Phase State and Relative Humidity Regulate the Heterogeneous Oxidation Kinetics and Pathways of Organic-Inorganic Mixed Aerosols

Inorganic species always coexist with organic materials in atmospheric particles and may influence the heterogeneous oxidation of organic aerosols. However, very limited studies have explored the role of the inorganics in the chemical evolution of organic species in mixed aerosols. This study examines the heterogeneous oxidation of glutaric acid- ammonium sulfate and 1,2,6-hexanetriol-ammonium sulfate aerosols by hydroxyl radicals (OH) under varied organic mass fractions (forg) and relative humidity in a flow tube reactor. Coupling the oxidation kinetics and product measurements with kinetic model simulations, we found that under both low relative humidity (RH, 30-35%) and high RH conditions (85%), the decreased forg from 0.7 to 0.2 accelerates the oxidation of the organic materials by a factor of up to 11. We suggest that the faster oxidation kinetics under low-RH conditions is due to full or partial phase separation, with the organics greatly enriched at the particle outer region, while enhanced "salting-out" of the organics and OH adsorption caused by higher inorganics could explain the observations under high-RH conditions. Analysis of the oxidation products reveals that the dilution of organics by the inorganic salts and corresponding water uptake under high-RH conditions will favor alkoxy radical fragmentation by a factor of 3-4 and inhibit its secondary chain propagation chemistry. Our results suggest that atmospheric organic aerosol oxidation lifetime and composition are strongly impacted by the coexistent inorganic salts.