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

Green leaf volatiles (GLVs) are biogenic C₅ to C₆ unsaturated oxygenated organic compounds that are emitted when vegetation is exposed to herbivores, pathogens, or harsh weather conditions. Increased GLV emissions when vegetation is subjected to biotic and abiotic stresses can lead to GLVs contributing substantially to the local secondary organic aerosol (SOA). GLVs can dissolve into atmospheric aqueous phases (e.g., aqueous aerosols, cloud and fog droplets), where they can be oxidized by aqueous oxidants. Aqueous SOA (aqSOA) mass yields as high as 88 % from aqueous reactions have been reported in previous studies, but these previous studies were mostly conducted under dilute aqueous conditions mimicking aqueous cloud/fog droplets. Little is currently known about the aqueous oxidation of GLVs under more concentrated aqueous aerosol-like conditions.  Here, we investigated the 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, focusing on the effects of pH, ionic strength, and sulfate on the reaction kinetics and aqSOA mass yields under cloud/fog-like vs. aqueous aerosol-like conditions. Our results showed that the aqueous reaction medium conditions governed the effects that pH, ionic strength, and sulfate had on the reaction kinetics and aqSOA mass yields. Higher reaction rates were observed at lower pH under dilute cloud/fog-like conditions, which could be attributed to the pH-dependent formation of reactive species from nitrate photolysis. Ionic strength and sulfate had insignificant effects on the reaction rates. In contrast, under concentrated aqueous aerosol-like conditions, higher reaction rates were observed at higher pH, and at higher ionic strength and sulfate concentration. Many of these differences could be attributed to sulfur-containing radicals produced from sulfate photolysis participating in the reactions of GLVs under aqueous aerosol-like conditions, but not in cloud/fog-like conditions. Nevertheless, similar aqSOA mass yield trends were observed for cloud/fog-like and aqueous aerosol-like conditions. Higher aqSOA mass yields were measured, likely due to increased production of oligomers from RO₂· and RO· combination reactions as a result of the higher concentrations of GLVs reacted. Higher aqSOA mass yields were measured at lower pH, likely a result of increased production of low volatility products from acid-catalyzed reactions. Lower aqSOA mass yields were measured at higher ionic strength and sulfate concentration, likely due to the increased importance of fragmentation pathways in the reactions of GLVs with sulfur-containing radicals formed from sulfate photolysis. These results provide new insights that can be used in modeling studies of the atmospheric fates of GLVs and their contributions to the SOA budget.