Air Pollution and Atmospheric Deposition

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The combustion of fossil fuels is not only necessary to provide energy for the extraction process, it also is the main way that stored chemical energy is converted from these fuels into useable forms, such as electricity and transportation. Air pollution was one of the first recognized problems with the burning of fossil fuels. Black smoke poisoned the air surrounding early industrial cities. It caused concern among citizens of urban areas. Beginning in the late 1800s, antipollution coalitions worked to raise awareness to public officials. These movements stimulated the first air pollution control laws in Europe and North America. During the latter half of the twentieth century, many countries implemented controls on the pollution emitted when fossil fuels are burned. However, there is still a long way to go. What are the pollutants that are released from fossil fuel combustion? What harmful effects do they have on our environment and human health? This section answers these questions.

The burning of fossil fuels releases a variety of harmful pollutants into the atmosphere. The most common emissions are carbon monoxide (CO), sulfur dioxide (SO2), nitrgen oxides (NOx), ozone (O3), and particulate matter. In addition, toxic metals (such as mercury and lead), hydrocarbons, and volatile organic compounds (VOCs) are released into the atmosphere. Many of these chemicals have been shown to have severely negative effects on human health. Others create hazy smog, which reduces visibility in many areas. Table 2.1 summarizes the different categories of air pollutants.

Different types of fossil fuels release different pollutants when combusted. Coal is the most polluting fuel; however, because it varies in chemical composition depending on the region it is from, the amount of pollution produced varies. Coal combustion produces large quantities of fly ash that most commonly contains CO, particulate matter, SO2, and NOx. In modern power plants much of this ash is collected before it is emitted from smokestacks. Since it is impossible to remove all of the pollutants from an emissions stream, regulations still allow for a permissible level of pollution to be emitted in a particular region. These regulations have reduced pollution significantly, but it is important to point out that not all countries have the same regulations. Recall the case of the former Soviet Union, whose archaic industries were not only inefficient, they were also highly polluting.

TABLE 2.1

Common Air Pollutants and Their Environmental and Health Effects

Pollutant

Impact on environment and human health

Sulfur dioxide (SO2)

Associated with and shown to exacerbate respiratory illnesses

(asthma, emphysema, bronchitis, etc.); causes acid rain when it

reacts with water in the atmosphere

Nitrogen oxides (NOx)

Irritates lungs, causes bronchitis and pneumonia, elevates

pulmonary edema levels, and lowers resistance to other respiratory

infections. Contributes to smog formation

Carbon monoxide (CO)

Affects the body's ability to assimilate oxygen; increases the risks

for heart disease and impacts brain functioning

Particulate matter

Scratches/damages the respiratory system; can cause acute or

chronic respiratory illness. Some particulates (e.g., benzo[a]pyrene)

can cause cancer

Hydrocarbons

Causes smog and contributes to the formation of ground-level ozone

Ozone

Irritates the eyes and mucous membranes of respiratory tract;

damages immune system; causes pulmonary congestion, chest pains,

coughing; can react with NOx to form smog.

Volatile organic compounds (VOCs)

Inhalation may cause cancer; can react with NOx compounds to form

smog

Toxic metals

Most cause neurological damage to humans and are especially

(Cd, Pb, Hg, Ar, Ni, Cr, etc.)

harmful to pregnant women, neurological development of fetuses

(increased instances of mental retardation, etc.), and children; can

contribute to high blood pressure, heart disease, respiratory illness.

Carbon monoxide is the most common pollutant that is formed when fossil fuels combust completely. In high concentrations, it can impair the ability to function properly, causing drowsiness and headaches. Most people are never exposed to this level of CO, but chronic exposure over time has been shown to increase the rate of heart disease.

Particulate matter (PM) is classified according to size. For example, PM-10 and PM-2.5 refer to particulate matter that is 10 and 2.5 micrometers in diameter, respectively (smaller than the period at the end of this sentence). Scientists have discovered that the smaller the particle size, the more easily it can become embedded in lung tissues of humans. PM can cause damage to the tissues of the respiratory system and is known to cause or exacerbate many respiratory infections. Sulfur and nitrogen oxides can also cause respiratory problems. Because of smokestack and vehicle emissions, urban air pollution can cause life-threatening health problems. Studies have shown that between 30,000 and 35,000 Americans die prematurely because of air pollution (Smith 2004, 84).

Smog is another problem that occurs with the release of these pollutants in the atmosphere. It forms when nitrous oxide molecules (NO2, N2O, etc.) react with ozone and water vapor in the atmosphere. Smog produces a brown haze that settles over areas where NOx is being emitted. It can cause hazy days and impact visibility. VOCs can also react with NOx to form ground-level ozone. Although ozone is a vital part of the upper regions of Earth's atmosphere, at the ground level it damages the mucous membranes of humans, creates respiratory problems, and can irritate the eyes.

Air pollution can also cause significant damage to crops. Crop yields can be reduced by as much as 10 to 15 percent in polluted areas. Ground-level ozone is the most damaging of all air pollutants to agricultural yields. For example, in the southeastern United States, where O3 concentrations can reach between 50 and 55 parts per billion (ppb), 10 percent reductions in cotton, soybean, and peanut crops were observed (Smith 2004, 86). In Spain, watermelon yields decreased by 19 percent when O3 concentrations were measured above regulatory limits.

Atmospheric deposition is one of the most pervasive and damaging consequences of using coal for the generation of electricity. Many bituminous coal resources contain high amounts of sulfur. When this sulfur is burned, it creates sulfur dioxide (SO2) gas, which is emitted with the exhaust from the combustion. Acid rain occurs when SO2 reacts with water vapor in the atmosphere to form sulfuric acid (H2SO4). During storm events, acid rain is deposited into water bodies and on land surfaces. It is damaging to ecosystems and plant life and can cause increased rates of erosion in some areas (Alexander 1996, 21).

Deposition of solid forms of pollution can also be damaging to ecosystems. Petroleum and coal contain many impurities, including trace metals (for example, lead and mercury) that are toxic to humans. This toxicity is the reason why unleaded gasoline fuels vehicles. But while lead concentrations have been regulated in petroleum products, mercury emissions are an increasingly important problem for countries that rely on coal-fired power plants to supply their electricity needs. Once mercury is emitted into the atmosphere, it can be transported long or short distances, depending on what chemical form it has assumed. Mercury is eventually deposited onto land or water surfaces, where it can settle into sediments and be chemically transformed by microorganisms. This process is called methylation (USEPA 1997, vol. 3, 2-5). Methylated mercury bioaccumulates in the tissues of organisms and becomes more concentrated as it moves higher up the food chain. Studies reveal that methylated mercury is extremely toxic for humans. It is considered to be a potent neu-rotoxin (it damages the nervous system) capable of causing developmental problems in human fetuses and cognition problems in adults. In ecosystems, methylated mercury can cause developmental damage to several species of migratory birds and can impair the developmental biology of fish (USEPA 1997, vols. 5-7). The pervasiveness of mercury is the reason many governments issue fish-eating advisories to their citizens.

The above example illustrates the complex nature of how air pollutants behave in the environment. There are many other pollutants emitted during the combustion of fossil fuels than those mentioned. These pollutants, which can damage humans and the environment, include other metals toxic to humans and volatile organic compounds, many of which are known to cause cancer. Discussion now turns to global climate change, which is one of the most pressing global environmental problems caused by fossil fuel combustion.

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