Photochemical Sources of Organic Acids. 1. Reaction of Ozone with Isoprene, Propene, and 2-Butenes under Dry and Humid Conditions Using SPME

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Abstract
Formation of C4 and smaller carboxylic acids from gas-phase ozonolysis of several alkenes under dry (relative humidity (RH) < 1%) and humid (RH = 65%) conditions have been investigated. We have developed a technique based on solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) to quantify the acids, as well as other products, and applied it to the reactions of ozone with propene, trans-2-butene, 2,3-dimethyl-2-butene, and isoprene. Acetic acid yields from propene and trans-2-butene ozonolysis in the presence of an OH scavenger were 2.7 ± 0.6 and 2.9 ± 0.6%, respectively, under dry conditions and 1.8 ± 0.4 and 2.3 ± 0.5% at 65% RH. Isoprene ozonolysis produced methacrylic and propenoic acids with yields of 5.5 ± 1 and 3.0 ± 1%, under dry conditions and 4.1 ± 1 and 1.5 ± 0.3% under wet conditions, respectively. That water inhibits acid formation indicates that the water reaction with stabilized Criegee intermediates is at most a minor source of acids. Acids that may form as coproducts of the OH radical elimination pathway, acetic acid from 2,3-dimethylbutene and isoprene, and propenoic acid from isoprene were also observed with significant yields (up to 10%), although the production of acetic acid was not a linear function of the alkene reacted. Carbonyl products are also reported.

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