Abstract
Abstract. We present a global carbonyl sulfide (OCS) data set covering the period June 2002 to April 2012, derived from FTIR (Fourier transform infrared) limb emission spectra measured with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on the ENVISAT satellite. The vertical resolution is 4–5 km in the height region 6–15 km and 15 at 40 km altitude. The total estimated error amounts to 40–50 pptv between 10 and 20 km and to 120 pptv at 40 km altitude. MIPAS OCS data show no systematic bias with respect to balloon observations, with deviations mostly below ±50 pptv. However, they are systematically higher than the OCS volume mixing ratios of the ACE-FTS instrument on SCISAT, with maximum deviations of up to 100 pptv in the altitude region 13–16 km. The data set of MIPAS OCS exhibits only moderate interannual variations and low interhemispheric differences. Average concentrations at 10 km altitude range from 480 pptv at high latitudes to 500–510 pptv in the tropics and at northern mid-latitudes. Seasonal variations at 10 km altitude amount to up to 35 pptv in the Northern and up to 15 pptv in the Southern Hemisphere. Northern hemispheric OCS abundances at 10 km altitude peak in June in the tropics and around October at high latitudes, while the respective southern hemispheric maxima were observed in July and in November. Global OCS distributions at 250 hPa (∼ 10–11 km) show enhanced values at low latitudes, peaking during boreal summer above the western Pacific and the Indian Ocean, which indicates oceanic release. Further, a region of depleted OCS amounts extending from Brazil to central and southern Africa was detected at this altitude, which is most pronounced in austral summer. This depletion is related to seasonally varying vegetative uptake by the tropical forests. Typical signatures of biomass burning like the southern hemispheric biomass burning plume are not visible in MIPAS data, indicating that this process is only a minor source of upper tropospheric OCS. At the 150 hPa level (∼ 13–14 km) enhanced amounts of OCS were also observed inside the Asian monsoon anticyclone, but this enhancement is not especially outstanding compared to other low latitude regions at the same altitude. At the 80 hPa level (∼ 17–18 km), equatorward transport of mid-latitude air masses containing lower OCS amounts around the summertime anticyclones was observed. A significant trend could not be detected in upper tropospheric MIPAS OCS amounts, which points to globally balanced sources and sinks. Simulations with the ECHAM-MESSy model reproduce the observed latitudinal cross sections fairly well.
Highlights
The first measurements of atmospheric carbonyl sulfide (OCS), which is the most prevalent sulfur-containing trace gas, were performed by Hanst et al (1975) and Sandalls and Penkett (1977)
A dedicated validation of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) OCS amounts measured in the tropical upper troposphere and extra-tropical lowermost stratosphere has been presented in the Supplement of Glatthor et al (2015a)
In support of the higher MIPAS OCS values we refer to Velazco et al (2011), who found Atmospheric Chemistry Experiment (ACE)-FTS OCS data to be biased low by ∼ 15 % compared to MkIV results in the altitude range 12–23 km, both data sets are retrieved in the same spectral region
Summary
The first measurements of atmospheric carbonyl sulfide (OCS), which is the most prevalent sulfur-containing trace gas, were performed by Hanst et al (1975) and Sandalls and Penkett (1977). According to these authors these trends are mainly driven by tropospheric processes, because for stratospheric OCS they found a weak trend of (2.0 ± 0.6) % decade−1 over the whole period 1995–2015 only In another investigation of groundbased FTIR measurements at three stations between 34.45 and 77.80◦ S, Kremser et al (2015) determined positive trends in total column OCS between (4.3 ± 0.2) % decade−1 and (7.3 ± 0.3) % decade−1 for the time period 2001–2014 and concluded that the OCS budget is not closed. Barkley et al (2008) presented upper tropospheric and stratospheric global OCS distributions of the time period 2004–2006 from spaceborne measurements of the Fourier Transform Spectrometer of the Atmospheric Chemistry Experiment (ACE-FTS) on the Canadian SCISAT satellite (Bernath et al, 2005; Boone et al, 2005). We will perform a dedicated analysis of the decennial trend
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