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

Recently, the influence of tropospheric clouds on zenith sky observations of atmospheric gases (O-3, NO2, O-4, H2O, and others) has been reported [Van Roozendael et al., 1994; 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 >100 km. This path length was much larger than expected from previous reports [Feigelson, 1981]. Thus 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 which were recorded in the Arctic, at midlatitudes, and the tropics. It is shown that the observed absorption enhancements of O-3, H2O, and O-4 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. Besides these observations made during specific meteorological conditions we also studied optical paths continuously for several weeks in Arctic winter/spring conditions. We derived a frequency distribution of path lengths for cloudy skies with an average mean optical path of 29 km.