Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, normally exceed by far the values predicted by the assumption of a photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. Here, we present budget calculations of HONO based on simultaneous measurements of all relevant species including HONO and OH at two different measurement heights, i.e. 1 m above ground and about 2 to 3 m above canopy (24 m above ground), conducted in boreal forest environment. We observed mean HONO concentrations during daytime of about 6.5 × 10 8 molecules cm −3 (26 ppt), more than twenty times higher than expected from the PSS, 0.2 × 10 8 molecules cm −3 (1 ppt). To close the budgets in both heights a strong additional source term during daytime is required. This unidentified source is maximal at noon (up to 1.1 × 10 6 molecules cm −3 s −1 , 160 ppt h −1 ) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and also other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO 3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.