Observations of several atmospheric species can be performed by measuring the absorption of visible and near‐ultraviolet light scattered from the zenith sky. The determination of vertical column abundances of molecules such as ozone and NO2 from such measurements is briefly reviewed. It is shown that the conversion of NO to NO2 can be of significance in the interpretation of measurements made near twilight. On the other hand, multiple scattering from the atmosphere or by clouds is likely to be a very small effect.

Abstract. Direct measurements of the absorption of downwelling visible radiation by nitrogen dioxide are presented. The data show that this gas can contribute significantly to local radiative forcing under certain conditions. The observed enhancements in nitrogen dioxide absorption are likely to be due both to pollution and to production by lightning in convective clouds. Case studies of several days of observations in Colorado reveal peak absorption of downwelling radiation by NO2 of up to 5-12%, corresponding to an estimated local radiative forcing that is likely to be in the range of 5-30 W/m 2. The amount of local forcing associated with thunderstorm activity depends strongly upon the cloud optical depth and on where the NO2 resides within the clouds. These case studies suggest that NO2 can play a significant role in the absorption of radiation (including but not limited to anomalous cloud absorption) either under polluted conditions or when electrically active storms are considered.

The column abundance of OClO at McMurdo Station, Antarctica, was measured by visible absorption spectroscopy during austral spring in 1986. Observations were obtained during the day using scattered sunlight and at night using direct light from the moon. The observed total column amounts in Antarctica were about 20–50 times larger than would be expected for standard homogeneous photochemistry, suggesting that the halogen chemistry of Antarctic spring is profoundly different from such standard theoretical expectations and may play a role in the development of the springtime Antarctic ozone “hole.”

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