Technical note: Novel estimates of the leaf relative uptake rate of carbonyl sulfide from optimality theory

In order to estimate the gross primary productivity (GPP) of terrestrial ecosystems from the canopy uptake of carbonyl sulfide (COS), leaf relative uptake rate (LRU) of COS with respect to carbon dioxide needs to be known a priori. Currently, the variability of LRU between plant species in different biomes of the world is poorly understood, making the choice of an appropriate LRU uncertain and hampering further progress towards developing COS as a tracer of GPP. Here we propose a novel approach for estimating light-saturated LRU based on plant optimality principles, validate it against in situ leaf gas exchange measurements and provide global monthly climatological estimates. The global vegetation season average simulated LRUs fall into the 95 % range of 0.68–1.58 and are thus lower than most other published global estimates. We advocate for these LRU estimates to be adopted by global modellers in order to test to what degree these are compatible with our current understanding of the sources and sinks in the global COS budget.