Industrial and Photichemical Smog
Industrial Smog. Various groups of air pollutants found in the air over cities can be classified as either industrial smog or photochemical smog. Although both types of smog are found to some degree in most urban areas, one type often predominates during at least part of the year as a result of differences in climate and major sources of air pollution.
Industrial smog consists mostly of a mixture of sulfur dioxide and SPM, including a variety of solid particles and droplets of sulfuric acid formed from some of the sulfur dioxide. These substances form a grayish haze, explaining why cities where this type of smog predominates are sometimes called gray-air cities. This type of air pollution tends to predominate during the winter (especially in the early morning) in older, heavily industrialized cities like London, Chicago, Philadelphia, St. Louis, and Pittsburgh, which typically have cold, wet winters and depend heavily on coal and oil for heating, manufacturing, and producing electric power.
Photochemical Smog: Cars + Sunlight = Tears. A combination of primary pollutants such as carbon monoxide, nitric oxide, and hydrocarbons and secondary pollutants such as nitrogen dioxide, nitric acid, ozone, hydrogen peroxide, peroxacyl nitrates (PANs), and formaldehyde, produced when some of the primary pollutants interact under the influence of sunlight, is called photochemical smog. Cities in which photochemical smog predominates usually have sunny, warm, dry climates. They are generally newer cities with few polluting industries and large numbers of motor vehicles, which are the major source of air pollution. Examples include Los Angeles, Denver, Salt Lake City, the USA, as well as Sydney, Australia; Mexico City, Mexico; and Buenos Aires, Argentina. The worst episodes from this type of smog tend to occur in summer months between noon and 4 p.m.
The first step in the formation of photochemical smog occurs during the early morning traffic rush hours, when NO from automobiles builds up and reacts with O2 to produce NO2, A yellowish-brown gas with a pungent, choking odor. This gas produces a characteristic brownish haze, explaining why cities such as Los Angeles, where photochemical smog predominates, are sometimes called brown-air cities. Then, as the sun rises, its ultraviolet rays cause a series of complex chemical reactions that produce the other components of this type of smog. The mere traces of ozone, PANs, and aldehydes that build up to their peak levels around noon and in the early afternoon on a sunny day can irritate people's eyes and respiratory tracts. During the summer months most industrial smog cities also experience photochemical smog.
Local Climate, Topography and Smog. The frequency and severity of industrial and photochemical smog in an urban area depend on local climate and topography, density of population and industry, and major fuels used in industry and for heating and transportation. In areas with high average annual precipitation, rain and snow help cleanse the air of pollutants. Winds also help sweep pollutants away and bring in fresh air. However, hills and mountains tend to reduce the flow of air in valleys below and allow pollutant levels to build up at ground level. Buildings in cities also slow wind speed and impede dilution and removal of pollutants.
During the day the sun warms the air near the earth's surface. Normally, this heated air expands and rises during the day, diluting low-lying pollutants and carrying them higher into the troposphere. Air from surrounding high-pressure areas then moves down into the low-pressure area created when the hot air rises. This continual mixing of the air helps keep pollutants from reaching dangerous levels in the air near the ground.
But sometimes a layer of dense, cool air is trapped beneath a layer of less dense, warm air in an urban basin or valley. This is called a temperature or thermal inversion. In effect, a warm-air lid covers the region and prevents pollutants from escaping in upward-flowing air currents. Usually these inversions last for only a few hours, but sometimes they last for several days when a high-pressure air mass stalls over an area. When this happens, air pollutants at ground level accumulate to harmful and even lethal levels. Most air pollution disasters-such as those in London and in Donora, Pennsylvania, occurred during lengthy thermal inversions during fall or winter in industrial smog areas.
Thermal inversions occur more often and last longer over towns or cities located in valleys surrounded by mountains, on the leeward sides of mountain ranges, and near coasts. A city with several million people and automobiles in an area with a sunny climate, light winds, mountains on three sides, and the ocean on the other possesses the ideal conditions for photochemical smog worsened by frequent thermal inversions. This describes the Los Angeles basin, which experiences almost daily inversions, many of which are prolonged during the summer months.
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