Aqueous photooxidation of isoprene hydroxyhydroperoxides (ISOPOOH)

Abstract:

Under pristine atmospheric conditions, Isoprene Hydroxyhydroperoxides (ISOPOOHs) are the main first generation products of isoprene oxidation. ISOPOOHs contain two highly hydrophilic functional groups that enable it to partition into atmospheric waters. There it can be further oxidized or act as a source of OH. In order to study this process, we conducted aqueous photooxidation experiments aimed at understanding the role of ISOPOOH, and hydroperoxides in general, in atmospheric waters. ISOPOOH standards were irradiated in the presence of H₂O₂ in order to simulate aqueous processing. The solutions were analyzed online via HR-ToF-CIMS and HR-ToF AMS. We were able to determine that the main products of ISOPOOH oxidation in water are isoprene epoxydiols (IEPOX). Side products formed during the photooxidation were detected as well, including products of direct photolysis and highly oxygenated compounds. ISOPOOH was also confirmed to produce OH in the condensed phase both, via direct photolysis of the peroxide bond and as a co-product of IEPOX formation. Competition experiments with Levoglucosan allowed for the measurement of ISOPOOHs OH reaction rate constant in the condensed phase. Finally, experiments using an OH trap enabled the measurement of the yield of OH from ISOPOOH photolysis that escapes the solvent cage. Together these experiments provide new insights into the role that hydrophilic products of atmospheric oxidation, especially peroxides, can play in SOA formation via aqueous processing.