High School Earth Science/Air Pollution

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Earth's atmosphere supports life by providing the necessary gases for photosynthesis and respiration. The ozone layer protects life on Earth from the Sun's ultraviolet radiation. People also use the atmosphere as a dump for waste gases and particles.

Pollutants include materials that are naturally-occurring but present in larger quantities than normal. In addition, pollutants consist of human-made compounds that have never before been found in the atmosphere. Pollutants dirty the air, change natural processes in the atmosphere, and harm living things. Excess greenhouse gases raise global temperatures.

Lesson Objectives[edit]

  • Describe the different types of air pollutants.
  • Discuss what conditions lead some cities to become more polluted than others.
  • Describe the sources of air pollutants.

Air Quality[edit]

Figure 22.1: A film crew recreates London smog in the Victorian Era.

Air pollution problems began centuries ago when fossil fuels began to be burned for heat and power. The problem grew into a crisis in the developed nations in the mid-20th century. Coal smoke and auto exhaust combined to create toxic smog that in some places caused lung damage and sometimes death.

In Donora, Pennsylvania in October 1948, 20 people died and 4,000 became ill when coal smoke was trapped by an inversion. Even worse, in London in December 1952, the "Big Smoke" killed 4,000 people over five days, and it is likely that thousands more died of health complications from the event in the next several months (Figure 22.1).

A different type of air pollution became a problem in Southern California after World War II. Although there was no coal smoke, cars and abundant sunshine produced photochemical smog. This smog is the result of a chemical reaction between some of the molecules in auto exhaust or oil refinery emissions, and sunshine. Photochemical smog consists of more than 100 compounds, most importantly ozone.

In the United States, these events led to the passage of the Clean Air Act in 1970. The act now regulates 189 pollutants. The six most important pollutants are ozone, particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, and the heavy metal lead. Other important regulated pollutants include benzene, perchloroethylene, methylene chloride, dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds. Some of these will be discussed in the following section.

Besides human-caused emissions, air quality is affected by environmental factors. A mountain range can trap pollutants on its leeward side. Winds can move pollutants into or out of a region. Pollutants can become trapped in an air mass as a temperature inversion traps cool air beneath warm air. If the inversion lasts long enough, pollution can reach dangerous levels. Pollutants remain over a region until they are transported out of the area by wind, diluted by air blown in from another region, transformed into other compounds, or carried to the ground when mixed with rain or snow.

As a result of the Clean Air Act, air in the United States is much cleaner. Visibility is better and people are no longer incapacitated by industrial smog. Still, in the United States, industry, power plants and vehicles put 160 million tons of pollutants into the air each year. Some of this smog is invisible and some contributes to the orange or blue haze that affects many cities (Figure 22.2).

Figure 22.2: Smog over Los Angeles as viewed from the Hollywood Hills.
Table 22.1: Smoggiest Cities, 2007
Rank City, State
1 Los Angeles, California
2 Bakersfield, California
3 Visalia-Porterville, California
4 Fresno, California
5 Houston, Texas
6 Merced, California
7 Dallas-Fort Worth, Texas
8 Sacramento, California
9 New York, New York
10 Philadelphia, Pennsylvania

Table 22.1 lists the smoggiest cities in 2007: six of the 10 are in California. The state has the right conditions for collecting pollutants including mountain ranges that trap smoggy air, arid and sometimes windless conditions, and lots and lots of cars.

Types of Air Pollution[edit]

Most air pollutants enter the atmosphere directly; these are primary pollutants. Secondary pollutants become pollutants only after undergoing a chemical reaction. Primary pollutants include toxic gases, particulates, compounds that react with water vapor to form acids, heavy metals, ozone, and greenhouse gases. Ozone is one of the major secondary pollutants. It is created by a chemical reaction that takes place in exhaust and in the presence of sunlight.

Primary Pollutants[edit]

Some primary pollutants are natural, such as dust and volcanic ash, but most are caused by human activities. Primary pollutants are direct emissions from vehicles and smokestacks. Some of the most harmful pollutants that go directly into the atmosphere from human activities include:

  • Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO2 are both greenhouse gases.
  • Nitrogen oxides are produced when nitrogen and oxygen from the atmosphere come together at high temperatures. This occurs in hot exhaust gas from vehicles, power plants or factories. Nitrogen oxide (NO) and nitrogen dioxide (NO2) are greenhouse gases. Nitrogen oxides contribute to acid rain.
  • Sulfur oxides include sulfur dioxide (SO2) and sulfur trioxide (SO3). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain.
  • Particles are solid particles, such as ash, dust and fecal matter. They are commonly formed from combustion of fossil fuels, and can produce smog. In addition, particulate matter can contribute to asthma, heart disease, and some types of cancers.
  • Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal causes can cause brain damage or blood poisoning.
  • Volatile organic compounds (VOCs) are mostly hydrocarbons, compounds made of hydrogen and carbon. Important VOCs include methane (a naturally occurring greenhouse gas that is increasing due to human activities), chlorofluorocarbons (human-made compounds that are being phased out because of their effect on the ozone layer), and dioxin (a byproduct of chemical production that serves no useful purpose, but is harmful to humans and other organisms).

Photochemical Smog[edit]

Any city can have photochemical smog, but it is most common in arid locations. A rise in the number of vehicles in cities worldwide has increased photochemical smog. This smog forms when car exhaust is exposed to sunlight. Nitrogen oxides are created in car combustion chambers. If there is sunshine, the NO2 splits and releases an oxygen atom (O). The oxygen ion then combines with an oxygen molecule (O2) to form ozone (O3). This reaction can also go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O2. The direction the reaction proceeds depends on how much NO2 and NO there is. If NO2 is three times more abundant than NO, ozone will be produced. If nitrous oxide levels are high, ozone will not be created.

Ozone is an acrid-smelling, whitish gas. Warm, dry cities surrounded by mountains, such as Los Angeles, Phoenix, and Denver, are especially prone to photochemical smog (Figure 22.3). Photochemical smog peaks at midday on the hottest days of summer. Other compounds in addition to ozone are found in photochemical smog. Ozone is also a greenhouse gas.

Figure 22.3: Counties with such high ozone levels that they do not attain federal air quality standards.

Causes of Air Pollution[edit]

Most air pollutants come from burning fossil fuels or plant material. Some are the result of evaporation from human-made materials. Nearly half (49%) of air pollution comes from transportation, 28% from factories and power plants, and the remaining pollution from a variety of other sources.

Fossil Fuels[edit]

Fossil fuels are burned in most motor vehicles and power plants (Figure 22.4). They fuel manufacturing and other industries. Pure coal and petroleum can theoretically burn cleanly, emitting only carbon dioxide and water, which are both greenhouse gases. But most of the time, these fossil fuels do not completely burn, so these incomplete chemical reactions produce pollutants. In addition, few fossil fuels are pure and so other pollutants are usually released. These pollutants include carbon monoxide, nitrogen dioxide, sulfur dioxide and hydrocarbons.

Figure 22.4: A power plant and its emissions before emission control equipment was added.

In large car-dependent cities such as Los Angeles and Mexico City, 80% to 85% of air pollution is from motor vehicles. Auto emissions are the most common source of ozone. Carbon monoxide is toxic in enclosed spaces like tunnels. Nitrous oxides come from the exhaust from a vehicle or a factory. Lead was once put in gasoline to improve engine knock, but is now banned in the United States. Still, enormous quantities of lead are released into the air every year from other sources.

A few pollutants come primarily from power plants or industrial plants. They pour out of smokestacks that burn coal or oil. Sulfur dioxide (SO2) is a major component of industrial air pollution. It is released whenever coal and petroleum are burned. SO2 mixes with H2O in the air to produce sulfuric acid (H2SO4). The heavy metal mercury is released when coal and some types of wastes are burned. Mercury is emitted as a gas, but as it cools, it becomes a droplet. Mercury droplets eventually fall to the ground. If they fall into sediments, bacteria convert them to the most dangerous form of mercury: methyl mercury. Highly toxic, methyl mercury is one of the metal's organic forms.

Biomass Burning[edit]

Fossil fuels are ancient plants and animals that have been converted into usable hydrocarbons. Burning plant and animal material directly also produces pollutants. Biomass is the total amount of living material found in an environment. The biomass of a rainforest is the amount of living material found in that rainforest.

The primary way biomass is burned is by slash-and-burn agriculture (Figure 22.5). The rainforest is slashed down and then the waste is burned to clear the land for farming. Biomass from other biomes, like savannah, is also burned to clear farmland. The pollutants are much the same as from burning fossil fuels: CO2, carbon monoxide, methane, particulates, nitrous oxide, hydrocarbons, and organic and elemental carbon. Burning forests increase greenhouse gases in the atmosphere by releasing the CO2 stored in the biomass and also by removing the forest so that it cannot store CO2 in the future. As with all forms of air pollution, the smoke from biomass burning often spreads far and pollutants can plague neighboring states or countries.

Figure 22.5: A forest that has been slash-and-burned to make new farmland.

Particulates result when anything is burned. About 40% of the particulates that enter the atmosphere above the United States are from industry and about 17% are from vehicles. Particulates also occur naturally from volcanic eruptions or windblown dust. Like other pollutants, they travel all around the world on atmospheric currents.


Volatile organic compounds (VOCs) enter the atmosphere by evaporation. VOCs evaporate from human-made substances, such as paint thinners, dry cleaning solvents, petroleum, wood preservatives, and other liquids. Naturally occurring VOCs evaporate off of pine and citrus trees. The atmosphere contains tens of thousands of different VOCs, nearly 100 of which are monitored. The most common is methane, a greenhouse gas. Methane occurs naturally, but human agriculture is increasing the amount of methane in the atmosphere.

Lesson Summary[edit]

  • Industrial pollution causes health problems and even death, though the Clean Air Act has decreased these health problems in the United States by forcing industry to clean their emissions.
  • The increase in motor vehicles in arid cities has increased ozone and other secondary pollutants in these regions.
  • Burning fossil fuels is the greatest source of air pollution.
  • Biomass burning is also a large source, especially in places where slash-and-burn agriculture is practiced.

Review Questions[edit]

  1. What is the difference between the type of smog experienced by cities in the eastern United States and that found in Southern California?
  2. London has suffered from terrible air pollution for at least seven centuries. Why is the city so prone to its famous "London fog"? What did London do to get rid of its air pollution?
  3. Imagine two cities of the same size with the same amount of industrialization and the same number of motor vehicles. City A is incredibly smoggy most of the time and City B usually has very little air pollution. What factors are important for creating these two different situations?
  4. What might be a reason why the city of San Francisco and its metropolitan area not on the list of smoggiest cities for 2007?
  5. Why are naturally-occurring substances, like particles or carbon dioxide, sometimes considered pollutants?
  6. How does ozone form from vehicle exhaust?
  7. What are the necessary ingredients for ozone creation, excluding those that are readily available in the atmosphere? Why could there be a city with a lot of cars but relatively little ozone pollution?
  8. Some people say that we need to phase out fossil fuel use and replace it with clean energy. Why is fossil fuel use becoming undesirable?
  9. Mercury is not particularly toxic as a metal but it is very dangerous in its organic form. How does mercury convert from the metal to the organic form?
  10. In what two ways does deforestation contribute to air pollution?


A heavy metal found in a large number of products. Exposure to too much lead causes lead poisoning, which harms people's brain and blood.
A heavy metal that enters the atmosphere primarily from coal-burning power plants. Mercury that has been converted to an organic form (methylmercury) is highly toxic.
Particles like ash, dust, and fecal matter in the air. Particulates may be caused by natural processes, such as volcanic eruptions or dust storms, or they may be caused by human activities, like burning fossil fuels or biomass.
photochemical smog
This type of air pollution results from a chemical reaction between pollutants in the presence of sunshine.
slash-and-burn agriculture
In the tropics, rainforest plants are slashed down and then burned to clear the land for agriculture.
volatile organic compounds (VOCs)
Pollutants that evaporate into the atmosphere from solvents and other humanmade compounds. Some VOCs occur naturally.

Points to Consider[edit]

  • Despite the Clean Air Act, the air over many regions in the United States is still not clean. Why?
  • How do pollutants damage human health?
  • In what ways does air pollution harm the environment?

Human Actions and the Atmosphere · Effects of Air Pollution