A ir P ollution COS Carbonyl sulphide. Several units are used. Two points should be noted. Firstly these deposition rates refer to both wet and dry deposition, including dry deposition of acid precursor gases. Deposition rates together with critical loads, have become the critical parameters in Europe in determining whether an ecosystem is under threat of acidification.
Units used are as for 'critical loads'.
Wet deposition it usually calculated from rainfall data and chemical analyses of rainfall ion composition. Estimates of both parameters are subject to numerous uncertainties e.g. canopy effects on throughfall composition, in the case of wet deposition, and variations in deposition velocities for different surface types, in the case of dry deposition. In practice where nightime 'decoupling' takes place the yearly average would be half this. Dry deposition Deposition of acids or acid precursors from the atmosphere onto plant foliage and other solid surfaces by adsorption and direct uptake in the absence of liquid water. The rate at which this occurs depends on the 'deposition velocity'.