During the DOMINO (<b>D</b>iel <b>O</b>xidant <b>M</b>echanism <b>I</b>n relation to <b>N</b>itrogen <b>O</b>xides) 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<sub>2</sub>, <i>j</i>(NO<sub>2</sub>) as a proxy for <i>j</i>(HONO). This allowed us to calculate values of the unknown HONO daytime source. This unknown HONO source, normalized by NO<sub>2</sub> mixing ratios and expressed as a conversion frequency (% h<sup>−1</sup>), showed a clear dependence on <i>j</i>(NO<sub>2</sub>) with values up to 43% h<sup>−1</sup> 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<sub>2</sub> conversion on soot surfaces and the reaction of electronically excited NO<sub>2</sub>* with water vapour, with the result that these two reactions normally contributed less than 10% (<1% NO<sub>2</sub> + soot + <i>h</i>ν; and <10% NO<sub>2</sub>* + H<sub>2</sub>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(<sup>1</sup>D)) integrated over the day.