Direct measurements of atmospheric CH₂O, HNO₂, O₃, NO₂, and SO₂ by differential optical absorption in the near UV

Measurements of CH 2 O, HNO 2 , O 3 , NO 2 , and SO 2 at four different sites in western Europe are reported. At the maritime sites (Loophead, Ireland, and Dagebüll, Germany) the mixing ratios of the pollution indicators SO 2 and NO 2 are <0.5–2 ppb, while concentrations of several tenths of a part per billion of CH 2 O are consistent with photochemical equilibrium. HNO 2 was only found at continental sites (Jülich and Deuselbach) with SO 2 and NO 2 mixing ratios up to 100 ppb. Surprisingly, NO 3 was found to be at least 10 times lower than expected in most cases.