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Reactions of Criegee Intermediates with Benzoic Acid at the Gas/Liquid Interface

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journal contribution
posted on 10.07.2018, 00:00 by Junting Qiu, Shinnosuke Ishizuka, Kenichi Tonokura, Shinichi Enami
Secondary organic aerosol (SOA) found in polluted mega-cities contains benzoic acid (BA) as a major organic acid in addition to a variety of species including alkenes. In polluted air, ozone could be a major oxidizer for SOA and induces subsequent reactions involving Criegee intermediates (CIs, carbonyl oxide, RR′C–O–O/RR′CO+–O) formed by the −CC– + O3 reaction at the gas/liquid interface. The possibility that abundant BA could be an effective scavenger of CIs at the interface remains to be investigated by direct experiments. Here, we showed that amphiphilic BA is able to compete with water molecules for the CIs produced in the prompt ozonolysis of β-caryophyllene on the surface of a water/acetonitrile solvent microjet by generating hitherto uncharacterized C22 ester hydroperoxide products. Competition between BA vs octanoic acid vs cis-pinonic acid toward CIs reveals that BA is a much less-efficient scavenger of CIs on aqueous organic surfaces. We attribute it to the surface-specific orientation of BA at the gas/liquid interface, where the reactive −C­(O)­OH group is fully hydrated and not available for CIs generated at the topmost layers.