Cloudy sky optical paths as derived from differential optical absorption spectroscopy observations

The influence of tropospheric clouds on zenith sky observations of atmospheric gases (O3, NO2, O4, H2O, and others) was reported by Van Roozendael et al. (1994) and Erle et al. (1995). For measurements under heavy cloud cover, Erle et al. (1995) determined from simultaneously measured absorptions of different tropospheric species an average optical path for photons reaching the instrument of more than 100 km. An important question was whether the magnitude of the observation was a typical situation rather than a rare exception. Here we add a large set of new 'cloud event' observations evaluated from data that were recorded in the Arctic, at midlatitudes, and the tropics. It is shown that the observed absorption enhancements of O3, H2O, and O4 perfectly coincide with indirect cloud indicators, i.e., changes in the average intensity and the color index, as well as direct observations of cloud passages detected by satellite. In particular, we demonstrate that the observed optical paths have the same magnitude as the path lengths reported by Erle et al. 1995, and thus optical paths under cloudy skies may indeed become much larger than previously reported. (Author)