Understanding the aqueous phase ozonolysis of isoprene: Distinct product distribution and mechanism from the gas phase reaction
The aqueous phase reaction of volatile organic compounds (VOCs) has\nnot been considered in most analyses of atmospheric chemical processes.\nHowever, some experimental evidence has shown that, compared to the\ncorresponding gas phase reaction, the aqueous chemical processes\nof VOCs in the bulk solutions and surfaces of ambient wet particles\n(cloud, fog, and wet aerosols) may potentially contribute to the\nproducts and formation of secondary organic aerosol (SOA). In the\npresent study, we performed a laboratory experiment of the aqueous\nozonolysis of isoprene at different pHs (3?7) and temperatures (4?25\n°C). We detected three important kinds of products, including carbonyl\ncompounds, peroxide compounds, and organic acids. Our results showed\nthat the molar yields of these products were nearly independent of\nthe investigated pHs and temperatures, those were (1) carbonyls:\n56.7 ± 3.7 % formaldehyde, 42.8 ± 2.5 % methacrolein (MAC), and 57.7\n± 3.4 % methyl vinyl ketone (MVK); (2) peroxides: 53.4 ± 4.1 % hydrogen\nperoxide (H2O2) and 15.1 ± 3.1 % hydroxylmethyl hydroperoxide (HMHP);\nand (3) organic acids: undetectable (<1 % estimated by the detection\nlimit). Based on the amounts of products formed and the isoprene\nconsumed, the total carbon yield was estimated to be 94.8 ± 4.1 %.\nThis implied that most of the products in the reaction system were\ndetected. The combined yields of both MAC + MVK and H2O2 + HMHP in\nthe aqueous isoprene ozonolysis were much higher than those observed\nin the corresponding gas phase reaction. We suggest that these unexpected\nhigh yields of carbonyls and peroxides are related to the greater\ncapability of condensed water, compared to water vapor, to stabilize\nenergy-rich Criegee radicals. This aqueous ozonolysis of isoprene\n(and possibly other biogenic VOCs) could potentially occur on the\nsurfaces of ambient wet particles and plants. Moreover, the high-yield\ncarbonyl and peroxide products might provide a considerable source\nof aqueous phase oxidants and SOA precursors.