OH has been detected in the lower troposphere by optical absorption of the Q1 2(A2&Sgr;+,v=O, X2II, v=O) line at 307.995 nm along a 7.8 km path above J¿lich (51¿ North, 6¿ East). The detection limit depended on the quality of the photographic exposure and reached 2⋅106 cm-3 in the most favourable cases.

OH generally remained below 4⋅106 cm-3, the average detection limit. But on several occasions during the observation period from mid August to November 1975 concentrations up to 7⋅106 cm-3 were observed.