The Earth's atmosphere is naturally composed of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.03% carbon dioxide, and 0-4% water vapor. In addition to these gases, smaller amounts of as many as forty trace gases-including ozone, helium, hydrogen, nitrogen oxides, sulfur dioxide, and neon-are also normally present. On any given day, the presence of small quantities of trace gases is harmless. It is when the balance is tipped and there is an unusually high concentration of these trace gases, that the air that we breathe becomes poisonous.

Emerging primarily as a result of the Industrial Revolution, industrial smog is perhaps often portrayed as huge gray smokestacks coming out chimneys of factories. Apparently, this is a logically depiction given that the main source of industrial smog was from the burning of coal for heat and energy (Nebel and Wright 527). As a result, the main pollutant of industrial smog is sulfur dioxide-a compound largely responsible for acid precipitation.

Photochemical smog, one of today's primary smog related concerns, arises mainly from the combustion process by motor vehicles, as well as the increased use of fossil fuels for heating, industry, and transportation (ibid.). These activities, along with slashing-and-burning of trees and agricultural organic wastes, led to large emissions of two major primary pollutants, volatile organic compounds (VOCs) and nitrogen oxides (Gow and Pidwirny). Interacting with sunlight, primary pollutants form various hazardous chemicals known as secondary pollutants-namely peroxyacetyl nitrates (PAN) and ground-level (tropospheric) ozone (ibid.). Present in small amounts, these secondary pollutants pose no threat to living things. Unfortunately, the starting threshold level for ground level ozone is determined to be approximately 82 parts per billion ("Smog"). Considering this with the fact that ground level ozone constitutes 90% of all smog found in urban areas, it could be seen where the heart of the concern regarding photochemical smog lies (ibid.).

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Generally, communities situated in valleys with surrounding hills and mountains tend to see less air circulation and more accumulation of air pollutants than their counterparts who are situated on an open range or near the ocean. In another case, wind patterns can affect the frequency of the replacement of local air with fresh air. Yet, almost all serious smog pollution cases are associated with temperature inversion (Davies 35).

During the day, the sun heats up the surface of the Earth such that warmer air usually resides closer to the surface of the Earth, with temperatures decreasing with increasing altitude. At night, the situation is reversed with cooler air near the surface and warmer resting on top-resulting in what is known as temperature inversion. Because basic physics dictates that warmer air rises and cooler air falls, air pollutants are simply carried up by the warmer air into the atmosphere where they are dispersed and diluted. Thus, the lower atmosphere is cleansed of any pollutants that accumulate overnight.

In a situation where temperature inversion exists during the day, the air near the surface of the Earth is not hot enough as to allow the warm air to rise into the atmosphere. In such a case, air pollutants accumulate near the surface of the Earth, reaching dangerous threshold levels. This is often easily linked to the problem of winter smog, in which the cold anti-cyclonic weather leads to an increased usage of heat and energy in homes and offices (Elsom 10). Consequently, increased amounts of air pollutants are released into the atmosphere in response to the generation of heat and energy. Because temperature inversion is present, it effectively acts as a thermal lid, thus preventing the air pollutants from rising and dispersing into harmless levels in the atmosphere. As a result, a smog siege can occur.

It is important to note that temperature inversion can also play an important role on warmer days as well. As noted above, sunlight reacts with nitrate oxides and VOCs to form ground-level ozone. Given stable weather conditions with low winds, a temperature inversion-in the form of a highly-pressurized air settling upon the cooler air beneath it-can lead to a buildup in the concentration of ground-level ozone. Unsurprisingly, urban areas, with their traffic-congested streets, are more likely to suffer from an air pollution disaster than their neighboring suburban and rural areas (ibid. 9).

In whichever case, the point of the matter is that while meteorological factors and topographical factors can increase the occurrence of smog sieges, they are not the root cause of air pollution disasters; if emissions were low enough as to not exceed threshold levels in the first place, a smog siege would not have occurred regardless of conditions.

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The United States Environmental Protection Agency (EPA) defines an air pollutant as any substance in the atmosphere that can "injure health, harm the environment and cause property damage" (EPA, "Air Quality"). What is important to realize, however, is that air pollutants can be either anthropogenic or natural.

For eons, natural phenomenon such as volcanic eruptions, fires, and dust storms have released smoke, gases, and particles into our atmosphere. However, natural processes and mechanisms have in most cases been able to convert these air pollutants into less toxic forms. Take the case of hydroxyl radicals (OH)-which are naturally occurring cleansers of the atmosphere. Through a chemical process, hydroxyl radicals react with pollutants in the atmosphere to oxidize them either into a less harmful substance or into a form, which can be precipitated back down onto Earth (Nebel and Wright 526). Once back down on Earth, decomposition by microorganisms can further continue the conversion of the substances into more neutral forms (ibid.). Without human disturbances, this process of addition and subtraction of air pollutants into the atmosphere just about balances such that pollutants are below threshold levels-the level of pollutants an organism can be exposed to without showing any ill effects (ibid. 527).

As can be predicted, hydroxyl radicals have played an important role in the removal of anthropogenic pollutants in the past. However, because the natural removal of pollutants can takes hours, days, and even months, and because human activities have led to increasing rates of emissions into the air, the amount of air pollutants in the atmosphere has accumulated to a detrimental level. Partly for this reason, and partly for other reasons-such as fear over the long-run effects of human disturbances on natural systems-concerns today over air pollutants have mainly rested upon those that are anthropogenic.

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