Constraining the atmospheric OCS budget from sulfur isotopes

Carbonyl sulfide (OCS), the most abundant sulfur-containing gas in the atmosphere, is used as a proxy for photosynthesis rate estimation. However, a large missing source of atmospheric OCS has been inferred. Sulfur isotope measurements (S-34/S-32 ratio and (delta S-34) 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 delta S-34 within Japan. The observed delta S-34 of OCS of 9.7 to 14.5 parts per thousand reflects source and sink effects. Particularly in winter, latitudinal decreases in delta S-34 values of OCS were found to be correlated with increases in OCS concentrations, resulting an intercept of (4.7 +/- 0.8)parts per thousand 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 delta S-34 of OCS in eastern Asia is consistent with the delta S-34 of OCS previously reported in Israel and the Canary Islands, suggesting that the background delta S-34 of OCS in the Northern Hemisphere ranges from 12.0 to 13.5 parts per thousand. 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.