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Carbonyl sulfide (COS) is a colorless gas with a sulfur-like odor. Like carbon dioxide (CO₂), its molecular structure is linear; but unlike CO₂ it is flammable. It decomposes slowly in water and more rapidly in the presence of base.

J. P. Couërbe described what he thought was COS in 1841, but the gas turned out to be a mixture of CO₂ and hydrogen sulfide (H₂S). In 1867, Hungarian Chemist Carl von Than characterized it correctly; he made it by the reaction between potassium thiocyanate (KSCN) and sulfuric acid (H₂SO₄), although other gases were coproduced.

For a simple molecule, COS has relatively few applications in research and manufacturing. But at �0.5 ppb, it is the most abundant sulfur-containing compound in Earth’s atmosphere, a circumstance that turns out to be surprisingly useful in Earth science.

About 10 years ago, J. Elliott Campbell, then at the University of California, Merced, and other scientists began to evaluate whether COS is a good surrogate for CO₂ for tracking how much carbon is taken up by photosynthesis worldwide. Even though COS’s concentration in the atmosphere is smaller than that of CO₂ by a factor of â‰�10⁶, its atmospheric “signalâ€� is 6 times greater than CO₂; and it avoids some of the complications that accompany CO₂measurements.

Earlier this year, Campbell, now at the University of California, Santa Cruz, and about two dozen coauthors at multiple institutions reported that  despite COS’s own complications. Their conclusion: “Pursuing multiple lines of evidence, including the COS technique, may yet provide a tractable path for addressing the pressing concern of carbon processes within the climate system.�