Quantification of the unknown HONO daytime source and its relation to NO(2)

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 + 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.