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Atmospheric concentrations of nitrous acid (HONO), one of the majorprecursors of the hydroxyl radical (OH) in the troposphere, significantlyexceed the values predicted by the assumption of a photostationary state(PSS) during daytime. Therefore, additional sources of HONO were intensivelyinvestigated in the last decades. This study presents budget calculations ofHONO based on simultaneous measurements of all relevant species, includingHONO and OH at two different measurement heights, i.e. 1 m above the ground andabout 2 to 3 m above the canopy (24 m above the ground), conducted in a borealforest environment. We observed mean HONO concentrations ofabout 6.5 × 108 molecules cm−3 (26 ppt) during daytime, more than 20 timeshigher than expected from the PSS of 0.2 × 108 molecules cm−3 (1ppt). To close the budgets at both heights, a strong additional source termduring daytime is required. This unidentified source is at its maximum at noon (upto 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and ingeneral up to 2.3 times stronger above the canopy than close to the ground.The insignificance of known gas phase reactions and other processeslike dry deposition or advection compared to the photolytic decompositionof HONO at this measurement site was an ideal prerequisite to study possiblecorrelations of this unknown term to proposed HONO sources. But neither theproposed emissions from soils nor the proposed photolysis of adsorbedHNO3 contributed substantially to the unknown source. However, theunknown source was found to be perfectly correlated to the unbalancedphotolytic loss of HONO

A comparison of HONO budgets for two measurement heights at a field station within the boreal forest in Finland