Infrared spectroscopic studies of the heterogeneous reaction of ozone with dry maleic and fumaric acid aerosol particles

Dicarboxylic acids, either directly emitted or formed in chemical processes, are found to be a significant component of tropospheric aerosols. To assess any potential chemical transformation of short unsaturated dicarboxylic acids in tropospheric heterogeneous chemistry, maleic and fumaric acid were selected as surrogates in this study. A novel aerosol flow tube apparatus is employed to perform kinetic studies of the oxidation of these organic compounds by gas-phase ozone. The system consists of a particle generation system, a vertically oriented glass flow tube and an infrared observation White cell with a Fourier transform infrared (FTIR) spectrometer for the detection system. A flow of single component organic aerosols with mean diameters ranging between 0.7 and 1.1 μm is introduced in a flow tube, in which the particles are subsequently exposed to a known concentration of ozone for a controlled period of time. A band assignment of infrared vibrational frequencies for dry maleic and fumaric acid aerosol spectra is presented. These studies are complemented with off-line analysis on the reaction products. The reaction exhibited pseudo-first-order kinetics on gas product formation, and the pseudo-first-order rate coefficients displayed a Langmuir–Hinshelwood dependence on gas-phase ozone concentration for both materials. By assuming a Langmuir–Hinshelwood behaviour, the following parameters were obtained: for the reaction of maleic acid aerosols, KO3 = (3.3 + 0.5) × 10−16 cm3 molecule−1 and kImax = (0.038 + 0.004) s−1; for the reaction of fumaric acid aerosols, KO3 = (1.6 + 0.5) × 10−16 cm3 molecule−1 and kImax = (0.048 + 0.007) s−1, where KO3 is a parameter that describes the partitioning of ozone to the particle surface and kImax is the maximum pseudo-first-order coefficient at high ozone concentrations. Apparent reactive uptake coefficients were estimated from the pseudo-first-order rate coefficient and a trend of decreasing uptake coefficients with increasing ozone concentrations was observed, in good agreement with literature values.