Aqueous Secondary Organic Aerosol Formation and Transformation by Nitrate-Mediated Photooxidation

Description

Aerosol pollution is a major concern to policymakers worldwide due to their adverse effects on air quality, climate, and health. Organic aerosols comprise a large mass fraction of submicron aerosols. Secondary organic aerosols (SOA) usually dominate the organic aerosol mass. Aqueous-phase chemistry of organic species in atmospheric aqueous phases (e.g., clouds, fog, aerosol water) plays an important, albeit still poorly understood, role in SOA formation and transformation. Inorganic nitrate is a ubiquitous component of aerosols, clouds and fog. Inorganic nitrate photolysis in aqueous phases produces ·OH radicals and a variety of reactive nitrogen species that can participate in reactions to form and transform aqueous SOA. Studies have shown that changes in climate and emissions will increase inorganic nitrate mass concentrations in clouds, fog, and aerosols globally. This will lead to nitrate-mediated photooxidation of organic species in atmospheric aqueous phases playing an increasingly important role in SOA formation and transformation.This proposal aims to provide new scientific insights into how nitrate-mediated photooxidation of organic species in aqueous phases can drive SOA formation and transformation. The first part of the proposal investigates SOA formation from aqueous-phase nitrate-mediated photooxidation of selected anthropogenic and biogenic SOA precursors. Phenolic compounds are chosen as anthropogenic SOA precursors because they are found in significant concentrations with inorganic nitrate in areas impacted by anthropogenic emissions. Terpenoic acids are chosen as biogenic SOA precursors because they are well-known oxidation products of monoterpenes, which are major contributors of biogenic SOA globally. The second part of the proposal investigates how aqueous-phase nitrate-mediated photooxidation transforms the water-soluble fraction of SOA formed from gas-phase VOC oxidation. Toluene and α-pinene will be used to generate representative anthropogenic and biogenic SOA via gasphase oxidation. Aqueous photooxidation experiments simulating nitrate-mediated SOA formation and transformation processes in atmospheric aqueous phases will be conducted. We will use different analytical techniques, including liquid chromatography-mass spectrometry to identify and quantify aqueous SOA components and oxidation products, UV-Visible spectroscopy to measure the evolution of SOA light absorption properties, and water-soluble organic carbon analysis to quantify aqueous SOA mass yields. The successful implementation of this proposal will fill critical knowledge gaps on the nitrate-mediated photooxidation of organic species in aqueous phases. Outputs of this proposal include measured reaction rate constants, kinetics, lifetimes, and mechanisms, which are useful inputs for atmospheric models used to predict aqueous SOA formation and transformation in areas with substantial inorganic nitrate concentrations in clouds, fog, and aerosols.