The uptake of atmospheric carbonyl sulfide (COS) by the lichen species Ramalina menziesii, representative for the open oak woodland in central California, was studied under laboratory conditions. By use of a dynamic cuvette system, the controlling parameters for the COS uptake were investigated under climate chamber conditions. The thallus water content, essential for the overall physiology of lichens, was found to be of basic importance for the trace gas exchange. A water content of 30% was the approximate minimum for COS uptake, with increasing activity up to a water content of 200%. Additionally, actual atmospheric mixing ratios have a significant influence on the exchange. The COS uptake was found to be a linear function of the ambient COS mixing ratio resulting in a compensation point as low as 37 ppt. A temperature optimum of 25 ¿C was indicative of a physiological basis of the COS uptake. The inhibition of the COS consumption in the presence of a specific inhibitor for the enzyme carbonic anhydrase proved this enzyme to be of key relevance for the uptake. All these variables controlling the COS deposition were integrated into an uptake algorithm to model the exchange behavior of this lichen. The applicability of the model to field data is demonstrated. Uptake rates on a dry weight basis normalized to optimized conditions (25 ¿C; 450 ppt COS) reached 0.17¿0.09 pmol g-1 s-1 (i.e. 4.2¿2.2 pmol m-2 s-1 thallus surface area, respectively). The contribution of lichens to the global COS sink strength is assigned to be about 0.3 Tg a-1, representing not a major but a significant sink. ¿ 2000 American Geophysical Union