Anthropogenic air pollution and pollutants

Although gaseous air pollutants can pose serious health hazards, only a few gases, such as carbon dioxide, warm the atmosphere. Particulate matter suspended in the air may have the opposite effect, blocking solar radiation and cooling the atmosphere.

Background

Air pollution has been a problem since humans began burning carbon-based fuels while living in large cities. The first known air-pollution ordinance was passed in London, England, in 1273 in an attempt to alleviate the soot-blackened skies caused by excessive combustion of wood in the heavily populated city. From the mid-eighteenth through the mid-twentieth centuries, the increasingly heavy use of coal for heat, electricity, and transportation resulted in filthy cities and an escalating crisis of respiratory diseases. It was not until the latter half of the twentieth century that governments began attacking the problem by enacting legislation to control noxious emissions at their source.

Before discussing air pollution, one must first define “clean air.” Earth’s atmosphere is approximately 78 percent nitrogen (N₂), 21 percent oxygen (O₂), and 1 percent argon. These concentrations may be reduced slightly by water vapor, which can make up between 1 percent and 3 percent of the atmosphere. In addition, there are many trace elements present in the atmosphere in concentrations so small that they are measured in parts per million. Among the trace elements near Earth’s surface are 0.52 part per million of oxides of nitrogen and 0.02 part per million of ozone, both of which occur both naturally and anthropogenically. This combination of N₂, O₂, argon, water, ozone, and oxides of nitrogen constitutes clean air. Any change in these concentrations or introduction of other compounds into the atmosphere constitutes air pollution, which occurs in one of two forms: gases and particulate matter.

Gaseous Air Pollutants

The primary gaseous pollutants are oxides of carbon, oxides of sulfur, oxides of nitrogen, and ozone. Carbon oxides occur whenever a carbon-containing fuel is burned; in general, a carbon fuel unites with oxygen to yield carbon dioxide (CO₂) and water vapor. If the combustion is incomplete as a result of insufficient oxygen, carbon monoxide will also be produced. Although CO₂ is a relatively benign compound, the vast amount of (coal, oil, and natural gas) burned since the began have raised its atmospheric concentration from about 280 parts per million to about 380 parts per million. CO₂ molecules, while transparent to visible light coming from the Sun, reflect infrared radiation emitted by the Earth when the visible light is absorbed and radiated as heat, thus raising Earth’s temperature in proportion to the amount of CO₂ in the atmosphere. As CO₂ concentrations increase, this greenhouse effect will increase Earth’s temperature, causing droughts, more severe storms of greater intensity, the shifting of climate zones, and rising sea levels.

Carbon monoxide (CO) is a toxic compound that can cause death by suffocation even when present in relatively small amounts. CO is two hundred times more reactive with hemoglobin than is oxygen; thus CO replaces oxygen in the bloodstream, depriving cells of their necessary oxygen. Deprived of sufficient blood oxygen, an organism will die in about ten minutes.

Since almost all coal contains sulfur, burning coal causes sulfur to react with oxygen to create sulfur dioxide (SO₂), which reacts with water vapor in the atmosphere to produce H₂SO₄, sulfuric acid. This pollutant reaches Earth’s surface as a component of rain ( acid rain), and it pollutes rivers, lakes, and other bodies of water.

Nitrogen oxides are synthesized whenever air is rapidly heated under pressure and then cooled quickly, as occurs in automobile cylinders and thermoelectric power plants. The two main compounds of this pollution are nitric oxide (NO) and nitrogen dioxide (NO₂); both are toxic, but NO₂ is worse (in equivalent concentrations, it is more harmful than CO). Nitrogen dioxide affects the respiratory system and can lead to emphysema, while nitric oxide often combines with oxygen to form nitric acid (NO₃), another component of acid rain.

NO₂ can also combine with oxygen to form NO and ozone (O₃), a very reactive and dangerous form of oxygen. Combustion-caused ozone is undesirable near Earth’s surface, but the compound occurs naturally in the upper atmosphere (about 19 kilometers above the surface) when energetic ultraviolet (UV) light from the Sun interacts with oxygen. Although the ozone composing it constitutes less than 1 part per million of Earth’s atmosphere, the plays an extremely important role. It prevents most of the Sun’s UV light from reaching Earth’s surface, a highly desirable effect since it is UV radiation that causes sunburn and skin cancer.

Chlorofluorocarbons

When first synthesized in the 1930s, chlorofluorocarbon (CFC) was hailed as an ideal refrigerant (Freon), because it was nontoxic, noncorrosive, nonflammable, and inexpensive to produce. Later, pressurized CFCs were used as the propellant in aerosol cans and as the working fluid for air conditioners. In 1974, the chemists Mario Molina and F. Sherwood Rowland proposed that the huge quantities of CFCs released into the atmosphere from aerosol sprays (500,000 metric tons in 1974 alone) and discarded refrigerant units were slowly migrating to the stratosphere. There, the CFCs were decomposed by the highly energetic UV radiation from the Sun, releasing large quantities of ozone-destroying chlorine.

Any decrease of the ozone layer could increase the incidence of skin cancer, damage crops, and decimate the base of the marine food chain. The reduction of ozone was most pronounced over Antarctica, where an “ozone hole,” first detected in the early 1970s, was increasing in size annually. Pressured by environmentalists and consumer boycotts, the US government imposed a 1978 ban on aerosol cans and refrigeration units utilizing CFC propellant, forcing the chemical industry to support the ban and to develop alternatives; several other nations soon followed suit. By 1987, the depletion of the ozone layer became so problematic that most CFC-using nations met in Montreal, Canada, to produce an international treaty calling for immediate reductions in all CFC use, with a complete phase-out by 2000. This Montreal Protocol, by 2001, had limited the damage to the ozone layer to about 10 percent of what it would have been had the agreement not been ratified.

Smog

The word “smog” is a melding of “smoke” and “fog.” When a local atmosphere becomes stagnant—for example, during a temperature inversion—pollution levels in the smog can become severe enough to call these smogs “killer fogs.” At least three times during the twentieth century, these killer fogs have caused a statistically significant increase in the death rate, particularly among the old and those with respiratory problems. The first documented killer fog occurred in 1948 at Donora, Pennsylvania, when a four-day temperature inversion stagnated a fog that became progressively more contaminated with the smoky effluents of local steel mills. The second documented case occurred in 1952 in London, England, when fog, trapped by another four-day temperature inversion, mixed with the smoke pouring from thousands of chimneys where coal was being burned. Many elderly people and people with respiratory ailments succumbed to these deadly events. Finally, during Thanksgiving, 1966, New York City experienced an increased death rate due to a choking smog.

A second, completely different type of smog is photochemical smog, a noxious soup of reactive chemicals created when sunlight catalyzes reactions of hydrocarbons and nitrogen oxides. This catalysis first occurred in Los Angeles in the late 1940s, when automotive traffic increased drastically, emitting thousands of metric tons of exhaust daily. As mentioned above, car engines, in addition to emitting carbon oxides, emit nitrogen oxides, ozone, and some residual unburned hydrocarbons from the fuel. When light acts on these chemicals, it produces photochemical reactions that create aldehydes (compounds, such as formaldehyde, that are well known for their obnoxious odors) and other dangerous compounds that can induce respiratory ailments, irritate eyes, damage leafy plants, reduce visibility, and crack rubber. Although photochemical smog was first observed in Los Angeles because of the abundant sunlight and heavy automotive traffic, it has since become prevalent in many other large cities.

Particulates

Particulate matter consists of soot, fly ash, or any other small particles or aerosols suspended in the air that can be breathed into the lungs or ingested with food. It is generated by combustion, dry grinding processes, spraying, and wind erosion. Particulate concentrations in the body can, over time, lead to cancer of the stomach, bladder, esophagus, or prostate.

The human respiratory system has evolved a mechanism to filter out and prevent certain sizes of particulates from reaching the lungs. The first line of defense is the nose and nasal passageway, whose mucus membranes and hairs will catch and remove particles larger than 10 microns (one one-hundredth of a millimeter). After passing through the nasal passages, air travels through the trachea, which branches into the right and left bronchi. Each bronchus is divided and subdivided about twenty times, terminating in the small bronchioles located inside the lungs. These end in 300 million tiny air sacs called alveoli, where oxygen is passed to the bloodstream and CO₂ removed for exhalation.

Particles ranging in size from 2 to 10 microns usually settle on the walls of the trachea, bronchi, and bronchioles, before reaching the alveoli. They are eventually expelled by ciliary action, a cough, or a sneeze. Particles smaller than 0.3 micron are likely to remain suspended in inhaled air and then removed from the lungs with exhaled air, similarly failing to enter the bloodstream. Humans thus have evolved a protective mechanism that shields them from particles of all sizes smaller than 0.3 micron and larger than 2 microns. No defense mechanism evolved for this intermediate size range, because during the long course of human evolution there were very few particles of this size in the environment. In recent centuries, however, many particles in this range—including coal dust, cigarette smoke, and pesticide dusts—have been added to the environment. Since no natural defense exists to eliminate these hazards from the human body, they coat the alveoli, causing such illnesses as black lung, lung cancer, and emphysema.

Context

The issue of whether or not global warming is caused by humans is still being debated, but strong measures were taken in the latter half of the twentieth century to control the noxious gases and particulate emissions known as air pollutants. When it was discovered that the ozone layer was being depleted by CFCs, the was ratified by most industrial nations. Both of these historic precedents indicate that strong, effective action and international cooperation is possible when a perceived threat to humanity and the environment is grave enough. Since the preponderance of scientific evidence seems to suggest that global warming is due to humanity’s excessive use of fossil fuels, perhaps it would be prudent to err on the side of caution and begin to curtail the disproportionate dependence on nonrenewable resources.

Key Concepts

• aerosols: minute particles or droplets of liquid suspended in Earth’s atmosphere
• anthropogenic: deriving from human sources or activities
• chlorofluorocarbons (CFCs): chemical compounds with a carbon backbone and one or more chlorine and fluorine atoms
• greenhouse effect: global warming caused by gases such as carbon dioxide that trap infrared radiation from Earth’s surface, raising atmospheric temperatures
• ozone: a highly reactive molecule consisting of three oxygen atoms
• parts per million: number of molecules of a chemical found in one million molecules of the atmosphere

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