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(O(3)) = (9 ± 4) × 10(-15) cm(3) molecule(-1) and k = (0.21 ± 0.01) s(-1); for the reaction of fumaric acid aerosols, K(O(3)) = (5 ± 2) × 10(-15) cm(3) molecule(-1) and k = (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 CO(2) and formic acid (HCO(2)H) through increased levels of hydroxyacetyl hydroperoxide intermediate.

Roles of the Phase State and Water Content in Ozonolysis of Internal Mixtures of Maleic Acid and Ammonium Sulfate Particles

“…Neither GA nor OA was detected by IC due to the low reaction rate. On the contrary, more than 90% of the observed MA mass in the aqueous droplets that reacted at 80% RH was consumed after 70 h of reaction ( Figure 1a), suggesting a much higher ozonolysis rate compared with that of solid particles Najera et al 2010). The presence of an aqueous phase is essential for the effective ozonolysis of MA.…”

“…To aid the discussion of the formation of the observed products in this study, we reproduce the mechanism of the Criegeetype ozonolysis of MA proposed by Najera et al (2010), illustrated in Figure 2. Gallimore et al (2011) presented a more detailed mechanism for ozonolysis of MA under dry and aqueous conditions with other immediate and minor products.…”

“…SCI may also react with water under elevated RH conditions (pathway C), forming hydroxyacetyl hydroperoxide (HAHP). This further enhances the removal of SCI and prompts formic acid production (Najera et al 2010). To show whether pathway A is important, we define molar conversion of GA here using the evaporation-corrected masses of MA and GA at Day 6 (Figure 1a and d): moles of GA produced/moles of MA reacted.…”

“…An 1800 g mm −1 grating was used to form the spectra in the 200-4000 cm −1 range with a resolution of 1.4 cm −1 . The assignments of Raman peaks were based on those used by Najera et al (2010) and Lin-Vien et al (1991).…”

“…Chemical characterizations of both the volatile and the particle-phase products are warranted. Najera et al investigated the ozonolysis of solid ) and aqueous MA (Najera et al 2010) using an aerosol flow tube coupled with an infrared spectroscopic detection system. They then examined the formation of gas-phase formic acid, an expected product of the Criegee mechanism to elucidate the reaction kinetics.…”