Tropospheric heterogeneous chemistry of H2O2 and SO2: processes mediated by mineral aerosols

Mineral aerosols play an important role in a variety atmospheric processes. They consist of fine particles of crustal origin advected from arid regions and are comprised primarily of silica, silicate minerals, iron and titanium oxides. Anthropogenic ZnO is also a component of tropospheric aerosol, arising from industrial activity. These oxides of Ti, Zn and Fe are semiconductors with bandgaps that are commensurate with ultraviolet light: therefore, they can be very effective photocatalysts for a very wide range of chemical processes. Accordingly, it is possible that photochemical or photocatalytic phenomena occurring on the surfaces of tropospheric mineral aerosols could play a significant role in atmospheric chemistry. Indeed we have recently demonstrated that gaseous HONO is produced by uv-illuminated TiO2 aerosols under conditions of temperature, partial pressures and relative humidity pertinent the troposphere. It is noteworthy that such light-driven heterogeneous processes have been largely neglected by the research community, despite their potential importance in the atmosphere. We therefore propose to investigate the formation and destruction of H2O2 and the processing of SO2 by H2O2 on the surfaces of relevant pure standard materials known to be present in the troposphere (TiO2, ZnO, Fe2O3) and natural samples that contain these materials, taken from the principal sources of tropospheric mineral aerosols (Arizona, Gobi and Sahara deserts). We shall investigate the efficiency of these processes in the absence and presence of uv light and will also study the effect of adsorbed organic species that can enhance the rate of photochemically generated peroxide. The intention is to obtain fundamental information about reaction rates, mechanisms and surface species: achieving this requires a cross-disciplinary approach involving application of realistic kinetic measurements and surface spectroscopies. The resulting information should be of use in atmospheric modelling where the effects of such systems have never been considered previously.