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/S ratio, δS) of COS may offer a way for improved determinations of atmospheric COS sources. However, measuring the COS δS at the atmospheric concentrations of ~0.5 ppb is challenging. Here we present high-accuracy δS 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 δS measurement for gas standards is ≤0.2‰, and that N and CO in the gas standard mixture had no effect on the measured δS. Natural air samples were collected in Israel and in the Canary Islands. The COS δS values in both locations were found to be 13.2 ± 0.6‰, and are believed to represent the background tropospheric value. This δS 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 δS value of atmospheric COS we measured to estimate that ~48% of it originates from the ocean.