Improved estimation of global vegetation uptake of carbonyl sulfide (COS) in a biosphere model (SiB4)

The atmospheric trace gas ‘Carbonyl Sulfide (COS)’ has been identified as a possible tracer to estimate global Gross Primary Production (GPP). This is because COS is taken up similarly to CO2 uptake through plant stomata and has almost no respiration. In addition to vegetation activity, COS is removed by soil and chemical reaction in the atmosphere. It enters the atmosphere by emission from oceans and anthropogenic sources (e.g., rayon production). Plants play an important role in the seasonal COS drawdown, but the models that simulate COS biosphere exchange have considerable uncertainty due to lack of observation. The uncertainty is mainly related to the poorly defined ambient COS mixing ratio and enzyme activity, processes that control COS uptake. The ambient COS molecules diffuse into stomatal pores and mesophyll cells, where the molecules are hydrolyzed by the enzyme Carbonic Anhydrase (CA). Due to the lack of understanding of CA activity, previous studies scaled the COS mesophyll uptake with the maximum Rubisco deposition velocity (Vmax), relevant for photosynthesis, and only used the associated temperature response.

This study will improve the estimation of COS vegetation uptake in the Simple Biosphere model version 4 (SiB4), corresponding to different COS mixing ratios from an atmospheric inversion and various environmental conditions. COS flux observations from Europe and North America will be applied. We aim to identify and present: (i) the response of the COS uptake to varying COS mixing ratios and environmental conditions (temperature, humidity, and light), (ii) application of the relationships derived by (i) to the estimation of global COS uptake by the SiB4 model, (iii) investigate the implications for SiB4 calculation of CO2 photosynthesis by the SiB4 model. Unlike the original SiB4 model, where CA uptake is proportional to Vmax and the temperature response identical for all plant types and environments, we will show the nonlinear CA responses to COS mixing ratio, temperature, humidity and light.