Isotopic constraints on the atmospheric carbonyl sulfide (OCS or COS) budget.

Abstract:

Carbonyl sulfide (OCS), the most abundant sulfur-containing gas in the atmosphere, is used as a proxy for photosynthesis rate estimation. Because of its long lifetime (longer than two years), OCS can be transported to the stratosphere, where it is converted to stratospheric sulfate aerosols (SSAs), consequently affecting Earth’s radiation balance and stratospheric chemistry. However, a large missing source of atmospheric OCS has been inferred.

Sulfur isotope measurements (³⁴S / ³²S ratio, δ³⁴S) on OCS are a feasible tool to distinguish OCS sources from oceanic and anthropogenic emissions. Here, we present the latitudinal (north–south) observations of OCS concentration and δ³⁴S within Japan. The observed δ³⁴S of OCS ranging from 9.7 to 14.5‰ reflects source and sink effects. Particularly in winter, latitudinal decreases in δ³⁴S values of OCS were found to be correlated with increases in OCS concentrations, resulting an intercept of (4.7 ±0.8)‰ in the Keeling plot approach. This result implies the transport of anthropogenic OCS emissions from the Asian continent to the western Pacific by the Asian monsoon outflow.

The estimated background δ³⁴S of OCS in eastern Asia is consistent with the δ³⁴S of OCS previously reported in Israel and the Canary Islands, suggesting that the background δ³⁴S of OCS in the Northern Hemisphere ranges from 12.0‰ to 13.5‰. Our constructed sulfur isotopic mass balance of OCS revealed that anthropogenic sources, not merely oceanic sources, account for much of the missing source of atmospheric OCS. Given that the historical estimation of GPP is sensitive to the estimate of the anthropogenic OCS inventory, a detailed update of the OCS budget constrained by this sulfur isotopic approach will enable precise estimation of global historical GPP changes and their interactions with global change.