(3) Air Pollution

 CHINA AIRPOCOLYPSE - Air Pollution is So Bad in China, Kills 500,000 People Each Year

AP5

Several Factors Can Decrease or Increase Outdoor Air Pollution

Five natural factors help reduce outdoor air pollution. First, particles heavier than air settle out as a result of the earth’s gravity. Second, rain and snow help cleanse the air of pollutants. Third, salty sea spray from the oceans can wash out much of the particulates and other water-soluble pollutants from air that flows over the oceans from land. Fourth, winds sweep pollutants away, diluting them by mixing them with cleaner air. Fifth, some pollutants are removed by chemical reactions. For example, SO2 can react with O2 in the atmosphere to form SO3, which reacts with water vapor to form droplets of H2SO4 that fall out of the atmosphere as acid precipitation.

Six other factors can increase outdoor air pollution. First, urban buildings can slow wind speed and reduce dilution and removal of pollutants. Second, hills and mountains can reduce the flow of air in valleys below them and allow pollutant levels to build up at ground level. Third, high temperatures promote the chemical reactions leading to photochemical smog formation. Fourth, VOC emissions from certain trees and plants such as some oak species, sweet gums, poplars, and kudzu, in heavily wooded urban areas can play a large role in the formation of photochemical smog. A fifth factor-the so-called grasshopper effect-occurs when volatile air pollutants are transported from tropical and temperate areas toward the earth’s poles, especially during winter. This explains why polar bears, whales, sharks, and other top carnivores and native peoples in the Arctic have high levels of DDT and other long-lived pesticides, toxic metals (such as lead and mercury), and PCBs in their bodies, even in the absence of industrial facilities and cars.

Sixth, temperature inversions can cause pollutants to build to high levels. During daylight, the sun warms the air near the earth’s surface. Normally, this warm air and most of the pollutants it contains rise to mix and disperse the pollutants with the cooler air above it. Under certain atmospheric conditions, however, a layer of warm air can temporarily lie atop a layer of cooler air nearer the ground, creating a temperature inversion. Because the cooler air is denser than the warmer air above it, the air near the surface does not rise and mix with the air above. This allows pollutants to build up in the stagnant layer of cool air near the ground.

Two types of areas are especially susceptible to prolonged temperature inversions. The first is a town or city located in a valley surrounded by mountains where the weather turns cloudy and cold during part of the year. In such cases, the surrounding mountains and the clouds block much of the winter sunlight that causes air to heat and rise, and the mountains block the wind. As long as these stagnant conditions persist, pollutants in the valley below will build up to harmful and even lethal concentrations.

The other type of area vulnerable to temperature inversions is a city with several million motor vehicles in an area with a sunny climate, light winds, mountains on three sides, and the ocean on the other side. Here, the conditions are ideal for photochemical smog worsened by frequent thermal inversions, and the surrounding mountains prevent the polluted surface air from being blown away by sea breezes. This describes the U.S. state of California’s heavily populated Los Angeles basin, which has prolonged temperature inversions, mostly during summer and fall.

 Acid Deposition Is a Serious Regional Air Pollution Problem

Most coal-burning power plants, ore smelters, and other industrial plants in developed countries use tall smokestacks to emit sulfur dioxide, suspended particles, and nitrogen oxides high into the atmosphere where wind can mix, dilute, and disperse them.

These tall smokestacks reduce local air pollution, but can increase regional air pollution downwind. The primary pollutants (sulfur dioxide and nitrogen oxides) emitted into the atmosphere above the inversion layer may be transported as far as 1,000 kilometers (600 miles) by prevailing winds. During their trip, they form secondary pollutants such as droplets of sulfuric acid, nitric acid vapor, and particles of acid-forming sulfate and nitrate salts.

These acidic substances remain in the atmosphere for 2–14 days, depending mostly on prevailing winds, precipitation, and other weather patterns. During this period they descend to the earth’s surface in two forms: wet deposition consisting of acidic rain, snow, fog, and cloud vapor and dry deposition consisting of acidic particles.

The resulting mixture is called acid deposition - sometimes termed acid rain-with a pH. Most dry deposition occurs within 2-3 days fairly near the emission sources, whereas most wet deposition takes place within 4-14 days in more distant downwind areas.

Acid deposition has been occurring since the industrial revolution. In 1872, British chemist Robert A. Smith coined the term acid rain after observing that rain was eating away stone in the walls of buildings in major industrial areas. Acid deposition occurs when human activities disrupt the natural nitrogen cycle, by adding large amounts of nitrogen oxides to the atmosphere, and disrupt the sulfur cycle by adding excessive amounts of sulfur dioxide to the atmosphere.

Acid deposition is a regional air pollution problem in areas that lie downwind from coalburning facilities and from urban areas with large numbers of cars. Such areas include the eastern United States and other parts of the world. Older coal-burning power and industrial plants without adequate pollution controls in the Midwestern United States emit the largest quantities of sulfur dioxide and other pollutants that can cause acid deposition. Because of these emissions, and those of other urban industries and motor vehicles, typical precipitation in the eastern United States is at least 10 times more acidic than natural precipitation. Some mountaintop forests in the eastern United States and east of Los Angeles, California, are bathed in fog and dews as acidic as lemon juice-about 1,000 times the acidity of normal precipitation. In some areas, soils contain basic compounds such as calcium carbonate (CaCO3) or limestone that can react with and neutralize, or buffer, some inputs of acids.

The areas most sensitive to acid deposition are those with thin, acidic soils that provide no such natural buffering and those where the buffering capacity of soils has been depleted by decades of acid deposition. Many acid-producing chemicals generated in one country are exported to other countries by prevailing winds. For example, acidic emissions from the United Kingdom and Germany blow into Norway and neighboring countries. The worst acid deposition occurs in Asia, especially China, which gets 69% of its total energy and 75% of its electricity from burning coal. In addition, air pollution that contributes to acid deposition and enhanced global warming is produced by the greatly increased use of cheap diesel generators to provide electricity for rural villages and power for irrigation pumps in China, India, and other developing countries.

 Acid Deposition Has a Number of Harmful Effects

 Acid deposition causes harm in several ways. It contributes to human respiratory diseases, and can leach toxic metals (such as lead and mercury) from soils and rocks into acidic lakes used as sources of drinking water. These toxic metals can accumulate in the tissues of fish eaten by people, other mammals, and birds. Currently 45 U.S. states have issued warnings to people (especially pregnant women) to avoid eating fish caught from some of their waters because of mercury contamination. Acid deposition also damages statues, national monuments, buildings, metals, and car finishes, and acidic particles in the air decrease visibility. Because of excess acidity, several thousand lakes in Norway and Sweden contain no fish, and many more lakes there have lost most of their acid-neutralizing capacity.

In Ontario, Canada, at least 1,200 acidified lakes contain few if any fish, and some fish populations in thousands of other lakes are declining because of increased acidity. In the United States, several hundred lakes (most in the Northeast) are threatened in this way. But some lakes are acidic because they are surrounded by naturally acidic soils.

Acid deposition (often along with other air pollutants such as ozone) can harm forests and crops by leaching essential plant nutrients such as calcium and magnesium from soils and releasing ions of aluminum, lead, cadmium, and mercury, which are toxic to the trees. This reduces plant productivity, tree growth, and the ability of soils to buffer acidic inputs. Acid deposition rarely kills trees directly, but can weaken them and leave them vulnerable to stresses such as severe cold, diseases, insect attacks, and drought. Mountaintop forests are the terrestrial areas hardest hit by acid deposition. These areas tend to have thin soils without much buffering capacity. And trees on mountaintops (especially conifers like red spruce and balsam fir) are bathed almost continuously in highly acidic fog and clouds. Most of the world’s forests and lakes are not being destroyed or seriously harmed by acid deposition. Rather, this regional problem is harming forests and lakes that lie downwind from coal-burning facilities and from large car-dominated cities without adequate pollution controls.

 We Know How to Reduce Acid

Deposition summarizes ways to reduce acid deposition. According to most scientists studying the problem, the best solutions are prevention approaches that reduce or eliminate emissions of sulfur dioxide, nitrogen oxides, and particulates. Controlling acid deposition is politically difficult. One problem is that the people and ecosystems it affects often are quite distant from those that cause the problem. Also, countries with large supplies of coal (such as China, India, Russia, and the United States) have a strong incentive to use it as a major energy resource. Owners of coal-burning power plants say that adding the latest pollution control equipment, using low-sulfur coal, or removing sulfur from coal would increase the cost of electricity for consumers. Environmental scientists counter that affordable and much cleaner resources are available to produce electricity. They also point out that the largely hidden health and environmental costs of burning coal are up to five times its market price. Including these costs in market prices would reduce coal use, spur the use of cleaner ways to burn coal, and help prevent acid deposition. Air pollution laws in the United States have reduced the acidity of rainfall in parts of the Northeast, Mid-Atlantic, and Midwest regions, but there is still a long way to go in reducing emissions from older coal burning power and industrial plants. Some plants have lowered SO2 emissions by switching from high-sulfur to low-sulfur coals. However, this has increased CO2 emissions that contribute to global warming, because low-sulfur coal has a lower heat value, which means that more coal must be burned to generate a given amount of electricity. Low-sulfur coal also has higher levels of toxic mercury and other trace metals, so burning it emits more of these hazardous chemicals into the atmosphere. Everything is connected.

 

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