Kinetic studies of the heterogeneous oxidation of maleic and fumaric acid aerosols by ozone under conditions of high relative humidity

In this paper, a kinetic study of the oxidation of maleic and fumaric acid organic particles by gas-phase ozone at relative humidities ranging from 90 to 93% is reported. A flow of single component aqueous particles with average size diameters in the range 2.6–2.9 µm were exposed to a known concentration of ozone for a controlled period of time in an aerosol flow tube in which products were monitored by infrared spectroscopy. The results obtained are consistent with a Langmuir–Hinshelwood type mechanism for the heterogeneous oxidation of maleic/fumaric acid aerosol particles by gas-phase ozone, for which the following parameters were found: for the reaction of maleic acid aerosols, KO3 = (9 ± 4) × 10−15 cm3 molecule−1 and kImax = (0.21 ± 0.01) s−1; for the reaction of fumaric acid aerosols, KO3 = (5 ± 2) × 10−15 cm3 molecule−1 and kImax = (0.19 ± 0.01) s−1. From the pseudo-first-order coefficients, apparent uptake coefficient values were calculated for which a decreasing trend with increasing ozone concentrations was observed. Comparison with previous measurements of the same system under dry conditions reveals a direct effect of the presence of water on the mechanism of these reactions, in which the water is seen to increase the formation of CO2 and formic acid (HCO2H) through increased levels of hydroxyacetyl hydroperoxide intermediate.