International ultraviolet explorer observation of Venus SO 2 and SO

Results of recent International Ultraviolet Explorer (IUE) observations of Venus made on January 20, 1987, and April 2 and 3, 1988, along with a reanalysis of the 1979 observations (Conway et al., 1979) are presented. The observations indicate that the amount of sulfur dioxide at the cloud tops of Venus declined by a factor of 8±4 from 380±70 ppb in 1979 to 50±20 ppb in 1987 and 1988. These values are consistent with the Pioneer Venus results (L. W. Esposito, A recalibration of the solar flux for Pioneer Venus results and a comparison of existing SO 2 measurements on Venus, unpublished manuscript, 1989). We identify absorption features of sulfur monoxide for the first time, and estimate the SO mixing ratio above the cloud level is 20±10 ppb for 1979. This is consistent with photochemical models by Winick and Stewart (1980) and Yung and DeMore (1982) and with the upper limit from Wilson et al. (1981).