Abstract. The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO 2 ) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO 2 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 NO 2 -concentration, reactive uptake coefficients γ rxn for the NO 2 →HONO conversion on the aerosol between γ rxn <10 −7 (in the dark) and γ rxn =6×10 −6 were observed. The observed uptake coefficients decreased with increasing NO 2 -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 (<20% RH) and high humidity (>60% RH). The measured uptake coefficients for the NO 2 →HONO conversion are too low to explain the HONOformation rates observed near the ground in rural and urban environments by the conversion of NO 2 →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.

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.

[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.