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HNO2 was investigated by long path optical absorption at three sites in Western Europe. It vas definitely identified and measured in moderately polluted air at Jülich. Mixing ratios as high as 0.8 ppb were observed before sunrise. After sunrise the HNO2 is photolyzed and considerable amounts of OH are produced. The formation of HNO2 is unclear and possible reactions are discussed.

Formaldehyde mixing ratios are reported for rural areas around Jülich and for maritime air at the north coast of Germany. The measurements were made using a ground‐based UV‐optical absorption technique which allowed the simultaneous determination of CH2O, NO2, and O3 with detection limits of 0.1, 0.1, and 1 ppb, respectively. In Jülich, which may be regarded as typical for central European background atmosphere, mixing ratios varied from 0.1 to 6.5 ppb from May through October 1978. In maritime air under conditions when photochemical equilibrium was expected, formaldehyde concentrations of 0.2 ppb were observed, which can be accounted for by photochemical oxidation of methane alone. However, the high concentrations of formaldehyde found at Jülich indicate other sources of formaldehyde.