Interhemispheric ratio and annual cycle of carbonyl sulfide (OCS) total column from ground‐based solar FTIR spectra

Abstract

We have made extensive measurements of total column OCS by ground‐based high‐resolution solar Fourier transform infrared absorption spectroscopy from two southern hemisphere sites to complement earlier measurements in the northern hemisphere and to investigate the interhemispheric ratio, variability, and seasonal cycles of OCS in the atmosphere. The measurements were made at Lauder, New Zealand (45.0°S, 169.7°E, 370 m asl, 547 spectra March 1993 through April 1997), and Wollongong, Australia (34.45°S, 150.88°E, 30 m asl, 358 spectra May 1996 through April 1997). The annual mean column amounts are 9.15×1015 molecules cm−2 above Lauder and 9.84×1015 molecules cm−2 above Wollongong, corresponding to tropospheric mixing ratios of 480 and 490 parts per trillion by volume, respectively, with the assumed mixing ratio vertical profiles. The secular trend in total column OCS is less than 1% per year. Variability of all measurements about the means implies an atmospheric lifetime for OCS of at least 2.8 years. Comparison with earlier measurements in the northern hemisphere yields a north/south interhemispheric ratio in the range 1.1–1.2. There are peak‐to‐peak apparent annual cycles in total column OCS of 6% at Lauder and 18% at Wollongong with a late summer maximum. Seasonal tropopause height variation accounts for a 5–6% amplitude, and the remainder of the amplitude in Wollongong is assumed to be due to changes in the tropospheric mixing ratio.