NIGHTTIME FORMATION AND OCCURRENCE OF NEW PARTICLES ASSOCIATED WITH OROGRAPHIC CLOUDS

The formation and occurrence of new ultrafine aerosol particles were s tudied in association with an orographic cloud during a field experime nt at Great Dun Fell (GDF), Northern England. Three size spectrometers to measure submicrometer aerosol particles were located upwind, on to p, and downwind of GDF Summit to investigate changes in the aerosol si ze distribution. During two nighttime cloud periods, ultrafine particl es were observed downwind of the hill while no particles were detected upwind of the hill. During one cloud event, there was some evidence o f entrainment. In this case, the occurrence of ultrafine particles may have been due to entrainment from aloft or by homogenous nucleation d ownwind of the hill. During the other cloud event, the formation of an ultrafine particle mode (nucleation mode) occurred probably after the cloud passage. There was no evidence of entrainment during this time period. Multicomponent homogeneous nucleation models were used to simu late the formation of new particle downwind of an orographic cloud. Po ssible homogeneous nucleation processes for this could be the formatio n of sulphuric acid or ammonium chloride due to outgassing of hydrochl oric acid. It was not possible, however, to simulate formation rates o f new particles as observed downwind the hill using a model for the bi nary or ternary homogeneous nucleation process of ammonia and hydrochl oric acid. During the first event with high sulphur dioxide concentrat ions, the formation of new particle via binary homogeneous nucleation of sulphuric acid and water could be only predicted using a high night time hydroxyl radical concentration. No formation of sulphuric acid pa rticle could be simulated during the second event with low sulphur dio xide concentrations. (C) 1997 Elsevier Science Ltd.