Ground‐based infrared measurements of carbonyl sulfide total column abundances: Long‐term trends and variability

Abstract

Total vertical column abundances of carbonyl sulfide (OCS) have been derived from time series of high‐resolution infrared solar absorption spectra recorded at the National Solar Observatory McMath solar telescope facility on Kitt Peak (altitude 2.09 km, latitude 31.9°N, longitude 111.6°W), southwest of Tucson, Arizona, and at the International Scientific Station of the Jungfraujoch (altitude 3.58 km, latitude 46.5°N, longitude 8.0°E), in the Swiss Alps. The analysis of both data sets is based on nonlinear least squares spectral fittings of narrow intervals centered on lines of the intense ν3 band of OCS, the P(37) transition at 2045.5788 cm−1 and the P(15) transition at 2055.8609 cm−1, with a consistent set of spectroscopic line parameters. The Kitt Peak measurements, recorded on 30 different days between May 1977 and March 1991, show a 10% peak‐to‐peak seasonal cycle with a summer maximum and a winter minimum and a trend in the total column abundance equal to (0.1 ± 0.2)% yr−1, 2σ. Jungfraujoch solar spectra recorded on 67 different days between October 1984 and April 1991 have been analyzed. The fitted trend in the Jungfraujoch total columns, (−0.1 ± 0.5)% yr−1, 2σ, is consistent with the Kitt Peak trend results within the errors. The Jungfraujoch total columns show a more complex seasonal variation than noted in the Kitt Peak data. The mean of the daily averaged total columns, 8.44 × 1015 molecules cm−2 above Kitt Peak and 6.41 × 1015 molecules cm−2 above the Jungfraujoch station, correspond respectively to mean tropospheric mixing ratios of 0.54 ± 0.04 and 0.52 ± 0.04 parts per billion by volume; these values are consistent with previously reported remote and in situ measurements. Taken together, the results from the two sites indicate that there has been no significant change in the OCS total column abundance at northern mid‐latitudes over the last decade.