A number of flow-through experiments have been performed at sites around the world, in which a hill-cap cloud in contact with the ground has been used as a "natural flow through reactor" to examine the interaction between clouds, aerosols and gases. The aims have been to examine the effects of the properties of the aerosol and trace gases in the airmass flowing into the cloud on the microphysics and chemistry of the cloud and to investigate the role of the cloud in modifying the aerosol and trace gases emerging downwind. The overall goal has thus been to increase understanding of and reduce uncertainties in the direct and indirect radiative effects of aerosols on the earth's radiation balance, so that the role that clouds and aerosols will play in future global climate change may be better assessed. Results from three such aerosol-cloud interaction experiments carried out at sites varying distances downwind of urban pollution sources will be presented and compared in this paper.

In Spring 1999 a PROCLOUD experiment was carried out at Holme Moss (a hilltop 30km east of the city of Manchester) to investigate the interaction of the Manchester urban plume with the cap cloud which forms at the site (Bower et al., 2001). In summer 1997 a "HILLCLOUD" experiment was carried out on Tenerife during the ACE-2 aerosol characterisation experiment. Tenerife is in a maritime region influenced at times by pollution outflow from continental Europe (1-2 days upwind) and desert dust from the Sahara (Bower et al., 2000). Recent results from the cloud-aerosol interaction experiment carried out on the South Korean Island of Cheju Do during ACE-Asia (Spring 2001) will also be presented. Here, aerosols are comprised of substantial amounts of Asian desert dust, particles generated from the high levels of combustion of coal and biomass, from industrial emissions, and from sea-salt picked up during transit across the ocean.

In all three experiments, multiple measurement sites were employed. Upwind, measurements were made of the aerosol properties and key trace gases. These included measurements of aerosol size distributions (DMPS and OPCs), aerosol composition using a impactors to obtain samples for later analysis of soluble inorganic and organic (in ACE-Asia,) components (by IC and other techniques), and measurements of aerosol hygroscopicity (using HTDMAs). Aerosol mass spectrometers were also deployed in ACE-Asia, providing high time and size resolved measurements of aerosol composition.. In-cloud, measurements of the cloud microphysics (droplet size distribution, liquid water content etc) were made, and cloud water collectors were deployed to obtain samples for later chemical analysis (by IC, and in ACE-Asia, for water-soluble organic species by a combination of chromatographic separation, spectroscopic and total organic carbon analysis). Other species (H2O2(aq)) and pH were measured on-line. Aerosol size distributions interstitial to the cloud were also measured.

Large variations in cloud droplet number are observed within the polluted plumes at the different sites. This and other differences will be explained in terms the measured aerosol properties, and the times the aerosol has spent ageing en-route to the different sites.