Quantification of the unknown HONO daytime source and its relation to NO₂

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.