7.1 Introduction to Air Pollution

Air pollution is the release of harmful gases and particles into the atmosphere from both human activities, especially fossil fuel and coal combustion, and natural sources like volcanoes. In AP Environmental Science, the key skill is identifying common pollutants, knowing their sources and health effects, and telling the difference between primary pollutants emitted directly and secondary pollutants formed by reactions in the atmosphere.

Why This Matters for the AP Environmental Science Exam

Unit 7 makes up about 7 to 10 percent of the AP Environmental Science exam, and this topic sets the foundation for everything else in the unit, including smog, thermal inversion, acid rain, and pollution reduction. On the exam you may be asked to identify a pollutant's source, describe its effect on human health or the environment, or sort pollutants as primary or secondary. You may also need to read graphs or data tables about pollutant levels and explain the patterns you see, and connect specific pollutants to the legislation that controls them.

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Key Takeaways

• Coal combustion releases carbon dioxide, sulfur dioxide, toxic metals, and particulates.
• Burning fossil fuels releases nitrogen oxides, carbon monoxide, hydrocarbons, and particulate matter; nitrogen oxides go on to form ozone, smog, and nitric acid.
• Sulfur dioxide is closely tied to burning fossil fuels, especially diesel fuels.
• Primary pollutants are emitted directly from a source; secondary pollutants form when primary pollutants react in the atmosphere, often with sunlight or water vapor.
• Through the Clean Air Act, the EPA regulated lead in fuels, which sharply reduced lead in the atmosphere.

Coal and Fossil Fuels

Burning coal and fossil fuels releases many gases and particles. Coal combustion releases carbon dioxide, sulfur dioxide, toxic metals (such as mercury and lead), and particulates. Fossil fuel combustion in general produces carbon monoxide, nitrogen oxides, sulfur dioxide, hydrocarbons, and particulate matter.

Because some of these gases exist in more than one form, they are often grouped as SOx (sulfur oxides), NOx (nitrogen oxides), and carbon oxides. The small "x" stands for the number of oxygen atoms in the chemical formula.

Sulfur dioxide deserves special attention because it is released mainly when fossil fuels are burned, especially diesel fuels. Air pollutants also come from other sources such as factories, volcanoes, and campfires, which release many of the same gases and particulates.

Primary and Secondary Pollutants

Air pollutants are sorted into two groups based on how they get into the air.

Primary pollutants are emitted directly from a source. Common sources include internal combustion vehicles, wildfires, factories, coal-burning power plants, agriculture, and volcanoes.

Secondary pollutants form when primary pollutants change in the atmosphere, often by reacting with sunlight and/or water vapor. Smog and acid precipitation are both examples of secondary pollutants.

Primary Pollutants

• Carbon monoxide (CO) is a colorless, odorless gas produced by the incomplete burning of fossil fuels such as gasoline and natural gas. It can cause headaches, dizziness, and nausea, and in severe cases coma and death.
• Nitric oxide (NO) is produced by burning fossil fuels and by the reaction of nitrogen and oxygen in the atmosphere. It can contribute to the formation of ground-level ozone and particulate matter.
• Nitrogen dioxide (NO2) comes from the same sources as nitric oxide. It contributes to ground-level ozone and particulate matter and can cause respiratory problems.
• Sulfur dioxide (SO2) is produced by burning sulfur-containing fossil fuels such as coal and oil. It contributes to particulate matter and acid rain and can cause respiratory problems.
• Ammonia (NH3) comes from the breakdown of organic matter and the use of fertilizers. It can contribute to particulate matter and reduce air quality.
• Volatile organic compounds (VOCs) are organic compounds that evaporate easily at room temperature. They come from industrial processes, paints, and cleaning products, and contribute to ground-level ozone and particulate matter.
• Particulate matter (PM) is a mixture of solid particles and liquid droplets suspended in the air. It is classified by size: PM10 is 10 micrometers or smaller, and PM2.5 is 2.5 micrometers or smaller. PM2.5 is especially harmful because it is small enough to be inhaled deep into the respiratory system.

Secondary Pollutants

• Sulfur trioxide (SO3) forms from burning sulfur-containing fossil fuels and can react with water vapor to form sulfuric acid.
• Sulfuric acid (H2SO4) forms when sulfur trioxide reacts with water vapor in the atmosphere. It contributes to particulate matter and acid rain.
• Nitric acid (HNO3) forms when nitrogen dioxide reacts with water vapor in the atmosphere. It contributes to particulate matter and acid rain.
• Ozone (O3) forms when nitrogen oxides and VOCs react in the presence of sunlight. Ground-level ozone can cause respiratory problems.
• Ammonium (NH4+) forms when ammonia reacts with acids in the atmosphere and can contribute to particulate matter.
• Particulate matter (PM) can be either primary (emitted directly) or secondary (formed by reactions in the air), so it shows up in both categories.

A quick way to keep these straight: if a pollutant is emitted straight from a tailpipe, smokestack, or volcano, it is primary. If it needed a chemical reaction in the atmosphere to form, it is secondary.

Air Quality and the Clean Air Act

Air quality depends on the amounts of different gases and particulates in the atmosphere. High levels of these pollutants can cause brown or grey smog, ozone warnings, or acid precipitation.

The Clean Air Act gives the Environmental Protection Agency (EPA) the authority to regulate pollutants that affect human health. One of its most important results was the regulation of lead in fuels, which dramatically reduced the amount of lead in the atmosphere. The same regulatory approach supports later efforts to reduce acid rain and protect the ozone layer.

How to Use This on the AP Environmental Science Exam

MCQ

• Be ready to match a pollutant to its source. For example, sulfur dioxide points to coal and diesel combustion, while nitrogen oxides point to vehicle and power plant exhaust.
• Sort pollutants as primary or secondary. Ozone, sulfuric acid, and nitric acid are classic secondary pollutants because they form from reactions in the atmosphere.
• Know that the Clean Air Act, enforced by the EPA, is the legislation behind the drop in atmospheric lead.

Free Response

• When asked to identify and describe a pollutant, name the pollutant, give its source, and state a specific health or environmental effect. Vague answers like "it is bad for you" do not earn points.
• If a prompt gives a graph or table of pollutant levels, describe the trend in words (increasing, decreasing, peaking at a certain time) before you explain why it happens.
• When proposing a solution, connect it back to a specific source. For example, reducing diesel combustion lowers sulfur dioxide and particulate matter.

Common Trap

• Do not assume every pollutant is human-made. Carbon dioxide and particulates also come from natural sources like volcanoes and wildfires.

Common Misconceptions

• Primary and secondary are not about how dangerous a pollutant is. The labels describe how it got into the air, not how harmful it is. A secondary pollutant like ozone can be very harmful.
• Particulate matter can be both primary and secondary. Some is emitted directly, and some forms from reactions in the atmosphere, so it does not fit only one category.
• Ground-level ozone is not the same as the ozone layer. Ground-level ozone is a harmful secondary pollutant you breathe, while stratospheric ozone high in the atmosphere protects us from UV radiation.
• The Clean Air Act did not ban all pollution. It sets standards and gives the EPA authority to regulate emissions; it is most famous here for cutting lead in fuels.
• Sulfur dioxide is not mainly from gasoline cars. It comes mostly from coal-burning power plants and diesel fuel, not typical gasoline vehicles.

Related AP Environmental Science Guides

• 7.2 Photochemical Smog
• 7.5 Indoor Air Pollutants
• 7.3 Thermal Inversion
• 7.6 Reduction of Air Pollutants
• 7.7 Acid Rain
• Unit 7 Overview: Atmospheric Pollution