The Link Between GPP and the Atmospheric Concentration of Carbonyl Sulfide (OCS)

Carbonyl sulfide (OCS) is an atmospheric trace gas that is taken up by leaves in parallel with photosynthetic uptake of CO2 (GPP). The latter is an important component of carbon cycle models and accurate measurements of photosynthesis are needed to test these models. However, it is difficult to obtain this information from atmospheric measurements of CO2 as air depleted of CO2 by GPP mixes very near the surface with air enriched in CO2 by respiration. Gradients in CO2, thus reflect the net sum of these two processes. In contrast, there is essentially no source of OCS in terrestrial ecosystems. The primary source of OCS is the oceans, thus mixing between this source and the photosynthetic sink occurs in the more remote atmosphere. This has led several researchers to postulate that gradients or time dependent changes in the concentration of OCS in air masses in contact with continental surfaces might be used to infer the rate of GPP of these surfaces. Several recent studies support this inference. However, it is important to note that these observations occur in a global context. The magnitude, spatial distribution and temporal dynamics of OCS sources and sinks are still poorly defined. This presentation will review recent efforts to improve our representation of the global cycle of OCS. We summarize new physiological measurements of exchange of OCS and CO2 by leaves including a wide range of species and environmental conditions, and report a new parameterization for OCS uptake for SiB3. We use these new simulations of surface exchanges in an atmospheric transport model, PCTM to simulate the concentration of OCS at points and times corresponding to flask measurements. While there remain important uncertainties about soil uptake and the ocean source, these simulations agree fairly well with observations.