Nitrous acid and OH were measured concurrently with a number of other atmospheric components and relevant photolysis frequencies during two campaigns at the Meteorological Observatory Hohenpeissenberg (980 m a.s.l.) in summer 2002 and 2004. On most of the 26 measurement days the HNO2 concentration surprisingly showed a broad maximum around noon (on average 100 pptv) and much lower concentrations during the night (~30 pptv). The results indicate a strong unknown daytime source of HNO2 with a production rate on the order of 2--4 ¿ 106 cm-3s-1. The data demonstrate an important contribution of HNO2 to local HOx levels over the entire day, comparable with the photolysis of O3 and HCHO. On average during the 2004 campaign, 42% of integrated photolytic HOx formation is attributable to HNO2 photolysis.