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[1] The nitrate formation on dust particles is considered as a sink for atmospheric NO y (such as HNO 3 ). However mineral dust is shown here to be an effective photocatalyst for transformation of nitrate anions into NO and NO 2 , without involving its photolysis. The photodecomposition of NO 3 À at the surface of synthetic mineral dust samples of SiO 2 , TiO 2 , mixed TiO 2 -SiO 2 and authentic sand doped with 6% NO 3 À was studied by means of a flow-tube at 298 K with UVillumination in the 340 -420 nm range at relative humidities between 5 and 80%. Both NO and NO 2 are observed during irradiation of films composed of either mixed TiO 2 -SiO 2 , pure TiO 2 and authentic minerals from the Sahara. The relative humidity strongly affects the concentration of NO x released into the gas phase. The photoinduced nitrate conversion into NO x is discussed as being a potential renoxification process of the atmosphere.

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