Heterogeneous reactions of volatile organic compounds on oxide particles of the most abundant crustal elements: Surface reactions of acetaldehyde, acetone, and propionaldehyde on SiO2, Al2O3, Fe2O3, TiO2, and CaO

The heterogeneous reactivity of volatile organic compounds (VOCs) on oxide particles has been investigated. Oxide particles composed of the most abund ant elements present in the Earth's crust were used to identify potentially important reactions of VOCs on mineral dust. In particular, heterogeneous reactions of acetaldehyde, acetone, and propionaldehyde on alpha -Al2O3, al pha -Fe2O3, TiO2, CaO, and SiO2 particle surfaces have been investigated. F ourier transform infrared and UV/visible spectroscopic measurements show th at these carbonyl compounds weakly and reversibly adsorbed on SiO2. However , on the more basic and acidic oxides investigated, the data show that thes e carbonyl compounds irreversibly adsorb and can, in part, react on the sur face to form larger molecular weight compounds. The kinetics of the heterog eneous reaction of acetaldehyde, acetone, and propionaldehyde on alpha -Al2 O3, alpha -Fe2O3, TiO2, CaO, and SiO2 were measured with a Knudsen cell rea ctor at a gas concentration of 6 x 10(10) molecules/cm(3). Using the Brunau er-Emmett-Teller surface area of the powdered sample, initial uptake coeffi cients were determined to be in the 10(-4) to 10(-6) range for the adsorpti on of carbonyl compounds on the oxides at 298 K. By using these values for the uptake coefficient, it is found that the heterogeneous loss of these tr ace volatile organic compounds is comparable to that due to photolysis and reaction with OH in the middle to upper troposphere. Although surface adsor ption can occur, the measured second-order pressure dependence to form high er molecular weight compounds suggests that these reaction products will le ss likely form under atmospheric conditions.