model, integrating MCM kinetics, vapor wall loss, and condensation dynamics, successfully reproduces the observed growth trends. The model reveals that the presence of SO2 not only increases particle surface area via enhanced nucleation but also induces an ∼8-fold increase in the effective condensation rate constant relative to SO2-free conditions, likely due to the enhanced gas-to-particle partitioning mediated by H2SO4. Additionally, SO2 reverses the size-dependent hygroscopicity trend, suggesting H2SO4 enrichment in smaller particles, which fundamentally alters growth and activation properties. These findings offer molecular-level insights into anthropogenic–biogenic interactions and highlight the critical role of trace SO2 in modulating the formation, composition, and cloud condensation nuclei activity of SOA in mixed environments.