Aerosols and clouds are important components of the Earth's atmosphere, influencing the radiation budget and chemical composition, and affecting human health. The impact of aerosols and clouds on global climate remains one of the largest single uncertainties in understanding previous climate observations and in predicting future climate change. Aerosols and clouds can scatter and absorb sunlight and terrestrial radiation, having a direct effect on climate by altering the balance of incoming solar radiation and outgoing infrared light. Aerosols also have an indirect effect on climate by influencing the albedo and lifetime of clouds, because cloud droplets form from the much smaller aerosol particle seeds on which water can condense. Changes in the number of aerosol particles in the Earth's atmosphere and their size distribution can lead to changes in the number of cloud droplets that form. This indirect effect is poorly constrained and generally counteracts the warming induced by increased levels of greenhouse gases in the atmosphere, exerting a cooling effect on the Earth's climate. The project "Reducing the Uncertainties in Aerosol Hygroscopic Growth", to which this project is linked, seeks to quantify the microphysical properties and processes that control the formation of cloud droplets from aerosol particles in a series of laboratory measurements on single, suspended, aerosol particles using state of the art techniques. These properties can then be used, in a much simplified form, in the computer models used to simulate atmospheric air quality and climate. One of these simplified methods is the "kappa-Köhler theory" created by our international partner (in the USA) on this project. Together, we will do the following: First, we will exchange staff between the Bristol Aerosol Research Centre and the laboratory of our international partner at North Carolina State University for a short period (one focus area will be the viscosity of aerosol components). This will enable an exchange of skills: our work is mostly fundamental, and laboratory-based, whereas our international partner participates extensively in field campaigns of atmospheric measurements. These areas of interest, and associated science, are complementary. Second we will work together to provide a database of values of the aerosol parameter kappa, and web-based tools to carry out calculations that are related to the uptake of water by atmospheric aerosols and their role in the formation of clouds. These tools will be publicly accessible on the Extended Aerosol Inorganics Model website. They should provide a focus for international efforts in this area, and help to spread best practice. Third, will hold a Workshop, hosted with our international partner and with invited experts in the measurement and use of kappa and kappa-Köhler theory, to discuss current problems in the field and to recommend where future effort should be directed. One problem area is the kappa values of the organic components of atmospheric aerosols, whose behaviour and composition are both very complex, making it difficult to relate parameter kappa to composition in a direct or reliable way. The participants will also review the website tools and database, and make recommendations for future development. In addition to the scientific benefits, UK participation and leadership in international atmospheric aerosol research will be advanced by the partnership and links created in this project.