Modelling of aerosol modification resulting from passage through a hill cap cloud

Data collected at the Great Dun Fell site are used to provide input and com parative output for a computer model of the hill cap cloud system. The aim of the report is to investigate the effect of in-cloud chemical processing on aerosols and trace gases, focusing on the production of S(VI) via the ox idation of SO2 by H2O2. Both airflow, cloud chemistry and cloud microphysic s are modelled and compared to upwind, downwind, and summit observations ma de during the Great Dun Fell field campaign in 1995, The results indicate t hat there exists a broader droplet size distribution than predicted due to the mixing of separate parcels of air with different trajectories and humid ities. Modification of the aerosol size distribution is predominantly due t o sulphate production at the expense of sulphur dioxide gas oxidised. by hy drogen peroxide. Predicted nucleation scavenging resulted in the loss of th e more hygroscopic particles of diameters 0.05-0.13 mu m, which by the addi tion of soluble mass grew to between 0.13 and 0.3 mu m in diameter. The les s hygroscopic mode comprised approximately 2% of the total mass input and t hus did not significantly contribute to the modified sections of the aeroso l spectrum. The modified particles were of a size suitable for nucleation s cavenging, increasing the number of CCN available for future droplet activa tion. The hygroscopic properties of the modified particles were also affect ed by the addition of soluble mass, such that they would require a lower cr itical supersaturation for activation (Swietlicki et al., 1999). The level of aerosol augmentation is dependent upon the activation history of the clo ud droplets, the concentrations of interstitial gas species, and the partit ioning of the aerosol ion species. (C) 1999 Elsevier Science B.V. All right s reserved.