Photoenhanced Uptake of NO₂ on Mineral Dust

Dust events are commonly observed every year and have been shown to strongly impact on the tropospheric ozone budget. This impact arises from the uptake of different gases, such as NO_y (mainly as HNO₃ or N₂O₅), on the solid surfaces exhibited by the uplifted minerals. While such “dark” processes have been deeply studied over the last years, dust particles contain a series of oxides that may be initiate photochemical process that have not been considered so far. In fact, in addition to quartz, illite, montmorillonite, and calcite, mineral dusts are heterogeneous mixtures of mineral oxides containing small levels of TiO₂. In order to mimic the properties that these oxides confer to mineral Saharan dust, TiO₂ and SiO₂ were mixed and their heterogeneous reactions with NO₂ studied using a horizontal wall flow tube. In addition, experiments were performed with real Arizona test dust in order to assess the importance of photochemical reactions under realistic atmospheric conditions. The effect of light (in the 380-700 nm range), temperature (in the 288-303 K range) and relative humidity have been determined. The uptake coefficient on TiO₂ mixing in SiO₂ increases with temperature and decreases with relative humidity. We found that despite its very low abundance, titanium dioxide (TiO₂) will strongly favour the photo-conversion of NO₂ on mineral dust, nitrogen dioxide being otherwise quite unreactive on these minerals. This photoenhanced uptake of gases will in turn impact on the ozone and/or HO_x budget during a dust event. In addition, such photo enhanced process will affect the nitrate content of the dust particles, impacting on the optical properties of the aerosols and their associated climatic impact. We therefore exemplified, on mineral dust, that photochemical conversion on solid surfaces encountered in the troposphere needs to be considered an important process of wide impact due to the ubiquitous presence of minerals in our environment as it will change the level of photo-oxidant and aerosol optical properties.