Climate-relevant modification of the aerosol size distribution by processes associated with orographic clouds

Cloud processes can modify optical properties of aerosol particles. As aerosols have been recognized to play an important role in the earthʹs radiation budget, the understanding of climate changes is also linked to cloud processes. During a field experiment at Great Dun Fell, Northern England in 1995, experimental evidence was found for the processing of aerosol by orographic clouds. During two periods (March 31, and April 3, 1995), a shift in the particle size distribution was detected when comparing data from upwind and downwind stations. For two periods of 1 and 3 h, when the shift was pronounced, the mass increase due to cloud processing was estimated to be 1.2 μg m−3 (±20%) and 1.7 μg m−3 (±50%). This equals roughly a quarter of the pre-existing submicron aerosol mass. As a major source of particulate matter, liquid-phase oxidation of sulphur dioxide was identified. Modelling results from Bradbury et al. [Bradbury, C., Bower, K.N., Choularton, T.W., Swietlicki, E., Birmili, W., Wiedensohler, A., Yuskiewicz, B., Berner, A., Dusek, U., Dore, C.-J., McFayden, G.G., 1999. Modelling of aerosol modification resulting from passage through a hill cap cloud.] strongly support such a reaction mechanism. Furthermore, large amounts of ultrafine particles were observed downwind the mountain ridge with almost none present at the upwind and summit sites. Some observations suggest that HCl may have degassed from the droplets when the cloud was evaporating. Another factor increasing the nucleation probability could have been the aerosol surface area which decreased by 35% prior to the occurrence of ultrafine particles.