Acid deposition in rain over hills

A numerical model of the feeder-seeder mechanism of orographic rain (Carruthers and Choularton, 1983) is used to describe the development of acid rain due to collection by initially 'clean' raindrops, falling from mid-level clouds, of cloud drops formed in low-level polluted air. The process is most efficient over hills where considerable low-level enhancement by the feeder-seeder mechanism occurs. We describe the distribution of this enhancement over hills of various sizes, the associated acidity and the deposition rate of H+ ions. The largest acid deposition rates may be very localized and more than an order of magnitude greater than other modes of deposition such as the turbulent diffusion of cloud droplets to ground.