A smog chamber study on the impact of aerosols on the photodegradation of chemicals in the troposphere

The interaction of aerosols with the photochemistry of the troposphere is simulated in a smog chamber. The aerosol smog chamber consists of a solar simulator (with up to three-fold light intensity of the lower troposphere) and a cylindrical glass vessel (2.4 cubic meters volume with thermostatted lid and bottom). It enables us to obtain long residence times of aerosols longer than 1 day. A photochemical smog is produced by irradiating a mixture of NO sub x (200 ppb NO2+100 ppb NO) and various n-alkanes (from propane up to n-nonane), iso-alkanes and aromatics (up to 30 compounds, 100 ppb each). The concentrations of the active constituents of the photosmog, NO sub x and O3, are monitored using chemiluminescent analysers, the concentrations of other active species are computed from the degradation rate of the hydrocarbons using known rate constants. In the presence and absence of SiO2 aerosol (Aerosil) the degradation behaviour of the hydrocarbons is observed to be dominated by reactions of OH radicals. Aerosol densities up to 1mg m -3 have been without any impact on the chemistry of OH radicals. This enables us to study the photodegradation of non-volatile organic compounds, adsorbed as a layer on the inert SiO2 carrier, by the reaction of the active species, OH and O3, from the photosmog. In contrast to the results in the presence of SiO2 aerosol in the presence of TiO2 (which is an UV-absorbing semiconductor material) the concentration of OH is enhanced by a factor of ten. (ITA)