Temperature dependent UV absorption cross sections for carbonyl sulfide

The ultraviolet absorption spectrum of carbonyl sulfide is investigated in order to assess the importance of the photolytic sink for OCS in the troposphere. Absorption cross sections were measured at wavelengths between 185 and 300 nm in a double-beam spectrometer at temperatures of 295 and 225 K. The results are found to be in good agreement with previous reports in the short wavelength region, but significantly lower than the value of 2 x 10 to the 22nd sq cm/molecule proposed by Rudolf and Inn (1981) between 280 and 300 nm, which may be attributed to trace contaminants or dimerization. A marked temperature effect is noted at wavelengths greater than 240 nm and below 190 nm. As no evidence of absorption at wavelengths greater than 270 nm was found, estimates of the solar photodissociation rate of tropospheric OCS based on the present cross sections yield a value of about 2 x 10 to the -9th/sec, corresponding to a tropospheric lifetime of about 20 years. It is therefore concluded that solar photolysis represents a negligible sink for OCS.