Carbonyl sulfide and the carbon budget
Understanding the processes that control the terrestrial exchange of carbon is critical for assessing atmospheric CO2 budgets. Carbonyl sulfide (COS) is taken up by vegetation during photosynthesis following a pathway that mirrors CO2 but has a small or nonexistent emission component, providing a possible tracer for gross primary production. Continuous high-resolution joint measurements of COS and CO2 concentrations in the boundary layer can be used alongside flux measurements to partition the influence that leaf and soil fluxes and entrainment of air from above have on the surface carbon budget.
We made field measurements of COS and CO2 mixing ratios in forest, senescent grassland, and riparian ecosystems using a laser absorption spectrometer installed in a mobile trailer. These data show 1) the existence of a narrow daytime uptake ratio of COS to CO2 across vascular plant species of 1.7, providing critical information for the application of COS to estimate photosynthetic CO2 fluxes and 2) a temperature-dependent uptake ratio of COS to CO2 from soils. Significant nighttime uptake of COS was observed in broad-leafed species and revealed active stomatal opening prior to sunrise. The results provide a number of critical constraints on the processes that control surface COS exchange, which can be used to diagnose the robustness of global models that are beginning to use COS to constrain terrestrial carbon exchange. This work was supported by a CIRES Innovative Research Program award (see page 64), and two prior CIRES Visiting Fellows collaborated in the research.