Particulates, also known as particulate matter, are fine particles of solid or liquid matter suspended in a gaseous medium. The particulates and the gas medium are collectively referred to as an aerosol. Particulates can be of natural or anthropogenic origin.

Natural sources of particulates include volcanoes, dust storms, and fires, while anthropogenic sources include by-products of industrial processes, such as the burning of fossil fuels.

Particulates are known contributory factors of climate change. Anthropogenic aerosols affect the climate by altering the atmospheric radiation field through scattering and absorption mechanisms. Aerosols can both absorb and scatter radiation (Botterill, 2003).

Whether a given aerosol is classified as radiation-absorbing or -scattering depends on its net effect and can be expressed by a single scattering albedo (SSA) value. Higher values indicate radiation scattering, whereas lower values indicate radiation absorption. For example, sea salt, a natural aerosol, has an SSA value of 1, whereas soot has an SSA value of 0.23, characterizing its immense atmospheric absorbing potential.

The manner and degree to which solar radiation is affected by atmospheric particulates depends on the chemical composition of both the particulates and the gaseous medium in which they are suspended.

Sulfate aerosols, for example, have a cooling effect due to their tendency to scatter light (Axelrod, 2004). It appears that the slight decrease in global temperature observed in the middle of the twentieth century can be attributed to sulfate aerosols. By contrast, soot, also known as black carbon or carbonaceous aerosol, has absorbing properties that contribute to a warming effect. According to the Intergovernmental Panel on Climate Change (IPCC), black carbon contributes a global mean radiative forcing (the difference between the incoming and outgoing radiation energy in a given climate system) of +0.2 watts per square meter.

Globally, approximately 10 percent of aerosols are generated as a result of human activity. The health effects associated with particulates and aerosols are well documented. The inhalation of particulates and aerosols has been positively correlated to the development of asthma, lung cancer, and cardiovascular disease. The size of a particle is the key determinant as to which area of the respiratory system it will first contact and primarily affect.

Large particles tend to become trapped in the nose and throat and are generally eliminated before harm is caused. However, smaller particles (around 10 micrometers) settle lower in the airway in the bronchi and lungs and can result in adverse health conditions. Particles smaller than 2.5 micrometers, can penetrate the gas-exchanging tissues of the lung, and those smaller than 100 nanometers can enter cardiovascular organs such as arteries, causing vascular disease.

References

1. Axelrod, Regina S., David Leonard Downie, and Norman J. Vig, eds. The Global Environment: Institutions, Law, and Policy. 2d ed. Washington, D.C.: CQ Press, 2004.

2. Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: CSIRO, 2003.

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