Atmospheric aerosols are very fine particles suspended in air. They are formed by the dispersal of material at the Earth's surface (primary aerosols), or by reaction of gases in the atmosphere (secondary aerosols). They include sulphates and nitrates from the oxidation respectively of sulphur dioxide and nitric oxide during the burning of fossil fuels, organic materials from the oxidation of volatile organic compounds (VOCs), soot from fires, and mineral dust from wind-blown processes. Natural aerosols, which also include sea salt and volcanic dust, are probably 4 to 5 times larger than man-made ones on a global scale, but regional variations in man-made pollution may change this ratio significantly in certain areas, particularly in the industrialised Northern Hemisphere. Although making up only 1 part in a billion of the mass of the atmosphere, they have the potential to significantly influence the amount of sunlight reaching the Earths surface, and therefore climate.
Removal of most aerosols is mainly achieved by rainfall (wet deposition) and by direct uptake at the surface (dry deposition). Explosive volcanic eruptions however, can inject large quantities of dust and gaseous material, such as sulphur dioxide, high into the atmosphere (the stratosphere). Here, sulphur dioxide is rapidly converted into sulphuric acid aerosols. Whereas pollution of the lower atmosphere is removed within days by the effects of rainfall and gravity, stratospheric pollution may remain there for several years, gradually spreading to cover much of the globe.
Like greenhouse gases, aerosols influence the climate. Atmospheric aerosols influence the transfer of energy in the atmosphere in two ways: directly through the scattering of sunlight; and indirectly through modifying the optical properties and lifetimes of clouds. The scattering of sunlight by aerosols is clearly demonstrated in the aftermath of a major volcanic eruption, when exceptionally colourful sunsets may be witnessed. The volcanic pollution results in a substantial reduction in the direct solar beam, largely through scattering by the highly reflective sulphuric acid aerosols. Overall, there is a net reduction of 5 to 10% in energy received at the Earth's surface. An individual eruption may cause a global cooling of up to 0.3oC, with the effects lasting 1 to 2 years.
Estimation of the impact aerosols have on longer-term global climate change however, is more complex and hence more uncertain than that due to the well-mixed greenhouse gases. This is largely because the geographical distribution of aerosols is highly variable and strongly related to their sources. The best estimates of global cooling attributable to man-made aerosols are based on computer models. These show that the global cooling effect of man-made aerosols could offset the warming effect of increased greenhouse gas concentrations by as much as 30%. The variable distribution of aerosols however, makes calculation of a global average difficult. Nevertheless, it is likely that aerosols may slow the rate of projected global warming during the 21st century.