Isotopic constraints on the tropospheric carbonyl sulfide budget

Carbonyl sulfide (OCS), the most abundant sulfur-containing gas in the ambient atmosphere, possesses great potential for tracer of the carbon cycle. Sulfur isotopic composition (³⁴S/³²S ratio, δ³⁴S) on OCS is a feasible tool to evaluate the OCS budget. We applied the sulfur isotope measurement for the tropospheric OCS cycle and distinguished OCS sources from oceanic and anthropogenic emissions.

 

Here, we present a developed measurement system of δ³⁴S of OCS and the result of latitudinal (north-south) observations of OCS within Japan using the method. The OCS sampling system was carried to three sampling sites in Japan: Miyakojima (24°8’N, 125°3’E), Yokohama (35°5’N, 139°5’E), and Otaru (43°1’N, 141°2’E). The observed δ³⁴Sof OCS ranging from 9.7 to 14.5‰ reflects the tropospheric OCS cycle. Particularly in winter, latitudinal decreases in δ³⁴Svalues were found to be correlated with increases in OCS concentrations, resulting in an intercept of (4.7 ± 0.8)‰ in the Keeling plot approach. This trend suggests 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. This sulfur isotopic constraint on atmospheric OCS is an important step together with isotopic characterizations and analysis using a chemical transport model, will enable detailed quantitative OCS budget and estimation of gross primary production.