Roles of pH, ionic strength, and sulfate in the aqueous nitrate-mediated photooxidation of green leaf volatiles

Abstract

Biotic and abiotic stresses can lead to terrestrial green plants releasing green leaf volatiles (GLVs), which can partition into atmospheric aqueous phases where they can undergo oxidation to form aqueous secondary organic aerosol (aqSOA). Anthropogenic emission changes have resulted in nitrate becoming an increasingly important component of atmospheric aqueous phases, which has significant implications for aqSOA formation since nitrate photolysis produces oxidants. Nevertheless, sulfate remains the main inorganic aqueous component in most regions, and thus it controls the pH and ionic strength of atmospheric aqueous phases. We report results from laboratory investigations of the effects of pH, ionic strength, and sulfate on the reaction kinetics and aqSOA formation of the aqueous nitrate-mediated photooxidation of four GLVs: cis-3-hexen-1-ol, trans-2-hexen-1-ol, trans-2-penten-1-ol, and 2-methyl-3-buten-2-ol. Our results showed that the aqueous reaction medium conditions, i.e., dilute cloud/fog vs. concentrated aqueous aerosol conditions, governed the effects that pH, ionic strength, and sulfate have on the GLV degradation rates and aqSOA mass yields. Most notably, reactions initiated by sulfate photolysis will have significant effects on the GLV degradation rates and aqSOA mass yields in aqueous aerosols but not in cloud/fog droplets. In addition to providing new insights into aqSOA formation from the aqueous reactions of GLVs in regions with substantial concentrations of nitrate in cloud, fog, and aqueous aerosols, this study highlights how nitrate and sulfate photochemistries can couple together to influence the reactions of water-soluble organic compounds and their aqSOA formation in aqueous aerosols, which have implications for our evaluations of aqueous organic aerosol lifetimes and composition.