OH ‐ Radicals in the lower troposphere

OH has been detected in the lower troposphere by optical absorption of the Q 1 2 (A²Σ + , v = O, X²Π, v = O) line at 307.995 nm along a 7.8 km path above Jülich (51° North, 6° East). The detection limit depended on the quality of the photographic exposure and reached 2 · 10 6 cm −3 in the most favourable cases. OH generally remained below 4 · 10 6 cm −3 , the average detection limit. But on several occasions during the observation period from mid August to November 1975 concentrations up to 7 · 10 6 cm −3 were observed.