Validation and development of carbonyl sulfide biosphere exchange in the Simple Biosphere Model (SiB4)

Abstract

<p>The uptake of carbonyl sulfide (COS) in plants is strongly dependent on stomatal conductance. The COS uptake is therefore strongly related to the photosynthetic uptake of CO₂ in plants. As there is a gap in the COS budget with a source missing (or an overestimated sink) in tropical regions, this asks for evaluation of all sources and sinks of COS to be able to apply COS as a photosynthetic tracer. The COS uptake by vegetation and soil is simulated by the Simple Biosphere Model (SiB4) but it has not been validated against ecosystem and vegetation fluxes across different biomes. We evaluated the SiB4 COS biosphere flux with observations and updated it with the latest insights, with the aim to get the best possible estimate of the global COS biosphere sink. Overall, we find good agreement of simulated diurnal and seasonal cycles of COS ecosystem fluxes with flux observations made over grasslands, evergreen needleleaf forest and deciduous broadleaf forests over Europe and Northern America. We improved the simulations of COS soil exchange with the implementation of the Ogee et al. (2016) soil model such that SiB4 is now capable of simulating COS emissions from soils. We found that accounting for varying COS mixing ratios (retrieved from an inversion by the TM5-4DVAR model) plays a large role in determining the global COS biosphere sink. With these modifications to the model, we find an average underestimation of the COS biosphere flux of 11 % compared to observations. Furthermore, our model modifications caused a drop in the global COS biosphere sink from 967 Gg S yr-1 in the original model to 788 Gg S yr-1 in the updated version. The largest drop in fluxes is over the tropical regions, mostly driven by lower COS mixing ratios and contributes towards closing the gap in the COS budget. However, given the underestimation of COS uptake in the boreal and temperature regions, it is unlikely that the remaining gap in the COS budget is caused by an overestimated tropical biosphere sink.&#160; &#160;</p><p>&#160;</p>