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

Abstract

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, K_O3 = (9 ± 4) × 10⁻¹⁵ cm³ molecule⁻¹ and k^I_max = (0.21 ± 0.01) s⁻¹; for the reaction of fumaric acid aerosols, K_O3 = (5 ± 2) × 10⁻¹⁵ cm³ molecule⁻¹ and k^I_max = (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.