CLOUDY SKY OPTICAL PATHS AS DERIVED FROM DIFFERENTIAL OPTICAL-ABSORPTION SPECTROSCOPY OBSERVATIONS

Recently, the influence of tropospheric clouds on zenith sky observati ons of atmospheric gases (O-3, NO2, O-4, H2O, and others) has been rep orted [Van Roozendael et al., 1994; Erle et al., 1995], For measuremen ts under heavy cloud cover, Erle et al. [1995] determined from simulta neously measured absorptions of different tropospheric species an aver age optical path for photons reaching the instrument of >100 km. This path length was much larger than expected from previous reports [Feige lson, 1981]. Thus an important question was whether the magnitude of t he observation was a typical situation rather than a rare exception. H ere we add a large set of new ''cloud event'' observations evaluated f rom data which were recorded in the Arctic, at midlatitudes, and the t ropics. 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 meteorologi cal conditions we also studied optical paths continuously for several weeks in Arctic winter/spring conditions. We derived a frequency distr ibution of path lengths for cloudy skies with an average mean optical path of 29 km.