Modelling Sulphate Deposition Onto Hills By Washout and Turbulence

Abstract

AbstractIt has recently been shown from model predictions and measurements at Great Dun Fell, that the variation of sulphate deposition with altitude associated with rainfall is much larger than had previously been thought. In this paper the model of deposition by the seeder‐feeder mechanism is extended to cover a wider range of atmospheric conditions and hill sizes and the results are compared with a model of turbulent deposition. It is shown that turbulent deposition rates are a factor of about 5 to 10 less than by the seeder‐feeder mechanism. the patterns of both types of deposition are strongly affected by atmospheric stability and wind speed. the model of deposition by washout of a hill cap cloud by rain predicts that for a steep hill, characteristic length 2 km, the maximum rainfall and sulphate deposition rates are downstream of the hill summit. In this case, the raindrop trajectory through the highest liquid water content region is short, but the condensation rate is large due to strong updraughts. Hence the greatest effect is the wind‐drift of the raindrops and consequently the rainfall and deposition maxima are displaced downstream. On a longer hill, about 10 km in length, cloud water is not so rapidly resupplied due to reduced updraught. the rainfall rate in the region of the summit is greater than for a steep hill because the region of high water content is large in horizontal extent and the effects of wind‐drift are small. As a result, the maximum rainfall and deposition rates occur somewhat upstream of the hill summit, depending on atmospheric stability. the generation of sulphate by the oxidation of sulphur dioxide within cloud can greatly influence the deposition patterns due to both turbulence and washout. If 1 p.p.b.v. of H2O2 is assumed present in the atmosphere prior to cloud formation (a summer concentration) with a higher concentration of SO2, then the sulphate loading within the cloud will rise by several μgm−3 and deposition will increase commensurately. the other main oxidant, ozone, may also contribute significantly to sulphate production. In this case, however, there are no limitations on its availability to inhibit sulphate generation; instead it is found that there is a rapid decrease in the rate of reaction below about pH 4.5. Its effectiveness is therefore strongly dependent on the buffering capacity of the cloud droplet system.