Abstract
Abstract. Globally, carbonyl sulphide (COS) is the most abundant sulphur gas in the atmosphere. Our chemistry-climate model (CCM) of the lower and middle atmosphere with aerosol module realistically simulates the background stratospheric sulphur cycle, as observed by satellites in volcanically quiescent periods. The model results indicate that upward transport of COS from the troposphere largely controls the sulphur budget and the aerosol loading of the background stratosphere. This differs from most previous studies which indicated that short-lived sulphur gases are also important. The model realistically simulates the modulation of the particulate and gaseous sulphur abundance in the stratosphere by the quasi-biennial oscillation (QBO). In the lowermost stratosphere organic carbon aerosol contributes significantly to extinction. Further, using a chemical radiative convective model and recent spectra, we compute that the direct radiative forcing efficiency by 1 kg of COS is 724 times that of 1 kg CO2. Considering an anthropogenic fraction of 30% (derived from ice core data), this translates into an overall direct radiative forcing by COS of 0.003 W m−2. The direct global warming potentials of COS over time horizons of 20 and 100 yr are GWP(20 yr) = 97 and GWP(100 yr) = 27, respectively (by mass). Furthermore, stratospheric aerosol particles produced by the photolysis of COS (chemical feedback) contribute to a negative direct solar radiative forcing, which in the CCM amounts to −0.007 W m−2 at the top of the atmosphere for the anthropogenic fraction, more than two times the direct warming forcing of COS. Considering that the lifetime of COS is twice that of stratospheric aerosols the warming and cooling tendencies approximately cancel.
Highlights
The anthropogenic increase of greenhouse gases causes a radiative forcing of climate of more than 3 W m−2 (IPCC, 2007)
Its anthropogenic emissions and that of CS2, the latter being converted into carbonyl sulphide (COS), contribute approximately 30 % to the global COS
Our EMAC model results indicate that the COS controlled sulphur flux into the stratosphere is about 0.15 million tonnes of sulphur (Mt S) yr−1 with about 0.035 Mt S yr−1 converted to aerosol, whereas in previous evaluations the same flux of S was obtained assuming direct transport of SO2, CS2 and H2S across the tropical tropopause
Summary
The anthropogenic increase of greenhouse gases causes a radiative forcing of climate of more than 3 W m−2 (IPCC, 2007). Aerosol particles backscatter solar radiation and their anthropogenic emissions exert a global mean negative radiative forcing of about −0.5 W m−2, being enhanced by a factor of two or more by indirect effects of the particles on clouds (IPCC, 2007). If the volcanic SO2 is lofted to high altitudes in the tropics a substantial fraction can reach the stratosphere where it is converted into sulphate particles that exert a negative radiative forcing. In the subsequent sections we present calculations with our chemistry-climate model EMAC of stratospheric aerosol and its forcings, and compute the global warming potential of COS. We conclude by assessing the role of COS in the stratospheric sulphur budget and the overall climate effects from anthropogenic emissions, including the chemically induced indirect effects through stratospheric aerosol
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