Processing of unsaturated carboxylic acids by ozone at the air-water interface: Implications for aerosol aging

Abstract

Org. matter contributes 20% to 90% of PM_(2.5) mass concns. in the troposphere. Unsatd. carboxylic acids are widespread in org. aerosols. Double bonds (C=C) in unsatd. species are generally oxidized by electrophilic ozone. In contrast with the ozonation of unsatd. species in the gas phase, the mechanism and products of their ozonation on the surface of aq. org. aerosols are unknown. In the gas-phase, O_3 adds to olefinic double bonds generating chem. activated ozonides, which rapidly decomp. into carbonyl oxide Criegee (CIs) intermediates. The fate of gas-phase Criegee intermediates (CIs) in the troposphere is detd. by their reactions with water and water clusters. Our recent studies on the CIs produced on air-water interfaces (AWIs) by the in situ ozonation of sesquiterpenes have showed that millimolar n-alkyl-COOH acids are able to compete for CIs with water. In this work, we investigate the ozonation of unsatd. carboxylic acids at air-water interface. Our work focuses on the impacts of mol. structure on the products of the interfacial ozonation of unsatd. carboxylic acid by O_3(g). Our studies were conducted by means of our online electrospray mass spectrometry (o-ESI-MS), which probes the compn. of reactive interfacial layers within 1 ms. We have used o-ESI-MS to investigate for the first time the ozonation of various aerosol components at the AWIs, such as phenol, α-humulene and β-caryophyllene. In this study, we explore the intramol. and intermol. products from reactions of the CIs derived from the interfacial ozonation of various unsatd. carboxylic acids. We present results on how products depend on the alkene structure (cyclic or linear) and the sepn. of olefinic double bonds from the -COOH functionality. The competition between intramol. and intermol. addn. of -COOH and the reaction of CIs with H_2O is discussed. This work evaluates the contribution of different unsatd. carboxylic acids to aerosol aging and the potential pathways to extremely low volatility org. compds. (ELVOCs) through multiphase reactions.

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