Key Concepts

1. After completing this chapter, you will be able to
2. Outline the major sources of emission of air pollutants associated with sulfur, nitrogen, and hydrocarbons.
3. Explain the difference between primary and secondary pollutants.
4. Contrast the environmental problems associated with stratospheric and tropospheric ozone.
5. Examine the importance of air pollutants to ecosystems and to human health.
6. Describe the patterns of pollution and ecological damage near point sources of air pollution.

Introduction

Gaseous air pollutants are emitted from various natural sources, such as volcanoes and forest fires. However, anthropogenic emissions of some gases may be greater than the natural ones, and are increasing because of population growth and industrialization.

In ancient times, there was not much pollution. Nevertheless, even then local problems would have occurred. For instance, smoky wood fires used for cooking and warmth would likely have caused poor-quality air to occur inside of badly ventilated dwellings. Hunting cultures often used fire to drive game animals and improve local forage, and those burns would have resulted in large emissions of particulate carbon (soot), carbon dioxide, and other gases that would have temporarily impaired air quality. Overall, however, these effects were relatively minor.

Of course, as people became more numerous and industrialized, air pollution increasingly developed into a much bigger problem. When the Industrial Revolution began (around 1750), coal quickly became the major fuel used to generate heat and energy for machines, and that resulted in increasingly worse air pollution. The widespread use of coal led to severe pollution by sulfur dioxide (SO₂) and soot in the industrial towns and cities of Europe and the Americas. Since 1900, burgeoning industrialization and new technologies such as power plants and automobiles have further increased the emissions of pollutants.

Air pollution can be especially severe in situations when the lower atmosphere is stable and calm. These conditions often occur beneath an atmospheric phenomenon called an inversion, in which a layer of cool air is trapped beneath a higher layer of warmer air (the more usual pattern is for temperature to cool with increasing altitude). An atmospheric inversion is a relatively stable condition that prevents polluted ground-level air from mixing with cleaner air from higher altitudes (see In Detail 20.1). If an atmospheric inversion is accompanied by fog, the pollution is known as smog (a word derived from “smoke” and “fog”). As recently as the 1950s, occasional so-called “killer smogs” rich in SO₂ and soot caused the deaths of thousands of urban people, and many more suffered from respiratory distress (pollution rich in SO₂ is also called reducing smog). The most famous killer smogs occurred in London, Glasgow, some other industrial centers of Europe, and near Pittsburgh in the U.S.

Once scientists recognized the severe damage that was being caused by air pollution, governments began to pass laws to decrease the emissions. Pollution control became particularly vigorous after medical researchers discovered clear evidence of links between air quality and human diseases and deaths. Particular attention was paid to air pollution in urban environments, where the killer smogs were most frequent. Governments typically responded with important control actions, such as the following:

• Switching from coal, which is a relatively “dirty” fossil fuel, to “cleaner” ones such as natural gas or oil, or to alternative energy technologies such as nuclear power and hydroelectricity
• Constructing tall smokestacks to spread emissions over a much wider area so that ground-level exposures become less common and less intense – this tactic is the “dilution solution to pollution”
• Centralizing energy production in large power plants to replace much of the relatively dirty and inefficient burning of coal in home fireplaces and furnaces, thereby permitting better control of emissions
• Treating waste gases to remove some of their pollutant content, thereby reducing emissions to the atmosphere

Because of these helpful regulatory actions, the importance of reducing smogs became much less in many countries. However, so-called oxidizing smog has become more important causes of damage in many regions. Oxidizing smog develops in the atmosphere through complex photochemical reactions in which hydrocarbons and nitrogen oxides are transformed into ozone (O₃) and other gases. Ozone and other oxidizing gases harm vegetation and irritate the respiratory system and eyes of people. Oxidizing smog develops under sunny conditions if hydrocarbons and nitrogen oxides are present from automobile and industrial emissions, and especially if the presence of an atmospheric inversion reduces dispersion of the polluted air mass.

In this chapter, we examine the most important gaseous pollutants. Their sources of emission, chemical transformations, and toxicity are described, and case studies are used to demonstrate the ecological damages that may be caused.

Sulfur Gases

Emissions and Transformations

Sulfur dioxide (SO₂) is one of the most important of the gaseous air pollutants. SO₂ is a colorless but pungent gas. Humans can detect its bitter taste at a concentration of only 0.3-1 ppm (parts per million; for SO₂, 1 ppm = 2.6 mg/m³). Hydrogen sulfide (H₂S), another sulfur gas, can be detected as a foul odor, reminiscent of rotten eggs, at concentrations lower than 1 ppb (parts per billion; for H₂S, 1 ppm = 1.4 mg/m³) (unless otherwise indicated, specific data cited in this and other chapters in this section on environmental damages are from Freedman, 1995).

After they are emitted to the atmosphere, SO₂ and H₂S become oxidized to other compounds, and ultimately form sulfate (SO₄^2–; see In Detail 20.2). Because the sulfate ion carries negative charges, it is an anion (this refers to any ion, atom, or molecule with a negative charge). Because H₂S is quickly oxidized to SO₂, its atmospheric residence time is less than one day (this is the time to complete disappearance of an initial amount). SO₂ oxidizes more slowly, at a rate of < 1-5% per hour, depending on sunlight, humidity, strong oxidants such as ozone, and the presence of metal-containing particulates that may act as catalysts. A typical residence time of SO₂ in the atmosphere is about four days. Consequently, SO₂ may disperse a long distance from its point of emission before it becomes oxidized or is deposited to a terrestrial or aquatic surface. This kind of dispersal is referred to as long-range transportation of air pollution (LRTAP).

Atmospheric sulfate, formed by the oxidation of SO₂ or H₂S, can combine with positively charged ions (cations) to form various compounds. Most atmospheric sulfate occurs as tiny particulates, especially ammonium sulfate ((NH₄)₂ SO₄). This is the most prominent component, along with ammonium nitrate (NH₄NO₃), of the fine particulate haze that often impairs visibility in cities. Haze also occurs in some rural areas where pollutants have been imported by LRTAP from emission sources elsewhere. Other cations that combine with sulfate include calcium (Ca²⁺), magnesium (Mg²⁺), and sodium (Na⁺). Often, however, there are not enough of these cations to balance all of the negative charges of the sulfate (SO₄^2–) present. Under such conditions, hydrogen ions (H⁺) serve to balance some of the negative charges, resulting in an aerosol containing sulfuric acid (H₂SO₄), the most important component of acidic precipitation (see Chapter 23).

Emissions of Sulfur Gases

Volcanoes are natural sources of emission of sulfur gases. On average, volcanoes emit about 12-million tonnes of sulfur gases per year, of which 90% is SO₂ and 10% is H₂S. However, the enormous 1991 eruption of Mount Pinatubo in the Philippines emitted 7-10-million tonnes of SO₂ (expressed as the sulfur content, or SO₂-S). The much smaller eruption of Mount St. Helens in Washington State in 1980 vented about 0.2-million tonnes of SO₂-S. Natural emissions of SO₂ also occur during wildfires.

The global anthropogenic emissions of SO₂ are about 150-million tonnes per year, or 3.8-times the natural releases. Fossil-fuel combustion accounts for more than half of the anthropogenic emissions (Figure 20.2). Coal and petroleum contain mineral compounds of sulfur, such as pyrite (FeS₂), as well as organic sulfur compounds. When these fuels are burned, the sulfur becomes oxidized to gaseous SO₂, which may be vented to the atmosphere. Hard coals mined in eastern North America contain 1-12% sulfur, softer coals from western regions have <0.3-1.5%, crude oil has 0.8-1.0% residual fuel oils such as bunker -C have 0.3-0.4%, and motor fuels such as diesel and gasoline have 0.04-0.05%. Other important sources of SO₂ emissions are manufacturing processes (23% of global emissions), the smelting of metal ores (7%), and the burning of natural habitats during agricultural conversions (most of the rest).

Figure 20.2. Global Anthropogenic SO₂ Emissions from 1850-2005. Source: Smith et al. 2011

Globally anthropogenic emissions of SO₂ have increased enormously since the beginning of the Industrial Revolution. Emissions in 1860 were about 5-million tonnes, compared with about 150-million tonnes in 2000. Since then, most of the wealthier countries have invested heavily in clean-air technologies for power plants and other industrial users of coal and oil in order to reduce emissions of damaging SO₂ to the atmosphere. The clean-air actions include the following:

• The installation of technologies to capture SO₂ from post-combustion waste gases (this is known as flue-gas desulphurization or scrubbing)
• The removal of some of the sulfur content of fuels (known as fuel desulphurization or coal washing)
• The installation of particulate-control devices (such as electrostatic precipitators) to greatly reduce the emissions of particulates (although this has little effect on SO₂ emissions)
• Switching to low- or no-sulfur fuels such as natural gas, or to no-sulfur energy technologies such as hydroelectricity and nuclear power (known as fuel-switching)
• Energy conservation to reduce the overall demand for fuel and associated emissions of pollutants
• Building taller smokestacks to disperse emissions of SO₂ more widely, which helps to decrease the local ground-level pollution

However, future global emissions are bound to increase. China, India, and other rapidly industrializing countries supply much of their burgeoning energy needs by burning coal and petroleum fuels. In China, coal is the major source of industrial energy, accounting for 58% of the energy supply in 2019 (U.S. Energy Information Administration 2020). Due to increasing industrialization, emissions of SO₂ in China increased from 10-million tonnes in 1980 to 34-million tonnes in 2000, a 3.4-fold increase in only 20 years.

The amounts of SO₂ emissions differ greatly among nations, depending on their population, their kind of industrialization, and the fuels they mostly use. For instance, Canadian emissions of SO₂ are about 22% of those of the U.S. However, the population of Canada is only about 11% that of the U.S. Therefore, per-capita emissions of SO₂ are about double in Canada than in the U.S.

About 66% of Canadian emissions of SO₂ are from large industrial sources, particularly metal smelters, while 24% are from “fuel combustion,” which is mostly fossil-fueled power plants. In comparison, 84% of U.S. emissions are from power plants, and 10% from industrial sources. Most of this difference is due to two factors: a relatively large proportion of electricity generation in Canada is from nuclear and hydro technologies, which do not emit SO₂, and metal smelting is a major industry in Canada.

Because of human-health and environmental damages associated with SO₂ and other air pollutants, most developed nations have acted to reduce their emissions. In Canada, emissions of SO₂ were reduced by about 41% between 1985 and 2012, compared with a 32% reduction in the U.S. (Figure 20.2). In both countries the reductions were achieved by several methods:

• Switching to low- or no-sulfur fuels for some major uses, especially for electricity generation
• Removing sulfur from some fuels prior to combustion, mostly by coal washing
• Reducing energy demands through conservation
• Installing scrubbers to remove SO₂ from post-combustion waste gases before they are vented to the atmosphere

The large reductions of SO₂ that have been achieved in North America during the past several decades cost many billions of dollars to achieve, and they should be regarded as an important “success story” of pollution control. Figure 20.3 depicts reduction in sulfur dioxide emissions for the U.S.

Figure 20.3. Reductions in Sulfur Dioxide Emissions in the U.S. from 2006 to 2015. Data from U.S. Energy Information Administration.

In contrast to SO₂, the global emissions of sulfide gases are mostly from natural sources. The largest sources are H₂S emitted from anoxic sediment in shallow marine and inland waters, and dimethyl sulfide ((CH₃)₂S) produced by phytoplankton and out-gassed from oceanic waters. The natural emission of H₂S is about 100-million tonnes per year, and dimethyl sulfide, 15-million tonnes per year (both expressed as sulfur equivalent, or as tonnes of S). Anthropogenic emissions of H₂S are about 3-million tonnes per year, and are mostly from chemical industries, sewage-treatment plants, and livestock manure. The global emission of all sulfur-containing gases is equivalent to almost 300- million tonnes of sulfur per year. About half of the global emission is from anthropogenic sources. Clean air typically contains less than 0.2 ppb of SO₂ or H₂S. Concentrations of SO₂ and H₂S in air that is polluted by emissions are highly variable. They are typically about 0.2 ppm in urban atmospheres but can exceed 3 ppm close to large emission sources.

Toxicity of Sulfur Gases

Concentrations of H₂S in the environment are rarely high enough to be toxic to plants. However, accidental emissions from sour-gas plants (where H₂S is removed from natural gas) may cause local vegetation damage. In contrast, concentrations of SO₂ in cities and near industrial sources are often high enough to injure wild and cultivated plants. Near certain metal smelters, vegetation has been severely damaged, as we examine later.

An exposure to 0.7 ppm SO₂ for one hour will result in acute injury to most plant species, as will 0.2 ppm over an eight-hour period. It is important to note, however, that certain species are extremely sensitive to SO₂ exposures (they are hypersensitive) and can suffer acute injuries at concentrations lower than those just noted.

In addition, plants will often exhibit a reduction in yield when exposed to concentrations of SO₂ that are lower than those required to cause an acute injury. This type of response, which occurs without symptoms of acute tissue damage, is referred to as “hidden injury”. Hidden injuries to wild and agricultural vegetation are measured by enclosing plants in experimental chambers and exposing them to either air containing ambient levels of SO₂, or that that has been filtered through charcoal to remove that gas. If productivity is greater in the filtered air, it follows that the ambient SO₂ was causing a hidden injury. Studies of pasture grasses have found that exposure to SO₂ concentrations averaging only 0.04 ppm would cause hidden injuries as reductions of yield.

The U.S. National Ambient Air Quality Standards (Table 20.1) sets the guideline for ambient SO₂ in the atmosphere at 75 parts per billion over a one-hour exposure time (EPA 2020a), and 0.5 ppm over a three-hour exposure time. These guidelines are intended to protect human populations (particularly those sensitive to air pollutants, e.g., those with asthma, children, and the elderly) as well as to prevent acute foliar (leaf) injuries agricultural plants. Although regions meeting these guidelines would not show much acute damage to vegetation, relatively sensitive species might be affected through hidden injuries, possibly resulting in significant losses of yield.

Table 20.1. National Ambient Air Quality Standards for Six Principal Pollutants. This table shows both the primary and secondary standards, which define the limits of the pollutants to minimize their harmful impacts on public health and the environment. Primary standards are the limits placed to protect public health, including protecting the “sensitive” populations (asthmatics, children, and the elderly). Secondary standards concern public welfare and environmental protections. Source: EPA.

Humans and most other animals are much less sensitive to SO₂ than plants are. Guidelines for allowable exposures of people to SO₂ and other potentially toxic gases accommodate the fact that, in terms of dose received, longer-term exposures to low concentrations can be as important as higher acute exposures.

Guidelines for occupational exposures to air pollutants are frequently greater than those for ambient exposures. In North America, it is recommended that occupational exposures to SO₂ be no higher than 2 ppm (5.2 mg/m³) over the long term, and no higher than 5 ppm (13 mg/m³) for shorter exposures. However, some people are relatively sensitive to SO₂, and concentrations less than 1 ppm (2.6 mg/m³) may cause them to suffer asthma or other distresses related to impaired lung function. In addition, some studies have suggested that long-term exposures of large human populations to sulfate particulate aerosols (which are ultimately derived from gaseous SO₂) in cities may result in small increases in the incidence of respiratory and circulatory diseases, most probably in hypersensitive people.

Nitrogen Gases

Emissions and Transformations

The most important of the nitrogen-containing gases are nitric oxide (NO), nitrogen dioxide (NO₂), nitrous oxide (N₂O), and ammonia (NH₃). NO and NO₂ are often considered together as a complex, referred to as NO_x.

Ammonia (NH₃), a colorless gas, is emitted mostly from wetlands, where it is produced during the anaerobic decomposition of dead biomass. Natural emissions of NH₃ are about 1.2-billion tonnes per year. Sources of anthropogenic emissions include fossil-fuel combustion (4-million tonnes per year) and animal husbandry (0.2-million tonnes per year). The residence time of NH₃ in the atmosphere is about seven days (the NH₃ is eventually oxidized to nitrate).

Nitrous oxide (N₂O) is a colorless, non-toxic gas that produces a mild euphoria when inhaled. This gas is also known as “laughing gas” and is used as a mild anesthetic in medicine, and sometimes as a recreational drug. Because N₂O is a rather unreactive compound, it has a long residence time in the atmosphere of about four years. Most N₂O emissions are associated with microbial denitrification in soil and water. These are equivalent to about 18-million tonnes per year, while industrial emissions are 12-million tonnes per year. Agricultural soil fertilized with nitrate can have high rates of N₂O emission, and modern agricultural practices are thought to have increased global emissions by about 40% (Figure 20.4).

Figure 20.4. Worldwide Nitrous Oxide Emissions by Sector. Nitrous oxide emissions are measured in tonnes of carbon dioxide equivalents (CO₂e) based on a 100-year potential global warming value. Source: visualization by OurWorldinData.org is licensed under CC BY 4.0.

Nitric oxide (NO) is a colorless and odorless gas, while nitrogen dioxide is reddish, pungent, and irritating to respiratory and eye membranes. Natural emissions of NO_x are about 430-million tonnes per year (expressed as NO; the same emissions expressed as NO₂ are 658-million tonnes per year). The most important natural emissions of NO_x are due to bacterial denitrification of nitrate in soil, fixation of atmospheric nitrogen gas (N₂) by lightning, and oxidation of biomass nitrogen during fires (see Chapter 5).

Anthropogenic emissions of NO_x, about 83-million tonnes per year (expressed as NO), result mostly from the combustion of fossil fuels, especially in automobiles and power plants. These emissions are mostly NO, which is secondarily oxidized to NO₂ by reactions in the atmosphere. Ultimately, most atmospheric NO_x gases become oxidized to nitrate (NO₃^–), an ion that is important in the acidification of precipitation and ecosystems (Chapter 23).

Toxicity of Nitrogen Gases

It is rare that concentrations of NH₃ or NO_x gases are high enough to injure vegetation. The environmental damage associated with NO_x is focused on the photochemical reactions by which ozone, a much more toxic gas, is produced (see below), and also the acidification of precipitation and ecosystems.

Ambient concentrations of NH₃ and NO_x are rarely high enough to bother humans. Guidelines for long-term exposures in an occupational setting are 25 ppm (34 mg/m³) for NO and 5 ppm (10 mg/m³) for NO₂. Occupational guidelines for short-term exposures are 35 ppm (47 mg/m³) and 5 ppm (10 mg/m³), respectively. Intense occupational exposures to NO_x can cause impaired pulmonary function in humans.

Organic Gases and Vapors

Emissions and Transformations

Hydrocarbons are a diverse group of chemicals whose molecular structures containing various combinations of hydrogen and carbon atoms. The simplest hydrocarbon is methane (CH₄), a gas. Larger hydrocarbons with greater weight and more complex structure may occur as vapors, liquids, or solids. Other volatile organic compounds (VOCs) may contain oxygen, nitrogen, and other light elements in addition to carbon and hydrogen, and include alcohols, aldehydes, and phenols.

The background concentration of methane in the atmosphere is about 1.7 ppm (1 ppm = 0.65 mg/m³), while all other hydrocarbons and volatile organics together amount to less than 1 ppb. Most emissions of CH₄ are natural and are associated with the fermentation of organic matter by microbes in anaerobic wetlands. Smaller amounts are out-gassed from deposits of fossil fuels, during wildfires, and from burping and flatulent ruminant animals (such as cows and sheep) and termites, which produce CH₄ as they digest their plant foods. Of total global CH₄ emissions, 50-65 percent are emitted from human activities, including energy, industry, agriculture, and waste management activities (EPA, 2020b). In the U.S., the largest source of CH₄ emissions is the agriculture sector.

Atmospheric hydrocarbons other than CH₄ are referred to as non-methane hydrocarbons. Natural emissions of these and many organics occur mainly as gases and vapors that evaporate from living vegetation, along with smaller quantities that out-gas from deposits of fossil fuels. The largest emissions from forests typically occur during hot, sunny days. Natural emissions of non-methane hydrocarbons are about 200-million tonnes per year, compared with anthropogenic emissions of 186-million tonnes per year. The most important anthropogenic sources are unburned fuel emitted from vehicles and aircraft, releases during fossil-fuel mining and refining, and evaporation of solvents.

Toxicity of Organic Gases and Vapors

Organic gases and vapors can be toxic, but atmospheric concentrations are rarely high enough to damage vegetation or animals. The environmental importance of these gases and vapors lies mainly in their role in the photochemical reactions that produce toxic ozone. In addition, CH₄ is an important greenhouse gas that affects global warming (Chapter 21). In some workplaces, however, relatively toxic organics such as benzene and formaldehyde may be important pollutants.

Ozone

There are two different ozone-related environmental issues: (1) O₃ in the stratosphere and (2) O₃ in the troposphere (ground-level ozone). High concentrations of ozone are naturally present in the upper-atmospheric layer known as the stratosphere, which begins at about 8-17 km above the Earth’s surface, depending on the latitude and season. Stratospheric O₃ causes no damage and is not an air pollutant. Rather, by absorbing solar ultraviolet radiation, stratospheric O₃ helps to protect organisms on the surface of the planet from many damaging effects of exposure to this harmful part of the electromagnetic spectrum. In contrast, O₃ in the lower atmosphere (the troposphere) is an important air pollutant that damages vegetation, materials, and human health. Ozone is removed from the atmosphere by interactions with other gases, organic vapors, and terrestrial and aquatic surfaces (including vegetation).

Ground-Level Ozone

Ground-level ozone (O₃) is the most damaging of the so-called photochemical air pollutants. Less important are peroxyacetyl nitrate (PAN), hydrogen peroxide (H₂O₂), and other oxidant gases. Oxidizing smog is rich in O₃ and the other oxidant gases. These chemicals are secondary pollutants, meaning they are not actually emitted to the atmosphere (as are primary pollutants such as SO₂ and NO_x). Instead, they are synthesized within the atmosphere by photochemical reactions (chemical reactions that require light, especially ultraviolet wavelengths; Figure 20.5). These proceed at faster rates and result in a buildup of oxidants if NO_x and hydrocarbons are present in high concentrations, a condition that is typically due to anthropogenic emissions.

Figure 20.5. Ground Level Ozone Formation. Ground level ozone is not emitted directly into the atmosphere, but instead is synthesized by photochemical reactions. Source: used with permission from the Minnesota Pollution Control Agency.

Some regions tend to develop a weak atmospheric inversion in the morning. Because an inversion is relatively stable and resists the in-mixing of cleaner air from above or beyond, this condition encourages the development of oxidizing smog during the morning and early afternoon. Later in the day, the inversion is typically broken up by stronger winds, and the air pollution is dispersed. Such inversions and their ozone-rich smog are common phenomena around the Los Angeles basin, Mexico City, Vancouver, and elsewhere.

The concentrations of ground-level O₃ vary greatly among different regions of North America. Average concentrations in the southwestern U.S. are relatively high, at about 100 ppb (1 ppb = 2 µg/m³), and they generally range from 40–60 ppb in other regions of the U.S. and in southern Canada.

Canada, the U.S., and other countries have developed air quality standards for O₃. These are intended to reflect concentrations that would prevent severe damage to agricultural and wild vegetation. For some time, the O₃ standard in the U.S. was 80 ppb (160 µg/m³) (for an average one-hour exposure), but in 1979 this was relaxed to 120 ppb (240 µg/m³). The authorities made the change because the original standard of 80 ppb was frequently exceeded over large regions and so was essentially unenforceable. In fact, even the 120 ppb criterion is commonly exceeded in some regions, particularly in the southwestern U.S.

The Los Angeles basin suffers especially intense photochemical air pollution. Concentrations of O₃ can exceed 500 ppb (one-hour average), and they typically exceed 100 ppb for more than 15 days during the summer. Maximum O₃ concentrations are lower in other cities of North America, typically reaching up to 150-250 ppb (one-hour average). These concentrations are well within the range at which O₃ can cause acute injury to plants, which is why O₃ is such an important air pollutant. The emissions of ozone precursors (NO_x and hydrocarbons) occur mainly in cities, but extensive ecological damage is caused when polluted air masses are transported to rural areas dominated by agricultural or natural vegetation.

Toxicity of Ozone

Humans and some animals are sensitive to O₃, which can irritate and damage membranes of the eyes and respiratory system and cause a loss of lung functioning. The guideline for long-term exposure to O₃ in an occupational setting is 100 ppb (196 µg/m³), and it is 300 ppb (589 µg/m³) for short-term exposures. However, sensitive people can be affected by O₃-related symptoms at lower concentrations. Exposure to O₃ can result in asthmatic attacks and can exacerbate bronchitis and emphysema.

Ozone causes important damage to wild and agricultural plants over widespread areas. Foliar injuries are often distinctive to O₃, and they diminish the photosynthetic capacity of plants and thereby reduce their productivity. Acute injuries are caused to most species by two- to four-hour exposures to 200-300 ppb O₃, while long-term exposures to only 40-100 ppb may cause hidden injuries (and reduced yield). However, many species are more sensitive and suffer acute and hidden injuries at lower concentrations. Some varieties of tobacco (Nicotiana tobacco), for example, can suffer acute foliar injuries from a two- to three-hour exposure to only 50-60 ppb, and spinach (Spinacea oleracea), from one to two hours at 60-80 ppb. Sensitive species of conifer trees may suffer acute injuries from exposures to 80 ppb O₃ over 12 hours.

Researchers have grown agricultural plants in experimental chambers that received ambient air, or air filtered through charcoal, which removes any O₃. These studies have been useful in defining the extent of damage caused to agricultural crops by exposure to ambient O₃. One series of field experiments demonstrated that crop yields were reduced in all regions of the U.S. The worst damage occurred in the southwest, where sunny conditions and large emissions of NO_x and hydrocarbons result in especially high O₃ concentrations. That study estimated that crop damage due to O₃ was equivalent to 2–4% of the total agricultural yield in the U.S., with economic losses equivalent to more than $5 billion per year. Because O₃-related damage to vegetation occurs over extensive areas of North America, it is by far the most important air pollutant in agriculture. Ozone is probably also the leading air pollutant causing damage to forests and other natural ecosystems.

Stratospheric Ozone

In contrast to ground-level ozone, ozone in the stratosphere protects life on Earth from the damaging effects of solar ultraviolet (UV) radiation. This is the reason why the fact that stratospheric ozone is being destroyed by anthropogenic emissions of certain gases is cause for alarm.

Ozone is produced in the stratosphere by natural photochemical reactions. They involve the absorption of solar UV radiation by oxygen molecules (O₂), which creates highly reactive oxygen atoms (O) that join with other O₂ molecules to form O₃ (see In Detail 20.2). These reactions proceed relatively quickly in the stratosphere because high-energy UV radiation is abundant there. As a result, O₃ concentrations are typically 200-300 ppb in the stratosphere, about 10 times greater than in the ambient troposphere.

Stratospheric O₃ provides a critical environmental service. It efficiently absorbs most of the incoming high-energy UV radiation, which can be extremely damaging to organisms. In particular, DNA is a strong absorber of UV and can be damaged by this radiation. This can increase the risk of developing skin cancers, including melanoma, an often-fatal malignancy. Other health risks from UV exposure include the development of cataracts in the eyes and suppression of the immune system. UV radiation also damages plants, in part because chlorophyll (the key photosynthetic pigment) is degraded by UV absorption, which may lead to decreases in productivity. The waxy covering of the cuticle of foliage is also damaged by UV radiation.

Stratospheric O₃ can be destroyed by various processes, including reactions with the trace gases NO_x and N₂O and with reactive ions of bromine, chlorine, and fluorine. Because of anthropogenic emissions, the concentrations of some of these O₃-consuming chemicals have been increasing in the stratosphere, leading to concerns about the depletion of stratospheric O₃. It is widely believed that emissions of chlorofluorocarbons (CFCs), particularly the industrial gases known as freons, have been especially important in this regard. Because CFCs are extremely unreactive in the troposphere, they eventually migrate up to the stratosphere, where they are bombarded with UV radiation and slowly degrade (photodissociate) to release free chlorine. The chlorine efficiently reacts with and destroys O₃.

The O₃-destroying reactions proceed most effectively under extremely cold and stagnant conditions in the stratosphere, such as those occurring above polar latitudes at the end of the Antarctic and Arctic winters. These polar-focused O₃ depletions result in the development of so-called ozone holes during the early springtime. These phenomena have been observed regularly since the early 1980s. The O₃ holes over Antarctica are particularly extensive and typically involve decreases of the O₃ concentration of 30-50% during the spring. Smaller depletions of O₃ occur above the Arctic, including northern Canada. The affected areas in the Northern Hemisphere are much smaller than their counterparts in Antarctica.

The sizes of the ozone holes in Antarctica vary from year to year, but there has been a trend of strong increases since 1980, when the first accurate data began to be collected (Figure 20.6). Although the seasonal losses of O₃ only take place over polar regions, lower latitudes are also affected when the O₃-depleted air becomes dispersed during the breakup of the holes. This temporarily reduces the stratospheric O₃ concentrations throughout the hemisphere, though not nearly to the same degree as occurs in the holes themselves.

Figure 20.6. Antarctic Ozone Hole Area. Changes in the maximum and minimum annual size of the ozone “hole” over Antarctica. Source: visualization by OurWorldinData.org, data from NASA Ozone Watch, is licensed under CC BY 4.0.

Air Pollution and Health

An extraordinary case of a natural emission of gas causing human deaths involved the release of a large volume of CO₂ from a lake in Cameroon, West Africa. Lake Nyos is a 200-m-deep volcanic lake in which the deep waters are naturally supersaturated with CO₂, similar to bottled soda water. One night in 1986, a large amount of sediment apparently slumped into the steep-sided lake, causing some of its bottom water to churn to the surface. The water de-gassed its CO₂ content as a dense air mass, which then flowed into low areas in the surrounding landscape. The CO₂-rich air asphyxiated about 1,700 sleeping people and 3,500 livestock as far as 25 km from the lake, plus uncounted wild animals. Atmospheric CO₂ is capable of causing severe toxicity at concentrations greater than 8-10%; its “normal” level is about 0.04%. Plants are much less vulnerable to CO₂ toxicity, so no vegetation was damaged by this rare and astonishing natural event.

Anthropogenic emissions of other gaseous pollutants have sometimes caused increases in human mortality and diseases. Some people, especially those with chronic respiratory or heart diseases, are especially vulnerable to the effects of air pollution. Exposures of people to toxic gases can occur within several contexts, including the following.

• The ambient environment: The urban atmosphere typically contains relatively high concentrations of potentially toxic chemicals. This is true in general, but air quality is especially bad during smog events, often caused by poor dispersion during an atmospheric inversion (Image 20.1). Consequently, city people living their normal lives are routinely exposed to higher concentrations of air pollutants than those living in cleaner, rural environments.
• The working environment: Many people are exposed to high concentrations of pollutants as a consequence of their occupation. Of course, the specific exposures depend on the job – workers in metal smelters may be exposed to sulfur dioxide and metallic particulates, auto mechanics may be affected by exhaust fumes containing carbon monoxide and hydrocarbons, and laboratory workers may inhale various organic solvents.
• The indoor environment: Buildings are often contaminated by gases and fumes. For example, space heaters, furnaces, and fireplaces burning wood, kerosene, or fuel oil may emit carbon monoxide into the indoor environment. All high-temperature combustions emit nitric oxide, and many synthetic materials and fabrics vent formaldehyde and other organic vapors. These chemicals can accumulate if indoor air is not exchanged frequently with cleaner, outdoor air.
• Tobacco smoke: The smoking of tobacco is a leading source of easily avoidable air pollution. Smoking is also the most important cause of preventable diseases, especially lung cancer and heart disease (see Chapter 19). People inhale a great variety of toxic gases and fumes when they smoke tobacco (and also marijuana). In addition, non-smokers are indirectly exposed to lower concentrations of those chemicals because of the lingering residues of “second-hand smoke” that may occur in indoor atmospheres.

Image 20.1. Smog in Denver, Colorado. Smog is becoming a growing problem in the western United States due to increased pollution from Asia. Source: National Renewable Energy Laboratory via NOAA.

All of these exposures to air pollutants have important implications for human health. However, the pollution of the ambient urban environment is the focus of the following paragraphs.

Since the beginning of the Industrial Revolution in Western Europe in the mid-18th century, people living in cities and working in certain types of factories have been exposed to high concentrations of air pollutants. Especially important have been sulfur dioxide, soot, and other emissions associated with the combustion of coal and other fossil fuels. The most severe exposures to pollutants in urban environments typically occurred during prolonged atmospheric inversions, which prevent the dispersion of emissions and result in smogs rich in SO₂ and particulates.

Coal has long been used in many places to heat homes and other buildings. The associated emissions have been regarded as a problem in cities and towns in Europe since at least 1500. With the beginning of the Industrial Revolution, which initially used coal as its principal energy source, air pollution worsened markedly. The first convincing link between air pollution and a substantial increase in the death rate of an exposed human population was made in 1909, in relation to a noxious smog during an inversion in Glasgow, Scotland, when about one-thousand deaths may have been caused.

The most infamous “killer smog” in North America occurred in 1948 in Donora, Pennsylvania. An inversion and fog persisted in the Donora Valley for four days, but emissions from several factories continued, resulting in a build-up of high concentrations of SO₂ and particulates in the atmosphere. The smog resulted in increased mortality in the local population (20 deaths in a population of only 14 100). An additional 43% of the population became ill, 10% severely so. The most common symptoms were irritation of the eyes and respiratory tract, sometimes accompanied by coughing, headache, and vomiting.

The world’s most notorious killer smog afflicted London, England, in 1952, when an extensive inversion and fog stabilized over southern England. In London, emissions of pollutants, mostly from coal combustion, transformed the natural “white fog” into a venomous “black fog.” Visibility was terrible – people lost their way while walking or driving, even falling off wharves into the Thames River, and airplanes became lost while trying to taxi at the airport. The smog lasted for four days, but it was followed by another 14 days with a higher-than-usual death rate. Overall, about 3,900 deaths were attributed to this episode of noxious pollution. Most of the affected people were elderly or very young, or had pre-existing respiratory or heart diseases.

Until the early 1960s, severe episodes of urban air pollution were common in the cities of North America and Western Europe. Most of the smogs were caused by the widespread burning of coal in fireplaces and furnaces in homes, electrical utilities, and factories. The poor-quality urban air affected the health of people and animals and also damaged vegetation. In many cities, only certain kinds of plants that can tolerate air pollution could grow. Examples of pollution-tolerant trees that are commonly grown in urban America include black locust (Robinia pseudoacacia), honey locust (Gleditsia triacanthos), salt cedar (Tamarix), linden (Tilia europaea), tree-of-heaven (Ailanthus altissima), and ginkgo (Ginkgo biloba) (Heggestad et al., 1972).

To deal with the problems of this kind of smog, governments brought in legislation that has required large reductions in the emissions of air pollutants, particularly in cities. In Canada, for example, the enactment of various federal, provincial, and municipal laws related to air emissions has substantially improved urban air quality. Air quality has been similarly improved under legislation enacted in the U.S., Britain, and other wealthier countries since the 1960s.

Of course, the killer smogs were particularly severe events of air pollution. More typically, the urban atmosphere is contaminated by much smaller concentrations of SO₂, NO_x, O₃, volatile organic compounds, and particulates. Many studies have investigated the effects of chronic exposures to those lower exposures to air contaminants on human health. The results of some studies suggest that modern urban air quality is sufficiently degraded to cause chronic damage to human health, especially by increasing the incidences of lung disease, asthma, and eye irritation. However, other studies have not found this to be the case. In any event, effective actions have been taken in the U.S. and other relatively wealthy nations. Visibly threatening, even lethal, episodes of air pollution like those described above no longer occur in those countries, although they could return if control standards were relaxed.

Unfortunately, in the cities of countries with rapidly growing economies, such as Brazil, China, India, Indonesia, and Mexico, poorly regulated industrial and urban growth is resulting in awful declines in air quality. Although not yet well studied in terms of human diseases, these appear to be modern tragedies of urban air pollution.

Conclusions

Gaseous air pollutants, such as sulfur dioxide and nitric oxide, are emitted from a variety of sources, which range from large power plants and smelters to individual automobiles and home furnaces. In contrast, secondary pollutants such as ozone are not emitted but are formed in the atmosphere by photochemical reactions involving sunlight and emitted oxides of nitrogen and hydrocarbons. If their concentrations are high enough, gaseous pollutants (often in combination with particulates) can be a risk to human health, and they may cause severe ecological damage. Because these risks are now well known, many governments have taken steps to reduce the emissions of the most important air pollutants. This is particularly the case for relatively developed countries, such as the U.S. Although the emissions of air pollutants in wealthy countries remain large, they are generally stabilizing or even decreasing. However, in rapidly growing economies, such as China and India, hasty and poorly controlled industrialization is resulting in rapidly worsening air pollution.

Questions for Review

1. Compare the natural and anthropogenic emissions of sulfur and nitrogen compounds. Why do the sources of emission vary between regions and countries?
2. Why are high concentrations of ozone in the lower atmosphere considered an environmental problem? Why does this differ from the stratosphere, where too little ozone is a problem?
3. Explain the differences between primary and secondary air pollutants? Give examples of each.
4. Why has air pollution decreased so much in the eastern U.S., and what have been the ecological responses to this environmental improvement?

Questions for Discussion

1. Existing clean-air technologies could be used to greatly reduce the emissions of air pollutants everywhere. Considering the damage that pollutants cause to human health, ecosystems, and other values, why are these technologies not being used more extensively? Consider factors associated with economics, politics, scientific uncertainty about pollution damage, and the benefits of having cleaner air. Contrast the lack of action with the successes achieved in controlling the emissions of ozone-depleting substances through the Montreal Protocol.
2. Epidemiological (statistical) research suggests that human health may be affected by ambient levels of air pollutants in urban areas, particularly through increased incidences of respiratory diseases, such as asthma. However, the statistical data are rather weak, and only a relatively small proportion of the urban population appears to be affected. What are some issues that decision makers must consider when deliberating about additional controls on the release of air pollutants in urban areas?

Exploring Issues

1. You have been asked to assess the potential ecological effects of building a new metal smelter in a region that is now wilderness. The smelter will emit sulfur dioxide to the atmosphere. Based on what you know about pollution damage, what would be the most important considerations to incorporate into the environmental impact assessment? Focus on the potential effects on terrestrial and aquatic ecosystems.

References Cited and Further Reading

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Ayres, J. and R.L. Maynard. 2006. Air Pollution and Health. World Scientific Publishing Company.

Barker, J.R. and D.T. Tingey. 1992. Air Pollution Effects on Biodiversity. Van Nostrand Reinhold, New York, NY.

Brimblecombe, P. 1996. Air Composition and Chemistry. 2nd ed. Cambridge University Press, Cambridge, UK.

Brimblecombe, P. 2005. The Effects of Air Pollution on the Built Environment. Imperial College Press, London, UK.

Centers for Disease Control and Prevention. 2020. Air Pollution. Climate and Health, Centers for Disease Control and Prevention. https://www.cdc.gov/climateandhealth/effects/air_pollution.htm. Accessed June 14, 2021.

Frias, L. 2016. County of Los Angeles Public Health, Environmental Health. http://publichealth.lacounty.gov/eh/tea/toxicepi/criteriaairpollutants.htm. Accessed June 14, 2021.

Department of Environmental Conservation (DEC) 2019. Air Quality Health Advisory Issued for Long Island. Department of Environmental Conservation, New York. https://www.dec.ny.gov/press/117675.html. Accessed June 14, 2021.

Environmental Protection Agency (EPA). 2019. Our Nation’s Air. https://gispub.epa.gov/air/trendsreport/2020/#home. Accessed June 14, 2021.

Environmental Protection Agency (EPA). 2020a. NAAQS Table. https://www.epa.gov/criteria-air-pollutants/naaqs-table. Accessed June 14, 2021.

Environmental Protection Agency (EPA). 2020b. Overview of Greenhouse Gases, Methane Emissions. https://www.epa.gov/ghgemissions/overview-greenhouse-gases#methane. Accessed June 14, 2021.

Environmental Protection Agency (EPA). 2020. Interactive Map of Air Quality. AirNow, U.S. Air Quality Index. https://gispub.epa.gov/airnow/. Accessed June 14, 2021.

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Heggestad, H.E., Santamour, F.S., and Bernstein, L. 1972. Plants that will withstand pollution and reduce it. American Society of Landscape Architects Foundation, United States Department of Agriculture. 16-22 https://naldc.nal.usda.gov/catalog/IND79000653.

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Seinfeld, J.H. and S.H. Pandis. 2006. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd ed. Wiley-Interscience, New York, NY.

Shriner, D.S. 1990. Responses of vegetation to atmospheric deposition and air pollution. In Acidic Deposition: State of Science and Technology. Vol. III. Terrestrial, Materials, Health, and Visibility Effects. Superintendent of Documents, U.S. Government Printing Office, Washington, DC.

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U.S. Energy Information Association. 2020. International Energy Statistics, China. September 30, 2020. https://www.eia.gov/international/analysis/country/CHN. Accessed June 14, 2021.

Vallero, D. 2014. Fundamentals of Air Pollution, 5th ed. Academic Press, San Diego. Wark, K. Jr., C.F. Warner, and W.T. Davis. 1998. Air Pollution: Its Origin and Control. 3rd ed. Addison-Wesley, Menlo Park, CA.

Watson, W.Y. and D.H. Richardson. 1972. Appreciating the potential of a devastated land. Forestry Chronicle, 48: 312-315.

Wise, W. 2001. Killer Smog: The World’s Worst Air Pollution Disaster. Author’s Guild, New York, NY.