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Laboratory studies on the heterogeneous conversion of nitrogen dioxide into nitrous acid on irradiated ice films containing humic acid are described. It was found that the presence of light in the visible range of the solar spectrum significantly enhances the rate of nitrous acid release from a humic acid doped ice film. This process might contribute to observed HONO production in snow, where the NO2 is thought to originate from nitrate photolysis. Analysis of the experimental data based on the Langmuir Hinshelwood model framework allowed quantification of the observed dependencies of the nitrous acid production rate on nitrogen dioxide concentration. The observed dependencies on the humic acid concentration as well as on the irradiation intensity were used to estimate light-driven HONO fluxes for environmental snow covers.

Ndour, Marieme Conchon, Pierre D'Anna, Barbara Ka, Oumar George, Christian; The nitrate formation on dust particles is considered as a sink for atmospheric NOy (such as HNO3). However mineral dust is shown here to be an effective photocatalyst for transformation of nitrate anions into NO and NO2, without involving its photolysis. The photodecomposition of NO3- at the surface of synthetic mineral dust samples of SiO2, TiO2, mixed TiO2-SiO2 and authentic sand doped with 6% NO3- was studied by means of a flow-tube at 298 K with UV-illumination in the 340-420 nm range at relative humidities between 5 and 80%. Both NO and NO2 are observed during irradiation of films composed of either mixed TiO2-SiO2, pure TiO2 and authentic minerals from the Sahara. The relative humidity strongly affects the concentration of NOx released into the gas phase. The photoinduced nitrate conversion into NOx is discussed as being a potential renoxification process of the atmosphere. Citation: Ndour, M., P. Conchon, B. D'Anna, O. Ka, and C. George (2009), Photochemistry of mineral dust surface as a potential atmospheric renoxification process, Geophys. Res. Lett., 36, L05816, doi: 10.1029/2008GL036662.

Nicolas, Melanie Ndour, Marieme Ka, Oumar D'Anna, Barbara George, Christian; The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coated-wall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 x 10(-6) to 3 x 10(-5). The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume.

[1] Mineral dust contains material such as TiO2 that is well known to have photocatalytic activity. In this laboratory study, mixed TiO2-SiO2, Saharan dust and Arizona Test Dust were exposed to NO2 in a coated wall flow tube reactor. While uptake in the dark was negligible, photoenhanced uptake of NO2 was observed on all samples. For the mixed TiO2-SiO2, the uptake coefficients increased with increasing TiO2 mass fraction, with BET uptake coefficients ranging from 0.12 to 1.9 × 10−6. HONO was observed from all samples, with varying yields, e.g., 80% for Saharan dust. Three-dimensional modeling indicates that photochemistry of dust may reduce the NO2 level up to 37% and ozone up to 5% during a dust event in the free troposphere.

The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn −7 (in the dark) and γrxn=6×10−6 were observed. The observed uptake coefficients decreased with increasing NO2-concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity (60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONO-formation rates observed near the ground in rural and urban environments by the conversion of NO2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles.

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 NOy (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 A. Mellouki and A.R. Ravishankara (eds.), Regional Climate Variability and its Impacts in the 219 Mediterranean Area, 219–233.