Ambient Air Quality

Ambient air quality refers to the concentration of gases, particles, or other elements in the outdoor atmosphere—from either natural or anthropogenic sources. The term quality is objective and gives an indication of what the concentration of a constituent of air is in relation to certain thresholds or standards. If a concentration is above the threshold, air quality is considered poor or, in some cases, even hazardous to human health. An applicable geographic area is often termed a “nonattainment” area. In these instances, the air is considered “polluted,” and the elevated constituent is a pollutant. Thresholds for different pollutants are determined with reference to human health. Standards are set by the World Health Organization (WHO), though in some countries thresholds have been modified by national or local authorities.

As an example, carbon dioxide (CO2) is a naturally occurring gas in the atmosphere, which in very small concentrations is necessary for life. If its concentration builds over certain thresholds, however, it can become detrimental and so will be termed a hazardous air pollutant. This process was tragically exemplified in 1986 by the Lake Nyos disaster in Cameroon, West Africa, when an upwelling of CO2 from the lakebed had lethal consequences for the local human population and [Page 69]animals. In this case, both the pollutant and the emission source were naturally occurring. Pollutants such as sulfur oxides, nitrogen oxides, and heavy metals, however, can only be caused by human activities.

Air quality issues came to the forefront of scientific and societal attention during the 1950s. In 1952, London's “killer fog” was linked to a large increase in human fatalities (4,000 excess deaths), especially among people with a history of cardio-pulmonary problems. The episode resulted in the implementation of air pollution mitigation measures by the London authorities. The air pollution in question was caused by smoke emissions from the burning of coal and other raw materials into a winter atmosphere already dense with mist and fog from the River Thames. This type of pollution is now termed London-type smog (smoke plus fog). In the same decade, another type of photochemical smog formed by chemical reactions in the atmosphere gained notoriety and became known as Los Angeles smog. Later, during the 1970s, acid rain emerged as a top environmental concern.

Most urbanized regions in the developed world have monitoring stations dedicated to assessing air quality on an hourly basis. Some, like Melbourne, Australia, even provide daily forecasts for ambient air quality, much like a typical weather forecast. This is made possible by the strong link between ambient air quality and weather.

Air quality is influenced by two major factors. The first is the emission of polluting substances into the air. Polluting emissions can be from urban, industrial, agricultural, or rural sources, or they can be totally natural. The second of these factors is the pollution potential of the atmosphere, or its ability to transport, diffuse, chemically transform, and remove pollutants. This involves three important processes: (1) dispersion, which is the horizontal and vertical spread and movement of pollutants; (2) transformation, which involves chemical reactions between pollutants or in pollutants under certain temperature and sunlight conditions; and (3) the removal of pollutants through mechanisms such as dry and wet deposition.

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