Air Pollution

Kategorie: Biológia (celkem: 966 referátů a seminárek)

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  • Přidal/a: anonymous
  • Datum přidání: 07. dubna 2007
  • Zobrazeno: 37103×

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Air Pollution

Air is a mixture of gases, including nitrogen (79%), oxygen (20%), carbon dioxide (0,03%), and several inert gases: argon (almost 1%), helium, xenon, neon, and crypton. Water vapor exists in varying amounts. Air is a finite resource capable of cleansing itself of many, but not all pollutants. There are six major pollutants: carbon monoxide, sulfur oxides, nitrogen oxides, particulates, hydrocarbons, and photochemical oxidants. The major air pollutants come from three principal sources: transportation, stationary sources (factories and power plants), and industrial processes. Air pollutants are released from vaporization (or evaporation), attrition (or friction), and combustion. Combustion is by far the major producer.
The cities can be generally divided into two categories, depending (based) on climate and the type of air pollution. Gray-air cities are generally located in cold, moist climates. The major pollutants are sulfur oxides and particulates. These pollutants combine with atmospheric moisture to form the grayish haze called smog, a term coined in 1905 to describe the mixture of smoke and fog that plagued industrial England. The gray-air cities depend greatly on coal and oil and are usually heavily industrialized. The air in these cities is especially bad during cold, wet winters, when the demand for home heating oil and electricity is heavy and atmospheric moisture content is high.
Brown-air cities are typically located in warm, dry, and sunny climates and are generally newer cities with few polluting industries. The major sources of pollution in these cities are the automobile and the electric power plant, the primary pollutants are carbon monoxide, hydrocarbons, and nitrogen oxides. In brown-air cities atmospheric hydrocarbons and nitrogen oxides from automobiles and power plants react in the presence of sunlight. A number of secondary pollutants such as ozone, formaldehyde, and peroxyacylnitrate (PAN) are formed. The reactions are called photochemical reactions because they involve both sunlight and chemical pollutants. The resulting brownish-orange shroud of air pollution is called photochemical smog. Ozone (O3) is the major photochemical oxidant, a highly reactive chemical, it erodes rubber, irritates the respiratory system, and damages trees.
Because the air laden with photochemical smog often drifts out of the city, the suburbs and surrounding rural areas usually have higher levels of photochemical smog than the city itself.

Major pollution episodes in brown-air cities usually occur during the summer months, when the sun is most intensive.
Air pollution levels are affected by numerous factors. Wind sweeps dirty air out of cities, rain washes pollutants from the sky. But these pollutants do not disappear. They are transferred from one medium to another. Airborne pollutants can travel hundreds, perhaps thousands, of kilometers to other cities or unpolluted wilderness.
Mountains and hills block the flow of winds and trap pollutants for days. Mountains also block the sun, which helps disperse pollutants.
During normal conditions, air temperature decreases with altitude, thus, pollutants ascend and mix with atmospheric gases. Because of this, ground-level pollution is reduced. Atmospheric mixing is brought about by sunlight. Striking the earth, sunlight heats the rock and soil. This heat is transferred to the air immediately above the ground. The warm air then rises, mixing with cooler air.
Under certain atmospheric conditions, mainly on winter days, the air temperature drops to a certain point. After that the temperature would begin to increase. This inverted temperature profile is called a temperature inversion. Temperature inversion creates a warm-air lid over cooler air. Because the cool dense ground air cannot mix vertically, pollutants become trapped near the ground, often reaching dangerous levels.
Breathing a polluted air may result in a number of diseases, including bronchitis, emphysema, and lung cancer. Symptoms of bronchitis include a persistent cough, mucus buildup, and difficult breathing. Cigarette smoking is a major cause of this disease, but urban air pollution is also a contributing factor. Sulfur dioxide, nitrogen dioxide, and ozone are believed to be the major causative agents.
Emphysema kills more people than lung cancer and tuberculosis combined. It causes the breakdown of small air sacs, or alveoli, in the lungs. This reduces the surface area for the exchange of oxygen with the blood. Breathing becomes more and more labored. Victims suffer shortness of breath when exercising even lightly. Ozone, nitrogen dioxide, and sulfur oxides are the chemical agents believed responsible for this disease.
Not all individuals are affected equally by air pollution. Particularly susceptible are the old and infirm, especially people with heart and lung disorders.

The health risk from air pollution is six times greater for children than for adults, because they are more active and therefore breathe more.
Air pollutants may severely damage metals, building materials (stone and concrete), paint, textiles, plastics, rubber, leather, paper, clothing, and ceramics. The two most corrosive, and therefore harmful, pollutants are sulfur dioxide and sulfuric acid. The economic damage caused by air pollution is immense, and the damage to statuary and other works of art cannot be calculated.

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