Air Pollution

Air pollution, or the contamination of the atmosphere with harmful substances, is generated from natural and human-made sources. Natural sources of air pollution include gases, particulate matter from volcanoes, decomposing organic matter, pollen from plants, and windblown dust. Human-made sources of air pollution include industrial and automobile emissions and airborne particles associated with human-induced abrasion.

Earth’s Atmosphere

Air pollution results from the unusual addition of gases, solids, and liquids to the atmosphere. The concentration of pollutants depends on prevailing atmospheric conditions as well as emission rates. Once pollutants are put into the atmosphere, it is impossible to control them to any significant degree. Thus, emissions at the local level contribute to regional and global air pollution problems, such as smog and photochemical oxidants, acid precipitation, the depletion of the ozone layer, and global warming associated with the intensification of the greenhouse effect. Although there are many air pollutants, the major ones are usually associated with burning, particularly the burning of fossil fuels and oil-carbon dioxide based products. They are generally unburned hydrocarbons, oxides of sulfur and nitrogen, carbon monoxide, various photochemical oxidants and reactive compounds, and particulate matter from many different sources.

The atmosphere is a mixture of gases, aerosols, and particulate matter surrounding Earth. The concentration of some of the gases in clean air is fairly constant both spatially and temporally. Consequently, these gases are referred to as stable or permanent gases. Nitrogen and oxygen, the two most abundant permanent gases, account for 78 percent and 21 percent of the total atmosphere by volume, respectively. Gases that experience noticeable temporal and spatial variations are termed variable gases. The two most abundant of these are water vapor and carbon dioxide. The average concentration of carbon dioxide is about 0.034 percent. It varies seasonally in response to the growth cycle of plants, daily in response to plant photosynthesis, and spatially in response to the burning of fossil fuels. Water vapor is also highly variable. Some variable gases have natural origins and tend to have relatively high concentrations in urban areas. They are methane, carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, ammonia, and hydrogen sulfide.

The atmosphere is stratified according to its vertical temperature gradient. From the ground surface up, the major layers are the troposphere, the stratosphere, the mesosphere, and the thermosphere. The troposphere contains the bulk of atmospheric gases and, under normal conditions, is characterized by a fairly uniform temperature decline from the surface upward. The uppermost limit of the troposphere is called the tropopause, a transition zone between the troposphere and stratosphere where temperatures stabilize with increasing altitude. The troposphere extends up to about 10 kilometers. The next layer encountered is the stratosphere, which extends from about 12.5 kilometers up to about 45 kilometers above the surface. In its lower layer, the temperature gradient is somewhat stable. At an elevation of about 30 kilometers, however, the temperature starts to increase. Located within the stratosphere about 24 to 32 kilometers above the earth’s surface is a zone with a relatively high concentration of ozone, a triatomic form of oxygen. This zone is called the ozone layer. It is important because the ozone absorbs most of the incoming ultraviolet rays emitted by the sun, preventing them from reaching the surface where they would have harmful effects on plant and animal life.

The two uppermost layers, the mesosphere and the thermosphere, have a distinctive temperature gradient. In the mesosphere, temperatures decline steadily with altitude, a condition that continues until its transition zone with the thermosphere, called the mesopause, is reached. At latitudes of 50 degrees north and higher, mesospheric clouds known as noctilucent clouds are sometimes seen during summer. These clouds may be anthropogenic in origin. The last layer, the thermosphere, slowly gives way to outer space and has no defined upper limit.

Atmospheric Inversions and Smog

Vertical and horizontal mixing of air is necessary to dilute pollutants in the atmosphere. Under normal conditions, temperatures decline with altitude in the troposphere. This decline in temperature with altitude is referred to as the thermal or environmental lapse rate. The warmer air near the surface rises, mixes with the air above it, and is dispersed upward by winds. This dilution process is important in reducing the concentration of pollution near the surface. Conversely, the vertical mixing of air is inhibited when the temperature profile in the troposphere inverts, developing one type of atmospheric inversion: a temperature or thermal inversion. When an inversion exists, a layer of warmer air becomes sandwiched between two layers of cooler air above and below, and the warmer, less dense air does not rise as it would normally. Pollutants can then accumulate below the warmer air as vertical mixing is prevented.

Conditions for temperature inversions develop when the earth readily radiates heat energy from its surface on clear nights or when air subsides and warms adiabatically from compression. On cool, clear nights, the earth readily radiates heat energy to space, cooling the surface. Air near the surface is, in turn, cooled by conduction, while the air above it is still relatively warm. This condition is referred to as a radiation inversion. Radiation inversions are common during autumn and are usually short-lived, as the rising sun in the morning heats the air near the surface, causing the inversion to dissipate as the day advances. Less frequent but more persistent subsidence-type inversions can occur when cooler air subsides in high-pressure systems or in valleys, as cooler, denser air descends along adjacent mountain slopes. Subsidence inversion episodes may last for days, allowing pollutants to concentrate to excessive levels, causing eye irritation, respiratory distress, reduced visibility, corrosion of materials, and soiling of clothes.

The atmosphere has inherent self-cleansing mechanisms. Pollutants are removed from the atmosphere through fallout due to gravitational settling, through rainout in condensation and precipitation processes, through washout as waterdrops and snowflakes accumulate pollutants as they fall to Earth, and through chemical conversion. Solar radiation, winds, and atmospheric moisture are important meteorological factors in these removal processes.

Chemical reactions between two or more substances in the atmosphere produce secondary pollutants, which are those created from other pollutants that have been released directly into the atmosphere from identifiable sources. Smog is a product of such reactions. Stability in the atmosphere that accompanies inversions provides favorable conditions for smog to develop. Smog is produced by chemical and photochemical reactions involving primarily sulfur oxides, hydrocarbons, and oxides of nitrogen. Smog that is characterized by sulfur oxides is called sulfurous smog and is associated with the burning of fuels having a relatively high sulfur content. This type of smog is common in developing countries. Photochemical smog develops when oxides of nitrogen and various hydrocarbons undergo photochemical reactions to produce ozone and other chemical oxidizers. Sunlight promotes the reactions, and automobile exhaust is a primary source of nitrogen oxides. This is the type of smog typically encountered in large cities and urban centers such as Los Angeles and Mexico City.

Acid Precipitation

While smog is a relatively localized phenomenon closely associated with urban areas, acid precipitation is a more widespread phenomenon. Its effects are observed in national parks, agricultural regions, forested areas, and lakes and other bodies of water as well as in urban centers. Acid rain develops when oxides of sulfur or nitrogen combine with water vapor in the atmosphere to form sulfuric and nitric acids that fall back to Earth in precipitation. Once released into the atmosphere, these oxides and the compounds formed from them can travel great distances before returning to Earth in precipitation or as dry particulates, as much as 1,000 to 2,000 kilometers over three to five days. This long-range transport allows time for chemical reactions to convert pollutant gases into components of acid precipitation. Evidence suggests that the pH values in precipitation have been dropping, becoming more acidic, for some years. Wet precipitation is not the only way pollutants find their way to the surface. Diffusion and settling enable acidic gases and particles to find their way to the ground even under dry conditions. It is now widely accepted that both wet and dry deposition can be traced to human activity.

Much evidence has been gathered documenting the damaging effects of acid precipitation. These effects include damage to wildlife in lakes and rivers, reduction of forest productivity, damage to agricultural crops, and deterioration of human-made materials. Acid precipitation, also suspected of promoting the release of heavy metals from soils and pipelines into drinking water supplies, has different effects on different ecological systems and is most damaging to aquatic ecosystems. Acidity in precipitation at a given time depends not only on the type and quantity of pollutants being produced but also on the prevailing and immediate atmospheric conditions. Stagnant air, resulting from upper-level inversions, tends to cause higher levels of acidity. Furthermore, prevailing and local atmospheric systems are associated with the spread of acid precipitation over broader areas. Higher exhaust stacks, while minimizing levels of air-borne pollutants locally, simply disperse the pollutants over larger areas, thus increasing their residence time in the atmosphere.

Ozone Depletion and Climate Change

The impact of air pollution is more far-reaching than the troposphere. Evidence indicates that pollutants making their way up to the stratosphere cause the ozone layer to break down or dissipate. Even though ozone constitutes a very small portion of the atmosphere, only about one part per million, it absorbs almost all of the ultraviolet rays from the sun, preventing them from reaching Earth’s surface. Research findings from satellite-based monitoring systems beginning in the 1970s showed that there had been a breakdown of the ozone shield over the Antarctic, where a hole was identified in the ozone layer. Subsequent research and satellite-based observation showed that the ozone layer also appeared to be thinning over the Arctic.

While early laboratory studies showed that oxides of nitrogen could attack ozone, attention later focused on chlorofluorocarbons (CFCs) as being responsible for the decline in ozone. These compounds were widely used as refrigerants in common household appliances and air conditioning systems, propellants in aerosol sprays, agents for producing foam, and cleansers for electronic products because they have low boiling points, facile compressibility, and very low chemical reactivity. Behaving much like inert gases, they do not degrade readily in the troposphere but eventually make their way into the stratosphere. Laboratory studies have shown that when the CFC molecules come in contact with ozone and ultraviolet light, they enter into a complex series of gas-phase reactions by which they are converted into more reactive gases, such as chlorine.

The United States banned the use of CFCs and the Montreal Protocol banned the substances effective in 1989. However, since these gases tend to linger in the atmosphere for many years, the ozone layer continued to decline for years. Stability was achieved in the 1990s and in the twenty-first century ozone layers began to increase.

Further evidence suggests that CFCs not only destroy the ozone but also trap heat energy radiated from the ground and contribute to heating the atmosphere. The trapping of sensible heat energy in the atmosphere by gases is called the greenhouse effect. One of the most important gases contributing to the greenhouse effect, however, is not a chlorofluorocarbon but carbon dioxide. Carbon dioxide moves in a continuous cycle throughout the environment. It provides a link between the organic and inorganic components of the environment. Reacting with water and solar energy through photosynthesis in plants, it forms glucose that is subsequently passed through the food chain as a source of energy required by essentially all animal species.

Carbon dioxide is given off by plants and animals to the atmosphere during respiration. When plant and animal remains decay, carbon dioxide is passed back to the atmosphere and hydrosphere through the most natural processes of decomposition. When fossil fuels are burned, however, those natural processes are short-circuited, and large amounts of carbon dioxide are released directly into the atmosphere.

About 0.04 percent of the total atmosphere is carbon dioxide. Molecules of carbon dioxide in the atmosphere absorb and retain infrared radiation as heat, in much the same way that the glass walls of a greenhouse reflect heat back into the structure rather than allowing it to escape. While it is transparent to shortwave radiation from the sun, carbon dioxide absorbs strongly in the sensible heat or longwave radiation band. It is hypothesized that an increase in atmospheric carbon dioxide causes a decrease in outgoing longwave radiation and thus an increase in the atmospheric temperature.

The consequences of rising global temperatures will greatly alter the earth’s surface. As the atmosphere warms, polar ice and glaciers will begin to melt and the rising oceans could flood many of the world’s coastal regions, devastating low-lying countries. As shorelines rise, saltwater intrusion will contaminate the drinking-water supplies of many cities worldwide. Agricultural regions of the middle-latitude countries will migrate farther northward, increasing the length of the growing seasons in nations such as Canada and Russia.

Study of Air Pollution

Methods of studying air pollution include controlled laboratory experiments, simulations in fluid-modeling facilities, computer simulations, and mathematical modeling. Controlled laboratory experiments are conducted in laboratories where gases are mixed to determine how they react. Laboratory experiments usually do not provide the definitive answer to what is actually occurring in the ambient environment because many variables cannot be replicated. These studies suggest what should be further studied and monitored in the natural environment. Some laboratory studies have simulated atmospheric conditions in a controlled environment, such as a biosphere where the impact of pollution on plants can be determined by introducing the pollutants at various levels. Simulation studies may also include gathering data from fluid-modeling facilities, where the environment is replicated using miniature models and atmospheric conditions are controlled. These studies often contribute to an understanding of the dispersion and deposition of air pollutants.

Monitoring the atmosphere is an essential component of air pollution studies involving computer simulations and mathematical modeling. These types of studies rely largely on data sources or values from the ambient environment and are constrained by difficulties of measuring ambient levels. Sometimes, vessels containing samples of air are collected and returned to the laboratory for analysis, but continuous monitoring devices that are placed in the ambient environment are more common.

Many of the monitoring devices involve a colorimetric or photometric technique. Air to be analyzed is isolated and subjected to conditions in which carefully selected gas phase reactions can produce specific compounds from the pollutants that are present. The reaction product is then analyzed by photometric techniques, in which the concentration of light-absorbing substances is indicated by the light intensity that reaches the photometer. Particulate matter can be measured by fairly simple collectors that may use adhesive coated paper or filtration, and measuring the increase in weight resulting from the trapped particles. Another method involves passing a known volume of air through filter paper and measuring the intensity of light passing through it. The intensity of light indicates the scattering and absorptive properties of aerosols; it is expressed as a coefficient. Instruments may be located at the surface, mounted on airplanes, or allowed to ascend in balloons. Acidity in precipitation is determined by standard measures of acidity using a pH indicator.

In 2016, researchers found that exposure to air pollution increased the risk of developing metabolic dysfunction and childhood obesity. Specifically, when very young children breathe polluted air, they are much more prone to develop diet-induced weight gain and are more susceptible to insulin resistance as adults. For the study, pregnant laboratory rats and their newborns were placed in chambers containing either the highly polluted air of Beijing, China, or air that had been filtered with most pollutants removed. After almost three weeks of exposure to polluted air, the pregnant rats' air had heavier lungs and livers and significantly higher cholesterol and triglyceride levels. Additionally, their insulin resistance level, which medical professionals know is a precursor to Type 2 diabetes, was significantly higher than their counterparts who were breathing the filtered air.

In 2019, the World Health Organization (WHO) held a study that proved about 90 percent of the global population lived in areas with higher levels of fine particulate matter (PM) than the WHO deemed safe for long-term exposure. In 2021, the WHO released updated guidelines to tame outdoor air pollution levels. The new guidelines included recommendations for reduced time spent in areas with higher PM_2.5 concentrations, as well as new guidelines levels for other pollutants, including ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. The same year, the New York Times and several other papers reported on studies done by the Environmental Protection Agency (EPA) that showed that these contaminants, and air pollutants in general, disproportionately affected people of color. The study showed that people of color, and specifically Black people, were exposed to greater-than-average concentrations of PM_2.5 from every type of source, including from industry, construction, and residential areas. In 2022, the WHO reported 3.2 million deaths occurred in 2020 due to household air pollution, and the combination of household and ambient air pollution killed 6.7 million individuals globally.

Also in 2022, the WHO reported that 99 percent of the global population breathed air that did not meet the organization’s air quality standards. By early 2024, the Washington Post reported that only seven countries (Australia, Estonia, Finland, Grenada, Iceland, Mauritius, and New Zealand) met WHO air quality standards governing particulate matter. The EPA previously linked particulate matter exposure to several health issues that predominantly involve the heart and lungs.

Significance

Many industry officials continue to downplay the threat of pollution to the atmosphere and dispute the extent of damage caused by pollution. Yet as greater amounts of pollutants are released into the atmosphere, it becomes increasingly difficult to control their levels and reverse any resulting damage. Issues such as increasing levels of smog and acid rain are all very real and threatening manifestations of air pollution. Yet, as the example of CFCs and the hole in the ozone layer show, effective mitigation of these threats is possible. Efforts to protect the atmosphere must be made on a worldwide basis, as gases cannot be confined to political boundaries. Air pollution may be lowered by reducing emissions or by extracting pollutants from the atmosphere through natural means. Thus, any plans to reduce air pollution should center on one or both of these approaches.

Principal Terms

acid rain: precipitation having elevated levels of acidity relative to pure water

atmosphere: the layer of mixed gases that surrounds Earth

carbon dioxide: CO₂, one of many minor gases that are natural components of the atmosphere; the product of the complete oxidation of carbon

greenhouse effect: the environmental process that results when heat energy is absorbed and retained in the atmosphere by various gases and is not radiated out into space

inversion: an unusual atmospheric condition in which temperature increases with altitude

off-gassing: the spontaneous emission of entrained or entrapped gases from within natural and artificial sources

oxides of nitrogen: several gases that are formed when molecular nitrogen is heated with air during combustion, primarily NO and NO₂

oxides of sulfur: gases formed when fuels containing sulfur are burned, primarily SO₂

ozone: a highly reactive compound composed of three atoms of oxygen, as O₃

photochemical oxidants: pollutants formed in air by primary pollutants undergoing a complex series of reactions driven by light energy

photochemical reaction: a type of chemical reaction that can occur in polluted air driven by the interaction of sunlight with various pollutant gases

Bibliography

"Air Topics." United States Environmental Protection Agency, 15 Dec. 2022, www.epa.gov/environmental-topics/air-topics. Accessed 19 Apr. 2023.

Akhtar, Raid, and Cosimo Palagiano, eds. Climate Change and Air Pollution: The Impact on Human Health in Developed and Developing Countries. Springer, 2017.

Bandy, A. R., ed. The Chemistry of the Atmosphere: Oxidants and Oxidation in the Earth’s Atmosphere. Royal Society of Chemistry, 1995.

Bolius, David. Paleoclimate Reconstructions Based on Ice Cores: Results from the Andes and the Alps. SVH-Verlag, 2010.

Brimblecombe, Peter. Air Composition and Chemistry. 2nd ed. Cambridge UP, 1996.

Brimblecombe, Peter, et al., eds. Acid Rain: Deposition to Recovery. Springer, 2010.

"Billions of People Still Breathe Unhealthy Air: New WHO Data." World Health Organization, 4 April 2022, www.who.int/news/item/04-04-2022-billions-of-people-still-breathe-unhealthy-air-new-who-data. Accessed 3 Apr. 2024.

Cho, Kelly Kasulis. "Only Seven Countries Met WHO Air Standards in 2023, Report Says." The Washington Post, 20 Mar. 2024, www.washingtonpost.com/climate-environment/2024/03/20/iqair-air-quality-report-who/. Accessed 3 Apr. 2024

"Exposure to Air Pollution Increases the Risk of Obesity." Duke Today, 19 Feb. 2016, today.duke.edu/2016/02/airfat. Accessed 15 Apr. 2023.

Gomez, Alan. "EPA Proposes New Limits to Ozone Air Pollution." USA Today, 26 Nov. 2014, www.usatoday.com/story/news/politics/2014/11/26/epa-ozone-air-pollution-limits/19527431. Accessed 15 Apr. 2023.

Haerens, Margaret. Global Viewpoints: Air Pollution. Greenhaven, 2011.

Hopler, Paul. Atmosphere: Weather, Climate, and Pollution. Cambridge UP, 1994.

McGranahan, Gordon, and Frank Murray, eds. Air Pollution and Health in Rapidly Developing Countries. Earthscan, 2003.

Nadadur, Srikanth S., and John W. Hollingsworth. Air Pollution and Health Effects. Humana Press, 2015.

"Particulate Matter (PM) Basics." United States Environmental Protection Agency, 11 July 2023, www.epa.gov/pm-pollution/particulate-matter-pm-basics#effects. Accessed 3 Apr. 2024.

Pendergrass, Angeline G., and Dennis L. Hartmann. "Changes in the Distribution of Rain Frequency and Intensity in Response to Global Warming." Journal of Climate, vol. 27, no. 22, 2014, pp. 8372–83. doi.org/10.1175/JCLI-D-14-00183.1. Accessed 15 Apr. 2023.

Sportisse, Bruno. Fundamentals in Air Pollution: From Processes to Modelling. Springer, 2009.

Stolarski, Richard S. “The Antarctic Ozone Hole.” Scientific American, vol. 258, Jan. 1988, pp. 30–36.

Tiwary, Abhishek, and Jeremy Colls. Air Pollution: Measurement, Modelling and Mitigation. Routledge, 2010.

Vallero, Daniel. Fundamentals of Air Pollution. 5th ed. Academic Press, 2014.