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 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 + hν and <10% NO_2~* + H_2O) 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.
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