Variations in the abundance of the atmospheric greenhouse gases have the potential for changing the global climate. Variations in another group of species, namely the atmospheric aerosols, can also affect climate. Aerosols are solid or liquid particles dispersed in the air (Kemp, 1994), and include dust, soot, sea salt crystals, spores, bacteria, viruses and a plethora of other microscopic particles. Collectively, they are often regarded as air pollution, but many of the aerosols have a natural origin (Jonas et al., 1995). Although atmospheric turbidity (the abundance of aerosols) varies over short time scales, for example after a volcanic eruption (Sear et al., 1987; section 2.6.3), over the long term it maintains a fair degree of equilibrium, owing to the natural cleansing mechanisms of the Earth's climate system. Cleansing is never complete, however, and there always remains a background level of atmospheric aerosols which reflects the dynamic processes involved with aerosol input and aerosol removal. Natural sources of aerosols are probably 4 to 5 times larger than anthropogenic ones on a global scale (Barry & Chorley, 1992), but regional variations of anthropogenic emissions may change this ratio significantly in certain areas, particularly in the industrialised Northern Hemisphere.