1University of Bayreuth, Atmospheric Chemistry Research Laboratory, Bayreuth, Germany
2Max Planck Institute for Chemistry, Atmospheric Chemistry Department, P.O. Box 3060, 55020 Mainz, Germany
3Max Planck Institute for Chemistry, Particle Chemistry Department, P.O. Box 3060, 55020 Mainz, Germany
4University of Bayreuth, Junior Professorship in Atmospheric Chemistry, Bayreuth, Germany
5Fraunhofer Institute for Toxicology and Experimental Medicine, Hannover, Germany
*now at: University Mainz, Institute for Atmospheric Physics, Mainz, Germany
Abstract. During the DOMINO (Diel Oxidant Mechanism In relation to Nitrogen Oxides) campaign in southwest Spain we measured simultaneously all quantities necessary to calculate a photostationary state for HONO in the gas phase. These quantities comprise the concentrations of OH, NO, and HONO and the photolysis frequency of NO2, j(NO2) as a proxy for j(HONO). This allowed us to calculate values of the unknown HONO daytime source. This unknown HONO source, normalized by NO2 mixing ratios and expressed as a conversion frequency (% h−1), showed a clear dependence on j(NO2) with values up to 43 % h−1 at noon. We compared our unknown HONO source with values calculated from the measured field data for two recently proposed processes, the light-induced NO2 conversion on soot surfaces and the reaction of electronically excited NO2* with water vapour, with the result that these two reactions normally contributed less than 10 % (<1 % NO2 + soot + hν; and <10 % NO2* + H2O) to our unknown HONO daytime source. OH production from HONO photolysis was found to be larger (by 20 %) than the "classical" OH formation from ozone photolysis (O(1D)) integrated over the day.