Abstract. COS uptake by trees, as observed under dark/light changes and under application of the plant hormone abscisic acid, exhibited a strong correlation with the CO 2 assimilation rate and the stomatal conductance. As the uptake of COS occurred exclusively through the stomata we compared experimentally derived and re-evaluated deposition velocities (V d ; related to stomatal conductance) for COS and CO 2 . We show that V d of COS is generally significantly larger than that of CO 2 . We therefore introduced this attribute into a new global estimate of COS fluxes into vegetation. The new global estimate of the COS uptake based on available net primary productivity data (NPP) ranges between 0.69-1.40 Tg a −1 . However, as a COS molecule is irreversibly split in contrast to CO 2 which is released again by respiration processes, we took into account the Gross Primary Productivity (GPP) representing the true CO 2 leaf flux the COS uptake has to be related to. Such a GPP based deposition estimate ranged between 1.4-2.8 Tg a −1 (0.73-1.50 Tg S a −1 ). We believe that in order to obtain accurate global COS sink estimates such a GPP-based estimate corrected by the different deposition velocities of COS and CO 2 must be taken into account.

Abstract. Carbonyl sulfide (COS) is an atmospheric trace gas that holds great promise for studies of terrestrial carbon and water exchange. In leaves, COS follows the same pathway as CO 2 during photosynthesis. Both gases are taken up in enzyme reactions, making COS and CO 2 uptake closely coupled at the leaf scale. The biological background of leaf COS uptake is a hydrolysis reaction catalyzed by the enzyme carbonic anhydrase. Based on this, we derive and test a simple kinetic model of leaf COS uptake, and relate COS to CO 2 and water fluxes at the leaf scale. The equation was found to predict realistic leaf COS fluxes compared to observations from field and laboratory chambers. We confirm that COS uptake at the leaf level is directly linked to stomatal conductance. As a consequence, the ratio of normalized uptake rates (uptake rates divided by ambient mole fraction) for leaf COS and CO 2 fluxes can provide an estimate of C i /C a , the ratio of intercellular to atmospheric CO 2 , an important plant gas exchange parameter that cannot be measured directly. The majority of published normalized COS to CO 2 uptake ratios for leaf studies on a variety of species fall in the range of 1.5 to 4, corresponding to C i /C a ratios of 0.5 to 0.8. In addition, we utilize the coupling of C i /C a and photosynthetic 13 C discrimination to derive an estimate of 2.8±0.3 for the global mean normalized uptake ratio. This corresponds to a global vegetation sink of COS in the order of 900±100 Gg S yr −1 . COS can now be implemented in the same model framework as CO 2 and water vapour. Atmospheric COS measurements can then provide independent constraints on CO 2 and water cycles at ecosystem, regional and global scales.