From AMS Glossary
Relatively unreactive sulfur gas (COS) that can persist in the atmosphere long enough for transport to the stratosphere to occur, where its oxidation is responsible for the maintenance of the background stratospheric sulfate layer.
This link between the biosphere and the stratosphere was recognized by Paul Crutzen in 1976. The atmospheric budget of COS is intimately linked to the biosphere, since it can be both released from, and taken up by, vegetation.
Crutzen, P. J. 1976. The possible importance of COS for the sulfate layer of the stratosphere. Geophys. Res. Lett.. 3. 73–76.