Coal and Fossil Fuels
The burning of coal and fossil fuels releases many gasses and particles. Coal combustion will release carbon dioxide, sulfur dioxide, metals such as mercury and lead, and particulates. Fossil fuel combustion generates carbon monoxide, nitrogen oxides, sulfur dioxides, hydrocarbons, and particulates
As there are various forms of some of these gasses, they are often referred to as the SOx (sulfur oxides), NOx (nitrogen oxides), and carbon oxides. The small ‘x’ denotes the number of oxygens in the chemical formula.
There are other sources of air pollutants such as factories, volcanoes, and campfires. They produce most of the same gasses and particulates as coal and fossil fuels.
Primary and Secondary Pollutants
Many of the gasses undergo changes and are therefore referred to as either primary or secondary pollutants. Primary pollutants are those that are emitted directly from a source. Primary sources include internal combustion vehicles, wildfires, factories, coal-burning power plants, agriculture, and volcanoes.
Secondary pollutants have undergone a change from a primary pollutant. These changes are often due to the gasses interacting with water vapor and/or sunlight. Smog and acid precipitation are both examples of secondary pollutants.
Primary Pollutants
- Carbon monoxide (CO) is a colorless and odorless gas that is produced by the incomplete burning of fossil fuels, such as gasoline and natural gas. It is a primary pollutant that can have serious health effects, including headaches, dizziness, and nausea. In severe cases, it can lead to coma and death.
- Nitric oxide (NO) is a gas that is produced by the burning of fossil fuels and the reaction of nitrogen and oxygen in the atmosphere. It is a primary pollutant that can contribute to the formation of ground-level ozone and particulate matter.
- Nitrogen dioxide (NO2) is a gas that is produced by the burning of fossil fuels and the reaction of nitrogen and oxygen in the atmosphere. It is a primary pollutant that can contribute to the formation of ground-level ozone and particulate matter. It can also have adverse effects on human health, including respiratory problems.
- Sulfur dioxide (SO2) is a gas that is produced by the burning of fossil fuels that contain sulfur, such as coal and oil. It is a primary pollutant that can contribute to the formation of particulate matter and acid rain. It can also have adverse effects on human health, including respiratory problems.
- Ammonia (NH3) is a gas that is produced by the breakdown of organic matter and the use of fertilizers. It is a primary pollutant that can contribute to the formation of particulate matter and the degradation of air quality. It can also have adverse effects on human health, including respiratory problems.
- Volatile organic compounds (VOCs) are organic compounds that evaporate easily at room temperature and can contribute to air pollution. They are emitted by a variety of sources, including industrial processes, paints, and cleaning products. VOCs are a primary pollutant that can contribute to the formation of ground-level ozone and particulate matter. They can also have adverse effects on human health, including respiratory problems and cancer.
- Particulate matter (PM) is a mixture of solid particles and liquid droplets that are suspended in the air. It is a primary pollutant that can have adverse effects on human health, including respiratory problems and cardiovascular disease. PM can be emitted by a variety of sources, including power plants, industrial processes, and transportation. It is classified by size, with PM10 being particles that are 10 micrometers or smaller in diameter, and PM2.5 being particles that are 2.5 micrometers or smaller in diameter. PM2.5 is particularly harmful because it is small enough to be inhaled and can penetrate deep into the respiratory system.
Secondary Pollutants
- Sulfur trioxide (SO3) is a gas that is produced by the burning of fossil fuels that contain sulfur, such as coal and oil. It can react with water vapor in the atmosphere to form sulfuric acid, which is a secondary pollutant.
- Sulfuric acid (H2SO4) is a strong acid that is produced by the reaction of sulfur trioxide with water vapor in the atmosphere. It is a secondary pollutant that can contribute to the formation of particulate matter and acid rain.
- Nitric acid (HNO3) is a strong acid that is produced by the reaction of nitrogen dioxide with water vapor in the atmosphere. It is a secondary pollutant that can contribute to the formation of particulate matter and acid rain.
- Ozone (O3) is a gas that is produced by the reaction of nitrogen oxides and volatile organic compounds in the presence of sunlight. It is a secondary pollutant that can have adverse effects on human health, including respiratory problems.
- Ammonium (NH4) is a compound that is produced by the reaction of ammonia with acids in the atmosphere. It is a secondary pollutant that can contribute to the formation of particulate matter.
- Particulate matter (PM) is described above.
Air Quality
Air quality is affected by the amounts of various gasses and particulates found in the atmosphere. These pollutants may cause brown or grey smog, ozone warnings, or acid precipitation.
The Clean Air Act was passed in 1963 in order to control what is released into the air. It has since gone through many changes. It is largely responsible for the reduction of lead in the atmosphere and currently helps to reduce acid rain and protect the ozone layer.
Watch: AP Environmental Science - Air Pollution
Frequently Asked Questions
What are the main sources of air pollution?
Main sources of air pollution fall into a few big categories you should know for the APES CED: - Fossil fuel combustion (power plants, cars, trucks, industrial boilers): releases CO2, NOx, CO, VOCs, SO2 (coal especially gives SO2, toxic metals, particulates). NOx leads to tropospheric ozone and photochemical smog; it also forms nitric acid → acid rain (EK STB-2.A.1–2, EK STB-2.A.3). - Diesel exhaust and other incomplete combustion: high particulate matter and SO2 impacts (diesel = major source of particulates). - Industrial processes and mining: emit heavy metals (lead, mercury) and particulates; historically leaded gasoline was a big source until EPA/Clean Air Act controls reduced atmospheric lead (EK STB-2.A.4). - Natural sources: wildfires, dust storms, volcanoes (natural CO2/particulates)—remember pollutants can be primary (direct) or secondary (formed in air). These are high-value CED facts to memorize for multiple-choice and FRQs. For a quick topic review see the Topic 7.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Why does burning coal release so many different pollutants?
Coal is a complex mix of carbon plus sulfur, nitrogen, and trace metals (like mercury and lead). When you burn it, several things happen: carbon oxidizes to CO2 (and CO if combustion’s incomplete), sulfur becomes sulfur dioxide (SO2), nitrogen in the fuel and air forms nitrogen oxides (NOx), and heat vaporizes or liberates toxic metals and creates particulates and volatile organic compounds. Those primary emissions can also make secondary pollutants: NOx and VOCs react in sunlight to form tropospheric ozone and photochemical smog, and SO2/NOx can oxidize to sulfuric and nitric acids that cause acid rain. That’s why coal combustion releases many different pollutants and why Topic 7.1 emphasizes both primary and secondary pollutants (CED EK STB-2.A.1–A.3). For a quick review, see the Topic 7.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and more practice at the Unit 7 page (https://library.fiveable.me/ap-environmental-science/unit-7).
What's the difference between primary and secondary air pollutants?
Primary pollutants are emitted directly from a source into the air. Think: coal combustion and vehicle exhaust release CO2, SO2, NOx, CO, VOCs, particulates, and lead—those are primary (EK STB-2.A, STB-2.A.2, STB-2.A.3, STB-2.A.4). Secondary pollutants form in the atmosphere when primary pollutants react chemically. For example, NOx and VOCs react in sunlight to make tropospheric (ground-level) ozone and photochemical smog; SO2 and NOx can convert to sulfuric and nitric acids, causing acid rain (EK STB-2.A.2). On the AP exam you should be able to identify sources and effects of each and give examples of transformations (primary → secondary). For quick review, see the Topic 7.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw), the Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7), and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How do nitrogen oxides from cars lead to smog formation?
Cars emit nitrogen oxides (NO and NO2) as primary pollutants. In sunlight NO2 photolyzes (NO2 → NO + O). The free O atom rapidly combines with O2 to form ozone (O3) in the troposphere—a key component of photochemical smog. Volatile organic compounds (VOCs) from vehicle exhaust react with NO to form peroxy radicals that convert NO back to NO2 without consuming ozone, allowing O3 to build up. Those radical-driven reactions also produce secondary pollutants (aldehydes, PANs) that make smog worse. So: vehicle combustion → NOx + VOCs (primary) + sunlight → ozone and other oxidants (secondary) = photochemical smog. This is exactly what the CED describes: NOx from fossil-fuel combustion leads to tropospheric ozone and photochemical smog (EK STB-2.A.2). For more on Topic 7.1 and related smog details, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7).
I'm confused about how fossil fuels cause acid rain - can someone explain?
Short answer: burning fossil fuels (especially coal and diesel) emits sulfur dioxide (SO2) and nitrogen oxides (NOx). In the atmosphere these are transformed into acids—SO2 → sulfuric acid (H2SO4) and NOx → nitric acid (HNO3)—by reactions with water and oxidants. Those acids mix with rain, snow, or dry particles and fall as acid rain, which lowers soil and water pH, dissolves toxic metals, and harms plants and aquatic life. Key AP terms: coal combustion releases SO2 and particulates (EK STB-2.A.1); fossil fuel combustion releases NOx that converts to nitric acid causing acid rain (EK STB-2.A.2). SO2 and NOx are primary pollutants that form secondary pollutants (acids) in the atmosphere. For exam prep, study the chemistry and ecosystem effects in Topic 7.1 and related Topic 7.2 (photochemical smog)—see the Topic 7 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What pollutants are released when we burn diesel fuel?
When you burn diesel fuel you release a mix of primary pollutants: nitrogen oxides (NOx), sulfur dioxide (SO2) (from sulfur in diesel), carbon monoxide (CO), volatile organic compounds/hydrocarbons (VOCs), and a lot of particulate matter (PM)—especially black carbon/soot and organic compounds like PAHs. Diesel exhaust also contains some toxic trace metals and other toxic organics. Many of these are primary pollutants, but NOx and VOCs react in sunlight to form secondary pollutants like tropospheric ozone (photochemical smog) and nitric acid (acid rain). These are the same pollutants called out in the AP CED (EK STB-2.A.2 and EK STB-2.A.3), so expect questions linking diesel to PM, SO2, NOx, ozone formation, and health effects on the exam. For a quick topic review, see the Topic 7.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw). For broader Unit 7 review or lots of practice problems, check the unit page (https://library.fiveable.me/ap-environmental-science/unit-7) and practice set (https://library.fiveable.me/practice/ap-environmental-science).
Why did the EPA ban lead in gasoline and how did that help air quality?
The EPA banned leaded gasoline because tetraethyl lead added to fuel emitted lead particles when burned—and lead is a toxic metal that causes neurological damage, especially in children (learning, behavior, IQ). Under the Clean Air Act the EPA phased out lead in fuels to protect public health and the environment (CED EK STB-2.A.4). Removing leaded gasoline cut the biggest airborne source of lead, so atmospheric lead concentrations, deposition to soils/water, and human blood-lead levels dropped substantially. That improved air quality by reducing a primary pollutant (lead particulates), lowered exposure-related health risks, and helped ecosystems that accumulate lead. For AP review, link this to primary vs. secondary pollutants and fossil-fuel combustion impacts (see the Topic 7.1 study guide: https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw). For extra practice, try problems at (https://library.fiveable.me/practice/ap-environmental-science).
What happens when sulfur dioxide gets released into the atmosphere?
When sulfur dioxide (SO2) is released—mainly from burning coal and diesel—it acts as a primary air pollutant that harms people and the environment. In the atmosphere SO2 can react with oxygen and water to form sulfuric acid and sulfate aerosols. That leads to acid rain (damaging lakes, soil, and buildings) and to fine particulate matter (PM2.5), which reduces visibility and aggravates respiratory and cardiovascular problems. SO2-derived sulfates are also secondary pollutants and can have a short-term cooling effect by reflecting sunlight. Because SO2 links to both acid deposition and particulate pollution, it’s a major target of air-quality regulation (e.g., Clean Air Act policies covered in the CED). For AP review, focus on sources (coal combustion, diesel), direct health effects (respiratory irritation, worsened asthma), and secondary processes (conversion to sulfuric acid and sulfate particulates). For a concise study guide and extra practice, see the Topic 7.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw), Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7), and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How do hydrocarbons from fossil fuels contribute to air pollution?
Hydrocarbons from fossil fuels (often called volatile organic compounds, VOCs) are primary air pollutants released by incomplete combustion and fuel evaporation. Alone they irritate airways and contribute to smog and respiratory problems, but their biggest exam-relevant role is as precursors to secondary pollutants. In sunlight VOCs react with nitrogen oxides (NOx) to form tropospheric (ground-level) ozone and photochemical smog—both harmful to lungs, plants, and materials (EK STB-2.A.2, photochemical smog/tropospheric ozone). Some hydrocarbons also oxidize to form secondary organic aerosols (particulate matter), worsening cardiovascular and respiratory disease. On the AP exam, be ready to ID hydrocarbons as primary VOC sources and explain how they react with NOx + sunlight to make ozone (secondary pollutant) and particulates (STB-2.A, primary vs secondary pollutants). For a focused review, see the Topic 7.1 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and practice related questions at (https://library.fiveable.me/practice/ap-environmental-science).
What are particulates and why are they dangerous to breathe?
Particulates (particulate matter, PM) are tiny solid or liquid particles released by combustion (coal, diesel), construction, wildfires, and dust. They're classified by size—PM10 (≤10 µm) and PM2.5 (≤2.5 µm). Smaller particles (especially PM2.5) can bypass the nose and throat, reach deep into the lungs and even enter the bloodstream. That makes them dangerous: they aggravate asthma, increase risk of chronic bronchitis, heart attacks, stroke, and can carry toxic metals (lead, mercury) or carcinogens from coal and fossil-fuel combustion (CED EK STB-2.A, STB-2.A.2). Particulates also reduce visibility and worsen overall air quality (important for AP questions about sources/effects). For a focused review, see the Topic 7.1 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw). Practice applying this to exam items asking you to ID sources and health impacts (STB-2.A).
Can someone explain photochemical smog in simple terms?
Photochemical smog is a type of secondary air pollution that forms when sunlight hits nitrogen oxides (NOx) and volatile organic compounds (VOCs) released from fossil fuel combustion (cars, power plants). Sunlight powers chemical reactions that convert NOx and VOCs into tropospheric ozone (O3) and other oxidants—that ozone is what makes smog harmful. Key points: it’s worst on sunny, stagnant days; nitrogen oxides are a primary pollutant but ozone is a secondary pollutant formed in the atmosphere; health effects include breathing problems and worsened asthma; it also damages plants and reduces visibility. For APES, know sources (fossil fuels), that ozone in the troposphere is bad (unlike stratospheric ozone), and that sunlight drives formation (Topic 7.1–7.2). Review the Topic 7.1 study guide for clear examples and the Unit 7 overview (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What's the connection between nitrogen oxides and ozone formation?
Nitrogen oxides (NOx), emitted when fossil fuels burn, are key players in making ground-level (tropospheric) ozone—a secondary pollutant. In sunlight, NO2 photolyzes (breaks apart) to form NO and a free oxygen atom (O). That free O quickly combines with O2 to make O3 (ozone). Volatile organic compounds (VOCs) react with NO to regenerate NO2 without consuming O3, so the cycle keeps producing ozone, especially on sunny days—this is photochemical smog. On the AP exam, recognize NOx as a primary pollutant from combustion that causes secondary tropospheric ozone and photochemical smog (EK STB-2.A.2; keywords: nitrogen oxides, photochemical smog, tropospheric ozone, VOCs). For a quick topic review, check the Topic 7.1 study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How does carbon monoxide from cars affect air quality?
Carbon monoxide (CO) from cars is a primary air pollutant produced by incomplete combustion of fossil fuels (especially from gasoline engines). It directly lowers air quality by increasing concentrations of a colorless, odorless toxic gas that the EPA regulates. Biologically, CO binds to hemoglobin more strongly than oxygen, reducing oxygen delivery to tissues—causing headaches, dizziness, and at high levels, death. In urban areas with lots of traffic CO can spike near roadways and contribute to unhealthy air for sensitive groups. CO itself doesn’t form secondary pollutants like ozone, but it’s listed with other combustion products (NOx, hydrocarbons, particulates) from cars that drive photochemical smog and tropospheric ozone formation (CED EK STB-2.A.2, STB-2.A.5). On the AP exam, expect questions linking source → pollutant type (primary/secondary) → health/ecosystem effects. For a quick topic review check the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why are toxic metals released when we burn coal such a big problem?
When coal burns it releases toxic metals like mercury and lead attached to particulates. They’re a big problem because they’re persistent (don’t break down), travel long distances in the air, and bioaccumulate and biomagnify in food webs—so small concentrations in air/water become much higher in fish and people who eat them. Mercury can be converted to methylmercury (a neurotoxin) and lead damages brains, especially in kids. Health effects include cognitive impairment, developmental delays, and cardiovascular/renal problems. Because these metals stick to particulates, controlling particulate emissions and coal combustion is key to reducing exposure. This ties directly to the CED: coal combustion = CO2, SO2, particulates, toxic metals (EK STB-2.A.1). For review, check the Topic 7.1 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and practice questions for Unit 7 (https://library.fiveable.me/ap-environmental-science/unit-7) to prep for AP-style questions.
I don't understand how the Clean Air Act actually reduced air pollution - what did it do?
Short answer: the Clean Air Act gave the EPA legal power to set and enforce limits on major air pollutants, and those rules forced real, measurable reductions. How it worked (big-picture): - Created National Ambient Air Quality Standards (NAAQS) for pollutants like SO2, NOx, particulates, ozone precursors, and lead, so areas had to meet clear concentration limits. - Required technology and emissions controls: scrubbers at coal plants (cuts SO2/particulates), catalytic converters on cars (cuts CO, NOx, hydrocarbons), and cleaner diesel standards. - Set emissions limits for industries, required state implementation plans, and enabled market tools (e.g., the SO2 cap-and-trade program). - Specifically regulated lead (including phasing out leaded gasoline), which dramatically reduced atmospheric lead levels (EK STB-2.A.4). Why it matters for APES: it shows regulation + tech + monitoring can turn primary pollutant sources into lower emissions—a key example for Unit 7 (Topic 7.1) and for questions on policy solutions. For a quick topic review, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-7/intro-air-pollution/study-guide/eciWz0AkkBZXeKAIGiRw) and more unit resources (https://library.fiveable.me/ap-environmental-science/unit-7). For practice, try the APES problem set (https://library.fiveable.me/practice/ap-environmental-science).