4 Air Pollution

Jean Brainard

The Atmosphere

Water (1-4% near the Earth’s surface) has so many unique properties (adhesion, surface tension, cohesion, capillary action, high heat capacity, high heat of vaporization…and more) that it is difficult for us to imagine any form of life on any planet which does not depend on it. As a major component of the hydrologic cycle, the atmosphere cleans and replenishes Earth’s fresh water supply, and refills the lakes, rivers, and oceans habitats for life (Figure 4.1). The Earth’s atmosphere thins but reaches away from its surface for 100 kilometers toward space; between about 15 and 35 km lies the Ozone Layer – just a few parts per million which shields life from the sun’s damaging Ultra-Violet radiation. Earth’s atmosphere appears ideal for life, and indeed, as far as we know it is the only planetary atmosphere which supports life.

Figure 4.1 A composite photo of satellite images shows Earth and its life-supporting waters and atmosphere.

As we noted in the History of Life chapter, the Earth’s atmosphere has not always been this hospitable for life. Life itself is probably responsible for many dramatic changes, including the addition of oxygen by photosynthesis, and the subsequent production of ozone from accumulated oxygen. Changes in CO2 levels, climate, and sea level have significantly altered conditions for life, even since the addition of oxygen some 2 billion years ago. On a daily time scale, dramatic changes take place:

  • most organisms remove O2 and add CO2 through cellular respiration
  • most autotrophs remove CO2 and add O2 through photosynthesis
  • plants transpire vast quantities of water into the air
  • precipitation returns it, through gentle rains or violent storms, to the Earth’s surface

On a human time scale, the daily dynamics balance, and the atmosphere remains at equilibrium – an equilibrium upon which most life depends.

This is what the atmosphere looks like viewed edge on from space. The image is of a small cross-sectional area, note the small curvature of the surface, yet the atmosphere is a small part of the whole. Looking closely, you can see tall thunderstorm clouds silhouetted against an orange layer of atmospheric gases backlit by the sun just below the horizon. Above this layer is the clear blue of the stratosphere and the blackness of space. From NASA Space Shuttle Flight 6 on 4 April 1983.

Composition of the Atmosphere

The atmosphere is composed of 78.08% nitrogen and 20.95% oxygen with small amounts of other gases: 0.93% argon, 0.038% carbon dioxide, 0.002% neon, and yet smaller concentrations of helium, methane, krypton, and hydrogen. Both nitrogen and oxygen exist in large quantities only because of life on earth, especially life in the ocean.

It would seem that the composition of the atmosphere would be stratified with different chemical composition at different heights. In fact, mixing in the atmosphere causes the composition to be nearly uniform up to about 80 km.

Ozone is a very important trace gas in the atmosphere. It exits in two places:

  1. In the stratosphere at heights around 20-30 km. This is good ozone. It protects all life on earth from dangerous solar ultraviolet radiation (energy).
  2. Close to the surface due to pollution. It is produced from nitrogen oxides and volatile carbon-based compounds when there is intense solar radiation (energy), above all in the spring and summer. This is bad ozone. It causes respiratory illness; it damages plants; and it attacks rubber.

Types of Air Pollutants

Despite the atmosphere’s apparent vastness, human activities have significantly altered its equilibrium in ways which threaten its services for life. Chemical substances, particulate matter, and even biological materials cause air pollution if they modify the natural characteristics of the atmosphere. Primary pollutants are directly added to the atmosphere by processes such as fires or combustion of fossil fuels (Figure 4.2). Secondary pollutants, formed when primary pollutants interact with sunlight, air, or each other, can be equally damaging. The chlorine and bromine which threaten the Ozone Layer are secondary pollutants, formed when refrigerants and aerosols (primary pollutants) decompose in the stratosphere (Figure 4.3).

Figure 4.2 – Burning fossil fuels

Figure 4.3 – Levels of sun-blocking aerosols declined from 1990 to the present. A corresponding return to pre-1960 levels of radiation suggests that pollution control measures in developed countries have counteracted Global Dimming. However, particulates are still a problem in developing countries, and could affect the entire global community again in the future. Aerosol increases in 1982 and 1991 are the result of eruptions of two volcanoes, El Chichon and Pinatubo.

The majority of air pollutants can be traced to the burning of fossil fuels. We burn fuels in power plants to generate electricity, in factories to power machinery, in stoves and furnaces for heat, in airplanes, ships, trains, and motor vehicles for transportation, and in waste facilities to incinerate waste. Since long before fossil fuels powered the Industrial Revolution, we have burned wood for heat, fireplaces, and campfires and vegetation for agriculture and land management. The resulting primary and secondary pollutants and the problems to which they contribute are included in Table 4.1 below.

Pollutant Example/Major Source Problem
Sulfur oxides (SOx) Coal-fired power plants Acid Rain
Nitrogen oxides (NOx) Motor vehicle exhaust Acid Rain
Carbon monoxide (CO) Motor vehicle exhaust Poisoning
Carbon dioxide (CO2) All fossil fuel burning Global Warming
Particulate matter (smoke, dust) Wood and coal burning Respiratory disease, Global Dimming
Mercury Coal-fired power plants, medical waste Neurotoxicity
Smog Coal burning Respiratory problems; eye irritation
Ground-level ozone Motor vehicle exhaust Respiratory problems; eye irritation

Beyond the burning of fossil fuels, other anthropogenic (human-caused) sources of air pollution are shown in Table 4.2.

Activity Pollutant Problem
Agriculture: Cattle Ranching
Herbicides and Pesticides
Methane (CH4)
Ammonia (NH3), Volatile Organic Chemicals(VOCs)
Persistent Organic Pollutants(POP): DDT, PCBs, PAHs*
Global Warming
Toxicity, Global Warming
Global Dimming
Industry (solvents, plastics)
Refrigerants, Aerosols
VOCs, POPs CFCs Cancer, Global Warming Ozone Depletion
Nuclear power and defense Radioactive waste Cancer
Landfills Methane (CH4) Global Warming
Mining Asbestos Respiratory problems
Biological Warfare Microorganisms Infectious Disease
Indoor Living CO, VOCs, asbestos, dust, mites, molds, particulates Indoor air pollution
  • DDT = an organic pesticide; PCB = poly-chlorinated biphenyls, used as coolants and insulators; DDT and most PCBs are now banned at least in the U.S., but persist in the environment; PAHs = polycyclic aromatic hydrocarbons – products of burning fossil fuels, many linked to health problems
 Many processes contribute to atmospheric pollution and trace gases. Click on image for a zoom. From US Strategic Plan for the Climate Change Science Program, Final Report July 2003Chapter 3 Atmospheric Composition.


The important sources of atmospheric pollution on a global or regional scale are:

  1. Automobiles. According to the U.S. Environmental Protection Agency (EPA), driving a car is the single most polluting thing that most of us do. Motor vehicles emit millions of tons of pollutants into the air each year. In many urban areas, motor vehicles are the single largest contributor to ground-level ozone, a major component of smog. The primary pollutants produced by automobiles are:
    1. Hydrocarbons. They come from the evaporation of fuel, especially on hot days, leaking fluids, and during refueling at gas stations.
    2. Nitrogen oxides. Produced by high heat during the burning of fuel.
    3. Carbon monoxide. Produced by the incomplete burning of fuel.

    Modern automobiles pollute much less than older models thanks to emission controls including catalytic converters. But the number of cars is so large, and they are driven so much, they are still major sources of pollution.

  2. Urban activity. The world’s population is concentrated more and more in mega cities, with five urban areas having more than 20 million people: Tokyo, Japan (34,997,000); Mexico City (22,800,000); Seoul, South Korea (22,300,000); New York (21,900,000); and São Paulo, Brazil (20,200,000). Urban air pollution is now common in all large cities, worse on some days, better on others, but never gone.
     From Air Pollution in Mexico City by Pierre Madl of Salzburg University Sound and Video Studio. It is caused not only by emissions from cars, trucks, buses and lawnmowers (operating a lawn mower for one hour produces as much pollution as driving a car 100 miles), but also by fumes from drying paint, charcoal fires (grills), and dry cleaners.
     A huge traffic jam backs up the streets in Bangkok. High population density in large urban areas leads to air pollution. Click on image for a zoom. From Patagonia.

    Urban activity leads to photochemical smog in many areas such as Los Angeles, Houston, Mexico City, and London, the archetype of a smoggy city (The term smog was coined by Dr. Henry Antoine Des Voeux in 1905, when he combined the words smoke and fog). In London, the smoke came from the burning of coal to heat thousands of houses. The London smog began in the middle ages, and extreme smog events led to periodic attempts to reduce air pollution. The great smog of 5-9 December 1952 killed more than 4,700 people during the event, and led to an additional 8,000 deaths in the year following the event. During the event, visibility was reduced to 20 m over an area of 20 by 40 km (Boubel et al, 1994) and deaths reached 900 per day. To ensure that such an event would never happen again, parliament passed the UK Clean Air Act of 1956. Photochemical smog is formed when sunlight acts on volatile carbon-based molecules and nitrous oxides trapped below inversions above cities. The sunlight powers chemical reaction that form harmful pollutants such as tropospheric ozone, aldehydes, and peroxyacyl nitrates (PAN). Here is an outline of some important chemical reactions leading to smog: The high temperature in automobile and diesel engines converts nitrogen gas to nitrous oxide. N2 + O2 —–> 2 NO (nitric oxide) In the atmosphere, nitric oxide is converted to nitrogen dioxide NO2, a brown gas which gives smog its characteristic color. 2 NO + O2 ——> 2 NO2 When nitrogen dioxide concentrations are high, sunlight leads to the formation of ozone. NO2 + sunlight ———-> NO + O O + O2 ———> O3. NO2 + O2 + hydrocarbons + sunlight ———-> CH3CO-OO-NO2 (peroxyacetylnitrate).

     Los Angeles smog on 29 January 2004. The top of the inversion layer is easily seen against the backdrop of distant mountains. Hilltops above the layer are visible at great distances, urban areas below the layer are obscured. Click on image for a zoom. Photo by Alan Clements, Middlesbrough, England.
     This is what the smog in Los Angeles looks like from the ground, a thick brown or slightly orange haze with a strong smell of ozone. In this scene, the inversion is below the top of the highest buildings. The exhaust from more than a million cars driven in the morning rush hour is trapped below this level. Click on image for a zoom. From Larvalbug article Choking on Air.


  3. Agricultural burning.
  4. Forest fires.

    The October wildfires that plagued Southern California.

    Wildfires and smoke on 23 October 2007 in southern California. The fires burned 800 square miles, and area almost 2/3 the size of Rhode Island. The smoke from the fires is clearly visible over the Pacific ocean on the left of the image. Red spots mark the location of the fires. Click on image for zoom. From NASA California Wildfires. Other similar images are at the NOAA site: Operational Significant Event Imagery. NOAA issues daily Fire Products, including a map showing all forest fires in North America, and information of Fire Events worldwide.

  5. Industrial activity Smelters, steel mills, oil refineries and chemical plants, paper mills, manufacturing plants, and power plants, especially coal-fired plants are the major sources. But even relatively clean industries such as semiconductor fabrication plants, which make computer chips, also contribute. Many of the worst polluters were in the format Soviet Union. Fortunately, industrial emissions are being greatly reduced as nations become richer.
     V.I. Lenin Steel mill, Magnitogorsk, 1991. From Monroe Gallery of Photography, photographed by Shepard Sherbell. Dust storms. Strong winds blowing across desert regions lift dust high into the troposphere. The higher-level winds then carry dust great distances. The Sahara, the Aral Sea, and Mongolia are notorious sources.
  6. Dust Storms (see the chapter titled Land Degredation and Desertification for more detalis).
     Dust blown from the Sahara across the Atlantic on 24 July 2005. Dust is colored yellow-brown in the image. From NOAA Dust Storm site. NASA has a catalog of images of dust storms.

Many pollutants travel indoors.

Pollutants from the air travel into building materials, furniture, carpeting, paints and varnishes, contributing to indoor air pollution. In 2002, the World Health Organization estimated that 2.4 million people die each year as a consequence of air pollution – more than are killed in automobile accidents. Respiratory and cardiovascular problems are the most common health effects of air pollution, but accidents which release airborne poisons (the nuclear power plant at Chernobyl, the Union Carbide explosion in Bhopal, and the “Great Smog of 1952” over London) have killed many people – and undoubtedly other animals – with acute exposure to radiation or toxic chemicals.

If you study the problems caused by air pollution (third column in the tables, above), you will note that beyond human health, air pollution affects entire ecosystems, worldwide. Acid Rain, Ozone Depletion, and Climate Change are widespread and well-recognized global concerns, so we will explore them in detail in independent sections of this lesson, – and an entire lesson on Climate Change. Effects of toxins, which poison wildlife and plants as well as humans, were addressed in discussions of soil and water pollution in the last chapter. Before we move on to the “Big Three,” let’s take a brief look at the problems caused by particulates and aerosols, since these are unique pollutants of air, rather than soil or water.

Global Dimming

“Global dimming” refers to a reduction in the amount of radiation reaching the Earth’s surface. Scientists observed a drop of roughly 4% between 1960 and 1990, and attributed it to particulates and aerosols (in terms of air pollution, aerosols are airborne solid particles or liquid droplets). These pollutants absorb solar energy and reflect sunlight back into space. The consequences for life are many:

  • Less sunlight means less photosynthesis.
  • Less photosynthesis means less food for all trophic levels.
  • Less sunlight means less energy to drive evaporation and the hydrologic cycle.
  • Less sunlight means cooler ocean temperatures, which may lead to changes in rainfall, drought and famine.
  • Less sunlight may have cooled the planet, masking the effects of Global Warming.

Recent measurements of sunlight-absorbing particulates show a decline since 1990, which corresponds to a return to normal levels of radiation. These data suggest that Clean Air legislation enacted by developed nations may have improved air quality and prevented most of the above effects, at least for now. Two caveats remain:

  1. If “Global Dimming” did indeed mask Global Warming for 30 years, predictions about future climate change may be too conservative. Keep this in mind when we address Global Warming in the next lesson.
  2. Population growth and industrialization of developing countries continues to increase levels of pollution.

Massive waves of pollution from Asian industry have blown across the Pacific by prevailing winds (Figure 4.4). On some days, atmospheric physicists at the Scripps Institution of Oceanography have traced nearly one-third of the air over Los Angeles and San Francisco directly to Asian sources. The waves are made of dust from Asian deserts combined with pollution from increasing industrialization, making the level of particulates and aerosols in Beijing, for example, reach levels 7 times World Health Organization standards. Scientists estimate that the clouds may be blocking 10% of the sunlight over the Pacific. By seeding clouds, the aerosols and particulates may be intensifying storms. In addition to direct effects on the global atmosphere (such waves can circle the Earth in three weeks), these pollution clouds can, as we stated above, mask Global Warming.

Figure 4.4 – A cloud of smoke and haze covers this region of China from Beijing (top center) to the Yangtze River (bottom right). At the top right, pollution is blowing eastward toward Korea and the Pacific Ocean. Aerosol pollution with large amounts of soot (carbon particles) is changing precipitation and temperatures over China. Some scientists believe that these changes help to explain increasing floods and droughts.

Light Pollution

One additional topic relates to atmospheric change. Light pollution (Figure 4.5) results from humans’ production of light in amounts which are annoying, wasteful, or harmful. Light is essential for safety and culture in industrial societies, but reduction in wasteful excess could mitigate its own harmful effects, as well as the amounts of fossil fuel used to generate it. Astronomers – both amateur and professional – find light interferes with their observations of the night skies. Some studies show that artificial spectra and excessive light exposure has harmful effects on human health. Life evolved in response to natural cycles and natural spectra of light and dark, so it is not surprising that our changes in both of those might affect us and other forms of life. Light pollution can affect animal navigation and migration and predator/prey interactions. Because many birds migrate by night, Toronto, Canada has initiated a program to turn out lights at night during spring and fall migration seasons. Light may interfere with sea turtle egg-laying and hatching, because both happen on coasts at nighttime. The behavior of nocturnal animals from owls to moths can be changed by light, and night-blooming flowers can be affected directly or through disruption of pollination. Zooplankton normally show daily vertical migration, and some data suggests that changes in this behavior can lead to algal blooms.

Figure 4.5 – When light produced by humans becomes annoying, wasteful, or harmful, it is considered light pollution. This composite satellite image of Earth at night shows that light is concentrated in urban

Solutions to problems caused by light pollution include

  • reducing use
  • changing fixtures to direct light more efficiently and less harmfully
  • changing the spectra of light released
  • changing patterns of lighting to increase efficiency and reduce harmful effects

Acid Rain

Acid rain is a common name for the deposition of acidic material from the atmosphere either as:

  1. Wet deposition of acid in precipitation (rain, snow, or fog); or
  2. Dry deposition of acidic material on dust, smoke, or other aerosols (small, microscopic particles in the air).

Acid rain was a major problem in Europe and the USA in the last few decades of the 20th century. Strong emission control laws have greatly reduced the problem in these areas. However, acid rain continues to be a major problem in some developing countries, especially China.

Here both types of deposition will be covered.

Do you remember the pH scale? Its range is 0-14, and 7 is neutral – the pH of pure water. You’ve probably measured the pH of various liquids such as vinegar and lemon juice, but do you know how important even very small changes in pH are for life? Your body maintains the pH of your blood between 7.35 and 7.45, and death results if blood pH falls below 6.8 or rises above 8.0. All life relies on relatively narrow ranges of pH, because protein structure and function is extremely sensitive to changes in concentrations of hydrogen ions. An important pollution problem which affects the pH of Earth’s environments is Acid Rain (Figure below).

Acidity of precipitation is measured in pH units, where

pH ≈ –log[H+]

where H+ is the dissolved hydrogen ion concentration in a weak solution in water. The lower the pH the more acidic the precipitation, the higher the pH the more basic the precipitation. Pure water water has a pH of 7.0, and pure rain has a ph of 5.6 because carbon dioxide dissolved in water forms a weak acid, carbonic acid, H2CO3.

H2O + CO2 –> H2CO3

 pH scale from Environmental Protection Agency, pH Scale.

The pH of precipitation from very polluted air can be less than 2 in extreme cases. Mostly, the pH of precipitation ranges from 4.4 to 5.8.

 Acidity of precipitation measured by the National Atmospheric Deposition Program in 2006. Notice that precipitation is most acidic downwind of the large concentration of power plants in the Ohio Valley.

Sources of Acid rain

The acidic materials come from sulfur dioxide (SO2), ammonia (NH3), nitrogen oxides (NOx) and acidic particles emitted into the atmosphere by burning of fossil fuels in power plants and cars. In the United States, roughly 2/3 of all SO2 and 1/4 of all NOx come from burning of fossil fuels, especially coal, in electric power plants.

  Left: Coal-fired power plants emit large quantities of acid pollutants into the atmosphere, although the volume of pollutants has been decreasing as scrubbers that remove pollutants from exhaust gases have become more widely used. The image shows exhaust from the American Electric Power’s Gen. James M. Gavin Plant in Chesire in Gallia County, in the Ohio Valley. It is one of the largest coal-fired power plants in Ohio. Most of the visible exhaust is condensed vapor, but the brownish haze includes acids. From Ohio.Com of the Akron Beacon Journal, article Ohio EPA cites area for soot problems. Right: Scrubber at base of Georgia Power’s Bowen Plant removes 95% of the sulfur dioxide in the plant’s exhaust gas. Click on the image to bring up a diagram of how a scrubber works from Scrubber freshens smokestack by Wade Rawlins, Staff Writer for the News Observer. From Rome-News Tribune.

Rain, snow, fog, dew, and even dry particles which have an unusually low pH are commonly considered together as Acid Rain, although more accurate terms would be acid precipitation or acid deposition. You will remember that a pH below 7 is acidic, and the range between 7 and 14 is basic. Natural precipitation has a slightly acidic pH, usually about 5, mostly because CO2, which forms 0.04% of the atmosphere, reacts with water to form carbonic acid:

CO2 + H2O ⇌ H2CO3 ⇌ HCO3 + H+
carbon dioxide water carbonic Acid bicarbonate hydrogen ion

This natural chemical reaction is actually quite similar to the formation of acid rain, except that levels of the gases which replace carbon dioxide are not normally significant in the atmosphere. The most common acid-forming pollutant gases are oxides of nitrogen and sulfur released by the burning of fossil fuels. Because burning may result in several different oxides, the gases are often referred to as “NOx and SOx.” This may sound rather affectionate, but it’s more accurate to think of it as obNOXious! Whereas the carbonic acid formed by carbon dioxide is a relatively weak acid, the nitric and sulfuric acids formed by NOx and SOx are strong acids, which ionize much more readily and therefore cause more damage. The reactions given below slightly simplify the chemistry (in part because NOx and SOx are complex mixtures of gases), but should help you see the acidic results of an atmospheric mixture of water and these gases.

NO2 + OH → HNO3 ⇌ NO3 + H+
nitrogen dioxide

hydroxide ion (from water)

nitric Acid nitrate hydrogen ion
SO3 + H2O → H2SO4 ⇌SO4-2 + 2H+
sulfur trioxide water sulfuric acid sulfate hydrogen ions

Nitrogen and sulfur oxides have always been produced in nature by volcanoes and wildfires and by biological processes in wetlands, oceans, and even on land. However, these natural levels are either limited in time or amount; they account for the slightly acidic pH of “normal” rain. Levels of these gases have risen dramatically since the Industrial Revolution began; scientists have reported pH levels lower than 2.4 in precipitation in industrialized areas. Generation of electricity by burning coal, industry, and automobile exhaust are the primary sources of NOx an SOx. Coal is the primary source of sulfur oxides, and automobile exhaust is a major source of nitrogen oxides.

Acid rain thus occurs when these gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. The result is a mild solution of sulfuric acid and nitric acid. When sulfur dioxide and nitrogen oxides are released from power plants and other sources, prevailing winds blow these compounds across state and national borders, sometimes over hundreds of miles. – What is Acid Rain, EPA.

Most acid rain falls downwind of power plants. In the USA, many are located in the mid-west, and acid rain is common there and throughout the east coast. As power plant emissions are increasingly regulated, the amount of acid rainfall has decreased. Total annual emissions of SO2 in the USA dropped from 28.8 × 106 metric tons in 1978 to 17.8 × 106 metric tons in 1998.

Acid rain deposition in the USA from 1983 through 1997. From: Driscoll (2001).

Affects on Vegetation and Animals

Because most life requires relatively narrow pH ranges near neutral, the effects of acid rain can be devastating. In soils, lowered pH levels can kill microorganisms directly, altering decomposition rates, nutrient cycles, and soil fertility. A secondary effect of increased acidity is the leaching of nutrients, minerals, and toxic metals such as aluminum and lead from soils and bedrock. Depletion of nutrients and mobilization of toxins weakens trees and other plants, especially at higher altitudes where higher precipitation and acid fog damage leaves and needles, as well (Figure below).

A mountain forest in the Czech Republic shows effects attributed to acid rain. At higher altitudes, effects on soils combine with direct effects on foliage of increased precipitation and fog.

The flow of acid rain through watersheds increases acidity, nutrients, and toxins in aquatic ecosystems. Fish and insects are sensitive to changes in pH, although different species can tolerate different levels of acidity (Figure below). Food chain disruption can compound even slight changes in pH; for example, acid-sensitive mayflies provide food for less-sensitive frogs.

Aquatic species show varying sensitivity to pH levels. Colored bars show survival ranges. Trout are more sensitive to increasing acidity than frogs, but mayflies, which frogs consume, are even more sensitive. Consequently, changes in a lake

In some regions, especially regions where granite is close to the surface and where soils have been degraded by logging and forest fires, the soil has little ability to neutralize the acid. In these regions, acid deposition depletes the available plant-nutrient cations Ca2+, Mg2+, and K+, it increases the leaching of aluminum, and it increases the amount of sulfur and nitrogen in the soil. All lead to weakening of trees, leading to their death by bark beetle infestations and disease.

Effect on abiotic items

Another class of victims of acid rain is entirely within the realm of human culture and history. Acid’s ability to corrode metal, paints, limestone, and marble has accelerated erosion of buildings, bridges, statues, monuments, tombstones, and automobiles (Figure below).

Acid rain accelerates erosion of statues, monuments, buildings, tombstones, bridges, and motor vehicles.

Attempts to solve the problem of acid rain began with building taller smokestacks. These only sent the polluting gases higher into the atmosphere, relieving local problems temporarily, but sending the damage to areas far from their industrial sources. Today in the U.S. and other western nations, smokestacks increasingly use “scrubbers” which remove as much as 95% of SOx from exhausts; the resulting sulfates “scrubbed” from the smokestacks can sometimes be sold as gypsum (used in drywall, plaster, fertilizer and more), but may also be landfilled. Catalytic converters and other emission control technologies remove NOx from motor vehicle exhaust. However, population growth and development throughout the world is increasing pressures to use more fossil fuels and high-sulfur coal, often without these expensive technologies.

Ozone Depletion

Ozone is found in two regions of the atmosphere:

  1. In the stratosphere at heights around 20–30 km, where it is produced by sunlight. This is good ozone. It is critical for life because it protects all life on earth from dangerous solar ultraviolet radiation, especially UVB, a band of ultraviolet radiation with wavelengths from 280–320 nanometers produced by the sun. Ultraviolet radiation with wavelengths from 320–400 nanometers, UVA, is not absorbed, and it is much less dangerous to life.
  2. Close to the surface, where it is produced by sunlight acting on atmospheric pollutants. It is produced from nitrogen oxides and volatile carbon-based compounds when there is intense sunshine, above all in the spring and summer. This is bad ozone. It causes respiratory illness; it damages plants; and it attacks rubber.

Many people confuse the “hole in the ozone” with “global warming.” Although the two are related in part, they are separate problems with separate effects and only partially overlapping causes, so they require separate solutions.

At altitudes less than 5 kilometers, respiratory irritant

Ozone is both a threat and a gift (Figure below). As a ground-level product of the interaction between sunlight and pollutants, it is considered a pollutant which is toxic to animals’ respiratory systems. However, as a component of the upper atmosphere, it has shielded us and all life from as much as 97-99% of the sun’s lethal UV radiation for as long as 2 billion years. The “hole” in the ozone develops in this thin upper Ozone Layer. How long will that protection continue? Let’s explore the problem of ozone depletion.

The ozone cycle involves the conversion of oxygen molecules to ozone (1 and 2) a slower reconversion of ozone molecules to oxygen (3). Interactions among ozone molecules or the presence of other reactive gases trigger the loss of ozone.

The Ozone (O3) Layer forms when UV radiation strikes oxygen molecules (O2) in the stratosphere, between 15 and 35 kilometers above the Earth’s surface. Even the highest concentrations of ozone are only about 8 parts per million, but ever since photosynthesis oxygenated the Earth’s atmosphere, allowing ozone-forming chemical reactions, this thin Ozone Layer has shielded life from the mutagenic effects of ultraviolet radiation – especially the more damaging UV-B and UV-C wavelengths (Figure above).

Total global monthly ozone levels measured by three successive spectrometers (TOMS) show both seasonal variations and a general decline.

The thickness of the Ozone Layer varies seasonally and across the Earth – thicker in Spring than in Autumn, and at the Poles compared to near the Equator. Ozone depletion describes two related declines in stratispheric ozone. One is loss in the total amount of ozone in the Earth’s stratosphere – about 4% per year from 1980 to 2001 (Figure below). The second, much larger loss refers to the ozone hole – a seasonal decline over Antarctica (Figures below and 14), which has now lost as much as 70% of pre-1975 ozone levels. A much smaller “dimple” overt the North Pole has also shown a 30% decline. The Antarctic ozone hole occasionally affects nearby Australia and New Zealand after annual breakup. A secondary effect is the decline in stratosphere temperatures, because when ozone absorbs UV radiation, it is transformed into heat energy.

On September 24, 2006 the seasonal ozone hole over the Antarctic covered a record daily area (29.5 million square kilometres or 11.4 million square miles). Blue and purple areas show the lowest ozone levels, and green, yellow, and red indicate successively higher levels.

Lowest annual values of ozone in the ozone hole decreased dramatically between 1980 and 1995. Before 1980, values less than 200 Dobson units were rare, but in recent years, values near 100 units are common. Unusually high temperatures in the Antarctic stratosphere may have caused the high reading in 2002.

Ozone is Good Up High Bad Nearby. It is good when it occurs in the stratosphere, where it absorbs ultraviolet radiation (energy) from the sun. It is bad when it occurs close to the ground in the troposphere, where it is a pollutant. Tropospheric ozone irritates the respiratory system, aggravates asthma and bronchitis, and it inflames the lining of the lungs. It harms vegetation and agricultural crops, and it damages rubber and other materials.

Ozone is the major component of smog. It is produced from nitrogen oxides and volatile carbon-based compounds when there is intense solar radiation (energy), above all in the spring and summer. See The Physics and Chemistry of of Ozone. For more information: see the Environmental Protection Agency’s page

Chemical reactions leading to ozone formation in cities. Click on image for a zoom. From Environmental Protection Agency.


The causes of ozone depletion are gases which unbalance the ozone cycle (Figure above) toward the breakdown of ozone. Chlorine and bromine gases have increased due to the use of chlorfluorocarbons (CFCs) for aerosol sprays, refrigerants (Freon), cleaning solvents, and fire extinguishers. These ozone-depleting substances (ODS) escape into the stratosphere, and when UV radiation frees chlorine and bromine atoms, these unstable atoms break down ozone. Scientists estimate that CFCs take 15 years to reach the stratosphere, and can remain active for 100 years. Each chlorine atom can catalyze thousands of ozone breakdown reactions.

Ozone depletion and the resulting increase in levels of UV radiation reaching earth could have some or all of the following consequences:

  • effects on human health
  • increase in skin cancers, including melanomas
  • increased incidence of cataracts
  • decreased levels of vitamin A
  • possible increase in levels of vitamin D produced by the skin
  • reduced abundance of UV-sensitive nitrogen-fixing bacteria
  • loss of crops dependent on these bacteria
  • disruption of nitrogen cycles
  • loss of plankton (supported by a supernova-related extinction event 2 million years ago)
  • disruption of ocean food chains

Most of these effects are based on the ability of UV radiation to alter DNA sequences. It is this potential which has made the Ozone Layer such a gift to life ever since photosynthesis provided the oxygen to fuel its production. Its total loss would undoubtedly be devastating to nearly all life.

In 1987, 43 nations agreed in the Montreal Protocol to freeze and gradually reduce production and use of CFCs. In 1990, the protocol was strengthened to seek elimination of CFCs for all but a few essential uses. Today, Hydrochlorofluorocarbons (HCFCs – similar compounds which replace one chlorine with a hydrogen) have replaced CFCs, with only 10% of their ozone-depleting activity levels. Unfortunately, HCFCs are greenhouse gases (see next lesson), so their role as alternatives is a mixed blessing. HFCs (hydrofluorocarbons) are another substitute; because these contain no chlorine, they have no ozone-depleting activity, and their greenhouse effect is less than HCFCs (though still significant). One HFC is currently used in automobile air conditioners in the U.S.

If ozone-depleting substances have been virtually eliminated, is ozone depletion no longer a problem?

Unfortunately, we have not yet reached that point. Levels of CFCs in the atmosphere are beginning to decline, and ozone levels appear to be stabilizing (Figures above and 14) for years after 2000). Scientists predict that ozone levels could recover by the second half of this century; the delay is due to the long half-life of CFCs in the stratosphere. However, recovery could be limited or delayed by two unknowns:

  1. Developing countries outside the Montreal Protocol could increase their use of CFCs.
  2. According to scientists, global warming would cool the stratosphere and increase ozone depletion because cooler temperatures favor ozone decomposition.

Preventing Air Pollution

Throughout this lesson, we have discussed solutions to specific problems for our atmosphere. A quick recap of ways to maintain our atmosphere and its ecosystem services from this chapter includes:

  • Reducing use of fossil fuels
  • Switching to cleaner fuels, such as nuclear power
  • Switching to renewable energy sources
  • Increasing fuel efficiencies
  • Supporting legislation for fuel efficiencies
  • Supporting national and international agreements to limit emissions
  • Utilizing pollution control technologies: e.g., scrubbers on smokestacks and catalytic converters for motor vehicles
  • Creating and supporting urban planning strategies

As always, costs are high and tradeoffs must be considered. The classic example is nuclear power, whose effects on the atmosphere are less than those of fossil fuels. Unfortunately, it has high potential for health damage and high costs – both economic and environmental – for storage and transport of nuclear waste.



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Air Pollution by Jean Brainard is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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