Sorgel, M.M.SorgelRegelin, E.E.RegelinBozem, H.H.BozemDiesch, J.M.J.M.DieschDrewnick, F.F.DrewnickFischer, H.H.FischerHarder, H.H.HarderHeld, A.A.HeldHosaynali-Beygi, Z.Z.Hosaynali-BeygiMartinez, M.M.MartinezZetzsch, C.C.Zetzsch2022-03-042022-03-042011https://publica.fraunhofer.de/handle/publica/22717210.5194/acp-11-10433-2011During 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 NO(2), j(NO(2)) as a proxy for j(HONO). This allowed us to calculate values of the unknown HONO daytime source. This unknown HONO source, normalized by NO(2) mixing ratios and expressed as a conversion frequency (% h(-1)), showed a clear dependence on j(NO(2)) 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 NO(2) conversion on soot surfaces and the reaction of electronically excited NO(2)* with water vapour, with the result that these two reactions normally contributed less than 10% (<1% NO(2) + soot + hv; and <10% NO(2)* + H(2)O) 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((1)D)) integrated over the day.en610551journal article