Sulfate Formation via Cloud Processing from Isoprene Hydroxyl Hydroperoxides (ISOPOOH).

The oxidation of sulfur dioxide (SO2) by peroxides leads to the formation of sulfate in cloudwater, contributing to particulate matter (PM) formation. The reaction with hydrogen peroxide (H2O2) is considered to be the main cloud oxidation pathway. Previous studies have examined the oxidation of SO2 in cloudwater by small organic peroxides with one functional group; however, oxidation by multifunctional organic hydroperoxides, which are expected to have higher water solubility and reactivity, has not been examined. We investigate the aqueous oxidation of SO2 by the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH), the primary low-NOx isoprene oxidation products in the atmosphere. Having large Henry's law constants and being among the most abundant multifunctional hydroperoxides, they are among the most important organic hydroperoxides present in clouds. The pH dependence of the reactions was investigated at cloud relevant pH of 3-6, and the results reveal their importance compared to the oxidation of SO2 via H2O2. Model simulations in GEOS-Chem, updated with the chemistry described herein, highlight the importance of these pathways for sulfate formation in regions with high isoprene emissions and low-NOx atmospheric conditions, especially if they maintain significant SO2 emissions.