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 (34S/32S ratio, δ34S) of COS may offer a way for improved determinations of atmospheric COS sources. However, measuring the COS δ34S at the atmospheric concentrations of ~0.5 ppb is challenging. Here we present high-accuracy δ34S 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 δ34S measurement for gas standards is ≤0.2‰, and that N2 and CO2 in the gas standard mixture had no effect on the measured δ34S. Natural air samples were collected in Israel and in the Canary Islands. The COS δ34S values in both locations were found to be 13.2±0.6‰, and are believed to represent the background tropospheric value. This δ34S 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 δ34S value of atmospheric COS we measured to estimate that ~48% of it originates from the ocean.

Forests exchange CO and O with the atmosphere at similar molar ratios. Correspondingly, the apparent respiratory quotient (CO/O flux ratio, ARQ) is expected to be ≈1 given the stoichiometry of organic substrates in soils and plants. However, measured ARQ values often deviate from ≈1, and it is still unclear how CO and O fluxes are balanced among ecosystem components, and what are the sources of ARQ variability. Here we measured ARQ of soil pore space air (ARQ), and in headspace air from incubations of bulk-soil (ARQ), tree stem-cores (ARQ) and roots in 10 measurement campaigns over 15 months in a Mediterranean oak forest. Mean (range) values were: ARQ = 0.76 (0.60-0.92), ARQ = 0.75 (0.53-0.90), and ARQ = 0.39 (0.19-0.70). As expected, ARQ was usually higher than ARQ and lower than the ARQ of incubated roots (range of 0.73-0.96). Variability in ARQ was correlated with soil moisture parameters. Temperature positively correlated with ARQ and ARQoutside the growing season. Abiotic O uptake by Fe was demonstrated to reduce ARQ, but this effect would be significant under field conditions only if respiration rates are very low. We hypothesize that low measured ARQ values likely result from selective decomposition of reduced compounds and physical protection of oxidized compounds. ARQ, measured at two stem positions, was lower than expected from oxidation of any possible substrate, indicating partial retention of respired C. The overall ARQ <1 reveals an imbalance of stem-soil CO and O fluxes that is unexpected at the ecosystem level.