Tropospheric aerosols are known to provide a reactive surface for the uptake atmospheric trace gases such as nitric acid. These atmospheric particulate matter contains various metal oxides in proportions that vary with their source region. Components such as TiO2, reported to reach over 4% by mass in aerosol formed during dust events, are known to be photoactive with atmospheric impact despite its relatively low proportion. Because TiO2 is a semiconductor with a band-gap above the cutoff of solar radiation in the atmosphere, it can trigger additional pathways in heterogeneous processes. In this work, we evaluate the photochemistry of nitric acid, a trace atmospheric gas, chemisorbed on TiO2 and compare it with that on γ-Al2O3, a well-known reactive insulator system. All experiments were carried out in a state-of-the-art heterogeneous photochemistry chemistry system consisting on a coupled surface and gas phases FTIRs, with all experiments carried out under vacuum. In addition, the effect of relative humidity (RH) and co-adsorbed water was examined. Our results indicate that the semiconductor, while not having a direct correlation with the rate of the heterogeneous reaction, can play a relevant role in the heterogeneous reaction mechanism, leading to nitrogen oxide products with a lower average oxidation number. Experiments at different RH indicate that H2O co-asorption changes the proportion of the products in the heterogeneous photochemistry of nitrates. In addition, as the RH increases, water will compete with nitrates for adsorption sites, resulting in less nitrate coverage. Kinetic constant, reaction yields and gas-phase product fraction are reported.

Heterogeneous photochemistry of nitrate chemisorbed on TiO2 as a function of relative humidity

249th National Meeting (2015)

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