Quantifying the Effects of Aerosols on Climate, and Their Behaviour in the Atmosphere

Summary for General Audience Atmospheric aerosols - small particles suspended in the air - include seasalt (from breaking waves), wind-blown dust, and sulphates and organic compounds from both natural and anthropogenic sources. Aerosols can affect the earth's budget of solar radiation, first, by scattering and absorbing incoming solar and outgoing terrestrial radiation (the direct effect). Second, aerosols can affect the optical properties of clouds - which both scatter solar radiation back to space, and absorb outgoing long wave radiation - by changing the numbers of water droplets present (the indirect effect). The total magnitude of the aerosol effect on the surface temperature of the earth is similar to that of the greenhouse gases (mainly CO2), but is believed to be a net cooling rather than warming. However, the exact size of the effect is very uncertain. Aerosol particles also affect human health and provide surfaces for chemical reactions to take place on, many of which are important in determining the concentration of surface ozone which is harmful. The U.K. Meteorological Office carries out climate research and is the joint leader of a programme to build and evaluate a new global computer model suitable for a range of topics in climate and environmental change research. One of the key tasks is to improve the representation of atmospheric aerosols and their effects in the model. Important aerosol properties include the uptake of water by the aerosol particles in response to atmospheric relative humidity and temperature changes, the size of the aerosol particles, and the partitioning between the aerosols and surrounding gas phase of volatile compounds such as ammonia, nitric acid, and some organic compounds. Improving the treatment of aerosols in the computer model is an essential step to representing both the direct and indirect aerosol effects more accurately, and to making improved climate change predictions, and formulating effective public policy for adaptation and remediation. This project assists the Met Office in this task. The formation, properties, and behaviour of atmospheric aerosols are currently the subject of active investigation in the laboratory, by field measurement campaigns, and by modellers around the world. The Met Office does not have expertise in the detailed thermodynamics of gas/aerosol which can be used to develop and test new aerosol schemes that can be incorporated into their climate model. The primary purpose of this knowledge transfer project is to address this gap by providing web-based tools incorporating state-of-the-art aerosol models developed by us over a period of more than 5 years. These embody the results of internationally recognized research into the behaviour of the components of tropospheric and stratospheric aerosol systems. They will enable the Met Office to evaluate and test representation of aerosol properties in their global climate and other models, guide their future development, and ultimately reduce the uncertainties associated with the aerosol influence on climate change. These web-based tools can be accessed freely, and our past experience demonstrates that they are likely to be used in aerosol science research and teaching worldwide.