An aerosol is a collection of particles suspended in a gas. The term refers collectively to both the particles and the gas in which the particles are suspended. The particle size ranges from 0.002 µm to more than 100 µm, that is from a gathering of a few molecules to the size where the particles no longer can be carried by the gas.

Atmospheric aerosols

Some aerosols occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. Averaged over the globe, anthropogenic aerosols—those made by human activities—currently account for about 10 percent of the total amount of aerosols in our atmosphere.

Radiative forcing from aerosols

Aerosols, natural and anthropogenic, can affect the climate by changing the way radiation is transmitted through the atmosphere. Direct observations of the effects of aerosols are quite limited so any attempt to estimate their global effect necessarily involves the use of computer models. The Intergovernmental Panel on Climate Change, IPCC, says: While the radiative forcing due to greenhouse gases may be determined to a reasonably high degree of accuracy... the uncertainties relating to aerosol radiative forcings remain large, and rely to a large extent on the estimates from global modelling studies that are difficult to verify at the present time [1].

A graphic showing the contributions (at 2000, relative to pre-industrial) and uncertainties of various forcing is available here.

Sulphate aerosol

Sulphate aerosol has two main effects, direct and indirect. The direct effect, via albedo, is to cool the planet: the IPCC's best estimate of the radiative forcing is -0.4 Watts/m² with a range of -0.2 to -0.8 W/m² [2] but there are substantial uncertainties. The effect varies strongly geographically, with most cooling believed to be at and downwind of major industrial centres. Modern climate models attempting to deal with the attribution of recent climate change need to include sulphate forcing, which appears to account (at least partly) for the slight drop in global temperature in the middle of the 20th century. The indirect effect (via the aerosol acting as cloud condensation nuclei, CCN, and thereby modifying the cloud properties) is more uncertain but is believed to be a cooling.

Black carbon

Black carbon, soot, from fossil fuels is estimated by the IPCC in the Third Assessment Report of the IPCC, TAR, to contribute a global mean radiative forcing of +0.2 W/m² (was +0.1 W/m² in the Second Assessment Report of the IPCC, SAR), with a range +0.1 to +0.4 W/m².

Aerosol spray

Colloquially, an aerosol (short for aerosol spray) is a canister holding a liquid under pressure from a compressed gas. When a valve is opened, the liquid is forced out of a small hole and emerges as a mist. Typical liquids dispensed in this way are deodorants and paints. An atomiser is a similar device that is pressurised by a hand-operated pump rather than by stored gas.

References

• Article at earthobservatory.nasa.gov describing the possible influence of aerosols on the climate

See also

• Air pollution
• Biological warfare
• Freon
• Global warming
• Propellants
• Radiological weapon

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