A smog chamber for studies of the photochemical degradation of chemicals in the presence of aerosols

A smog chamber, especially designed for the simulation of the tropospheric photochemical degradation of chemicals adsorbed on aerosol surfaces, is described. With optimum aerodynamic diameters of the aerosols (0.5 μm) and by special thermostatization of the chamber residence times of the aerosols up to 2 days in the dark and up to 1 day in the presence of simulated sun irradiation are achieved. Experiments on the degradation of simple alkanes (up to n-nonane) and aromatics in the absence and presence of aerosols (SiO2 and TiO2) are reported. Rate constants for the reactions of OH with n-alkanes from n-butane to n-tridecane are determined to be (k ± 2 σ)/10−12 cm3s−3 = 2.54 ± 0.04, 4.13 ± 0.05, 5.68 ± 0.04, 7.31 ± 0.08, 8.79 (reference value), 10.3 ± 0.2, 12.4 ± 0.2, 13.3 ± 0.2, 13.9 ± 0.2, 15.5 ± 0.2), respectively, at 300 K. These rate constant measurements and the observed concentrations of OH remain unaffected in the presence of SiO2 aerosol at mass concentrations of l mg m−3. In the presence of TiO2 at 2 mg m−3 mass concentration, the steady-state concentration of OH is enhanced by an order of magnitude. In addition, a heterogeneous photodegradation process of the hydrocarbons occurs on the surface of the TiO2 aerosol.