1Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institut, 5232 Villigen, Switzerland
2Physikalische Chemie/FB C, Bergische Universität Wuppertal, 42097 Wuppertal, Germany
3National Research Centre, Dokki, Giza, Egypt
4IRCELYON, Institut de recherches sur la catalyse et l'environnement de Lyon, 2 avenue Albert Einstein, Villeurbanne, France
5CNRS, UMR5256, Villeurbanne, France
6Université Claude Bernard Lyon 1, Villeurbanne, 69626, France
7Institut für Chemie und Dynamik der Geosphäre II: Troposphäre, Forschungszentrum Jülich, 42425 Jülich, Germany
Abstract. 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 <10−7 (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 (<20% RH) and high 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 humic materials present 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.