Strong daytime production of OH from HNO 2 at a rural mountain site

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 HNO 2 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 HNO 2 with a production rate on the order of 2–4 × 10 6 cm −3 s −1 . The data demonstrate an important contribution of HNO 2 to local HO x levels over the entire day, comparable with the photolysis of O 3 and HCHO. On average during the 2004 campaign, 42% of integrated photolytic HO x formation is attributable to HNO 2 photolysis.