Carbonyl sulfide as a new window on the carbon cycle
Carbonyl sulfide (COS) is a promising new tracer to constrain the gross CO2 fluxes of land ecosystems. Since vegetation COS and CO2 uptake during photosynthesis is closely coupled, it should be possible to quantify the gross fluxes of photosynthesis and respiration from concurrent measurements of COS and CO2. We demonstrate the application of carbonyl sulfide (COS) measurements to obtain new estimates of canopy-scale Gross Primary Production (GPP) at two flux tower sites. We measured net ecosystem, leaf and soil fluxes. Typically, the daytime variations in COS mirrored those of CO2, however, COS uptake continued at night whereas the CO2 fluxes are small emissions. We also developed models to describe the exchange of COS simultaneously with that of CO2 and water. GPP estimates obtained from measured COS fluxes agreed well with those calculated using CO2-based partitioning methods based on daytime measurements. In contrast, one of the most widely used flux partitioning methods, using the relationship of night-time fluxes and temperature, did poorly due to lack of turbulence at night. Our results demonstrate that adding COS to flux tower measurements would provide valuable observation-based constraints on the land carbon cycle.