Daytime concentrations of HONO, NOx, OH and photolysis frequencies were measured during the ECHO field campaign in a mixed deciduous forest near J¿lich, West-Germany, in summer 2003. Midday measurements show clear evidence for a large, yet unexplained daytime source of HONO (~500 pptv/h), which represents an important net source of OH radicals due to ongoing HONO photolysis. The evidence for a large HONO daytime source is for the first time completely constrained by measured parameters, needed to determine the daytime budget of HONO. The large contribution of 33% to the primary OH production during noon at the top of the forest canopy suggests that the unexplained source of HONO could have an important impact on the photochemical transformation of biogenically emitted volatile organic compounds (VOCs) by OH into partly oxidized VOCs and secondary organic aerosols during their release from forest regions into the troposphere.