Sulfur isotopes ratio of atmospheric carbonyl sulfide constrains its sources

Carbonyl sulfide (COS) is the major long-lived sulfur bearing gas in the atmosphere, and is used to estimate the rates of regional and global (both past and current) photosynthesis. Sulfur isotope measurements (S-34/S-32 ratio, delta S-34) of COS may offer a way for improved determinations of atmospheric COS sources. However, measuring the COS delta S-34 at the atmospheric concentrations of similar to 0.5 ppb is challenging. Here we present high-accuracy delta S-34 measurements of atmospheric COS done by gas chromatograph (GC) connected to a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS), after pre-concentrating from 2-liters of air. We showed that the precision of COS delta S-34 measurement for gas standards is <= 0.2%, and that N-2 and CO2 in the gas standard mixture had no effect on the measured delta S-34. Natural air samples were collected in Israel and in the Canary Islands. The COS delta S-34 values in both locations were found to be 13.2 +/- 0.6%, and are believed to represent the background tropospheric value. This delta S-34 value is markedly different from the previously reported value of 4.9%. We estimate the expected isotopic signature of COS sources and sinks, and use the delta S-34 value of atmospheric COS we measured to estimate that similar to 48% of it originates from the ocean.