Review AP Environmental Science Unit 7 to understand how pollutants enter the atmosphere from fossil fuel combustion, natural sources, and indoor environments, and how they produce secondary effects like photochemical smog, acid rain, and thermal inversion. This unit covers the full chain from emission sources to environmental and health consequences and the strategies used to reduce them.
Use the topic guides, key terms, and practice questions available for this unit to work through all eight topics before your exam.
Atmospheric pollution connects the energy choices studied in Unit 6 to the global change consequences examined in Unit 9. In Unit 7, you trace how burning fossil fuels releases sulfur dioxide, nitrogen oxides, carbon monoxide, particulate matter, and toxic metals, and how those primary pollutants react in the atmosphere to form secondary pollutants like ground-level ozone, sulfuric acid, and nitric acid.
Coal combustion releases SO2, CO2, toxic metals, and particulates. Vehicle exhaust adds NOx, CO, hydrocarbons, and VOCs. Primary pollutants are emitted directly; secondary pollutants like tropospheric ozone and sulfuric acid form through atmospheric reactions. Natural sources including volcanic eruptions, wildfires, and decomposition also contribute CO2 and particulates.
Photochemical smog forms when NOx and VOCs react with heat and sunlight, peaking on summer afternoons in traffic-heavy cities. Thermal inversions trap that smog near the ground by reversing the normal temperature gradient. Acid rain forms when SO2 and NOx convert to sulfuric and nitric acid, lowering the pH of soils and water bodies downwind of coal plants.
Catalytic converters convert CO, NOx, and hydrocarbons into CO2, N2, and H2O. Wet and dry scrubbers and electrostatic precipitators remove SO2 and particulates from industrial exhaust. Vapor recovery nozzles prevent gasoline fumes from escaping at the pump. Regulatory tools like the Clean Air Act and EPA standards set enforceable emission limits.
Every topic in Unit 7 fits the same logic: an emission source releases a pollutant, atmospheric or indoor conditions determine how it behaves, and the result is a measurable effect on human health or ecosystems. Knowing that chain for each pollutant, from coal-plant SO2 to acid-rain soil acidification, or from vehicle NOx to afternoon ozone peaks, is the core skill the exam tests.
Identifies pollutants released by coal combustion and fossil fuel burning, distinguishes primary from secondary pollutants, and introduces the Clean Air Act and EPA regulation of lead and other emissions.
Explains how NOx and VOCs react with heat and sunlight to form ground-level ozone and smog, why ozone peaks on summer afternoons, and how catalytic converters and vapor recovery nozzles reduce smog precursors.
Describes how a warm air layer above cooler surface air reverses the normal temperature gradient, trapping smog and particulates near the ground and worsening air quality.
Covers natural sources of atmospheric CO2 (respiration, decomposition, volcanic eruptions) and natural particulate matter (volcanic ash, dust, sea salt, wildfire smoke, pollen).
Identifies indoor pollutants by source category: natural (radon, mold, dust), human-made (VOCs, formaldehyde, asbestos, lead paint), and combustion (CO, NOx, tobacco smoke). Radon-222 is the second leading cause of lung cancer in the U.S.
Covers pollution-control devices including catalytic converters, vapor recovery nozzles, wet and dry scrubbers, and electrostatic precipitators, plus regulatory and conservation approaches.
Traces acid deposition from SO2 and NOx emissions through atmospheric conversion to sulfuric and nitric acid, and explains effects on soils, lakes, and structures, including how limestone bedrock buffers acid inputs.
Defines noise pollution in terms of decibel levels and physiological harm, identifies urban sources, and describes effects on human hearing and stress as well as wildlife communication, hunting, and migration.
This snapshot uses Fiveable practice activity to show where students tend to miss questions and which review moves are worth prioritizing first.
Across 15k multiple-choice practice attempts for this unit.
Practice activity included in this snapshot.
Across 48 scored free-response attempts for this unit.
Review Atmospheric CO2 and Particulates with attention to how the concept appears in AP-style source and evidence questions.
Review Reduction of Air Pollutants with attention to how the concept appears in AP-style source and evidence questions.
Review Indoor Air Pollutants with attention to how the concept appears in AP-style source and evidence questions.
Review Acid Rain with attention to how the concept appears in AP-style source and evidence questions.
Coal combustion is the most pollutant-dense energy source, releasing SO2, CO2, toxic metals like mercury and lead, and particulate matter. Vehicle combustion adds NOx, CO, hydrocarbons, and VOCs. The Clean Air Act authorized the EPA to regulate these emissions, and phasing out leaded gasoline dramatically reduced atmospheric lead. The key conceptual split is primary vs. secondary pollutants.
| Pollutant | Source | Primary or Secondary | Key Effect |
|---|---|---|---|
| SO2 | Coal combustion, diesel | Primary | Acid rain, respiratory irritation |
| NOx | Vehicles, power plants | Primary | Smog precursor, acid rain |
| Tropospheric ozone | NOx + VOCs + sunlight | Secondary | Photochemical smog, lung damage |
| CO | Incomplete combustion | Primary | Asphyxiant, binds hemoglobin |
| Particulate matter | Combustion, dust | Primary | Respiratory disease, reduced visibility |
Photochemical smog is a secondary pollution event driven by the reaction: NOx + VOCs + heat + sunlight produces ground-level ozone and other oxidants including peroxyacyl nitrates (PANs). NOx concentrations peak in the morning rush; ozone peaks in the afternoon when sunlight is strongest. Urban geography, traffic density, summer heat, and stagnant air all amplify smog formation. Reductions come from catalytic converters, vapor recovery nozzles, reformulated fuels, and public transit expansion.
Normally, air temperature decreases with altitude, allowing warm surface air to rise and disperse pollutants. During a thermal inversion, a layer of warm air sits above cooler surface air, acting as a lid that prevents vertical mixing. Smog, particulates, and other pollutants accumulate near the ground where people breathe. Valleys and coastal basins surrounded by mountains are especially prone to inversions.
| Condition | Temperature gradient | Pollutant behavior |
|---|---|---|
| Normal atmosphere | Decreases with altitude | Warm surface air rises, dispersing pollutants |
| Thermal inversion | Increases with altitude (warm layer above) | Cool surface air trapped; pollutants accumulate near ground |
Not all atmospheric CO2 and particulate matter comes from human activity. Natural CO2 sources include cellular respiration by all organisms, decomposition of organic matter, and volcanic eruptions. Natural particulate sources include volcanic ash, windblown mineral dust, sea salt aerosols, wildfire smoke, pollen, and fungal spores. Recognizing these natural baselines is important for distinguishing background levels from anthropogenic additions.
Indoor air can be more polluted than outdoor air. Pollutants fall into three source categories: natural (radon, mold, dust), human-made materials (VOCs and formaldehyde from furniture and carpets, asbestos from insulation, lead from paint), and combustion (carbon monoxide, NOx, SO2, particulates, tobacco smoke). Radon-222, produced by uranium decay in soil and rock, infiltrates homes through basements and foundation cracks and is the second leading cause of lung cancer in the United States.
| Pollutant | Source category | Health effect |
|---|---|---|
| Radon-222 | Natural (uranium decay) | Lung cancer |
| Carbon monoxide | Combustion | Asphyxiation, oxygen deprivation |
| Formaldehyde | Human-made (building materials) | Respiratory irritation |
| Asbestos | Human-made (insulation) | Mesothelioma, lung disease |
| Mold | Natural | Allergic reactions, respiratory problems |
Air pollution reduction strategies fall into three categories: regulatory practices (Clean Air Act standards, EPA enforcement), conservation practices (driving less, energy efficiency), and alternative fuels (natural gas, renewables). Device-level controls are the most testable: catalytic converters, vapor recovery nozzles, wet and dry scrubbers, and electrostatic precipitators each target specific pollutants at specific emission points.
| Device | Pollutant targeted | Where used |
|---|---|---|
| Catalytic converter | CO, NOx, hydrocarbons | Vehicle exhaust |
| Vapor recovery nozzle | VOCs (gasoline fumes) | Gas station pump |
| Wet/dry scrubber | SO2, particulates | Industrial/power plant exhaust |
| Electrostatic precipitator | Particulate matter | Coal plant exhaust stack |
Acid deposition occurs when SO2 and NOx from coal-burning power plants and motor vehicles react with water vapor in the atmosphere to form sulfuric acid (H2SO4) and nitric acid (HNO3). These acids fall as wet deposition (rain, snow) or dry deposition (particles, gases). Communities downwind of major emission sources receive the highest acid loads. Effects include soil acidification, lake acidification and fish kills, and corrosion of marble and limestone structures. Limestone bedrock can buffer acid inputs by neutralizing acidity with calcium carbonate.
Noise pollution is sound at levels high enough to cause physiological stress and hearing loss. Urban sources include transportation (road traffic, aircraft, rail), construction (pile driving, jackhammers), and industrial and domestic activity. Effects on humans include hearing loss, stress, and sleep disruption. Effects on wildlife include stress responses, masking of communication and hunting sounds, hearing damage, and altered migratory routes. Noise pollution is measured in decibels (dB).
Try stimulus-based AP practice questions and written prompts after you review the notes.
| Term | Definition |
|---|---|
| Primary Pollutants | Substances emitted directly into the atmosphere from a source, such as SO2 from coal combustion or CO from incomplete fuel burning. |
| Secondary Pollutants | Pollutants formed in the atmosphere through chemical reactions involving primary pollutants, such as tropospheric ozone formed from NOx and VOCs in sunlight. |
| Nitrogen Oxides (NOx) | Reactive gases released by high-temperature combustion in vehicles and power plants; precursors to photochemical smog, tropospheric ozone, and nitric acid in acid rain. |
| Volatile organic compounds (VOCs) | Chemicals that evaporate at room temperature from gasoline, solvents, and some plants; react with NOx and sunlight to produce photochemical smog. |
| Tropospheric ozone | Ground-level ozone formed when NOx and VOCs react with heat and sunlight; a major component of photochemical smog and a respiratory irritant. |
| Particulate Matter | Tiny solid or liquid particles suspended in the air from combustion, dust, and natural sources; linked to respiratory disease and reduced visibility. |
| Sulfur Dioxide | Gas released primarily from coal combustion and diesel burning; a primary pollutant and the main precursor to sulfuric acid in acid deposition. |
| Clean Air Act | U.S. federal law that authorizes the EPA to set and enforce National Ambient Air Quality Standards for pollutants including SO2, NOx, CO, ozone, particulates, and lead. |
| radon | Naturally occurring radioactive gas produced by uranium decay in soil and rock; infiltrates homes through basements and cracks and is the second leading cause of lung cancer in the U.S. |
| Carbon Monoxide | Colorless, odorless gas from incomplete combustion; classified as an asphyxiant because it binds hemoglobin and reduces the blood's ability to carry oxygen. |
| Scrubbers | Industrial air pollution control devices that remove SO2 and particulates from exhaust streams using liquid spray (wet scrubbers) or dry sorbent injection (dry scrubbers). |
| acidification of soils | Process by which acid deposition lowers soil pH, reducing plant nutrient availability, mobilizing toxic aluminum ions, and degrading ecosystem function. |
| Formaldehyde | VOC off-gassed from pressed wood furniture, carpets, and building materials; a common human-made indoor air pollutant that irritates the eyes and respiratory system. |
| Peroxyacyl Nitrates (PANs) | Secondary smog pollutants formed from VOCs and NOx reactions; cause eye irritation and are toxic to plants. |
| sulfur oxides | Gaseous sulfur-oxygen compounds, primarily SO2, released from coal-burning power plants; react with water vapor to form sulfuric acid and contribute to acid deposition. |
SO2 and NOx are primary pollutants emitted directly from combustion. Tropospheric ozone, sulfuric acid, and nitric acid are secondary pollutants formed in the atmosphere. Students often misclassify ozone as primary because it is so commonly discussed.
Both smog and acid rain involve NOx, but photochemical smog also requires VOCs and sunlight, while acid rain requires SO2 or NOx reacting with water vapor. Sulfur dioxide is a major acid rain precursor but is not a direct smog ingredient.
During an inversion, the surface air is cooler and the air above is warmer, which is the reverse of normal. Students sometimes describe it as warm air at the surface trapping pollution, which is the normal condition that allows dispersion.
Radon is not a product of human activity. It forms from uranium decay in soil and rock and enters homes passively. Confusing it with combustion pollutants like CO leads to incorrect source identification on exam questions.
Limestone bedrock neutralizes acid inputs through calcium carbonate buffering, so lakes and soils in limestone regions are less damaged. Granite or other non-buffering bedrock regions suffer greater acidification. The exam tests this distinction directly.
AP Environmental Science questions frequently ask you to trace a pollutant from its emission source through atmospheric processes to an environmental or health effect. For Unit 7, practice writing that chain for photochemical smog (vehicle NOx and VOCs to ground-level ozone to respiratory harm) and for acid rain (coal plant SO2 to sulfuric acid to lake acidification), including how limestone buffering modifies the outcome.
Free-response questions often present a pollution scenario and ask you to identify a specific technology or policy that would reduce it and explain the mechanism. For Unit 7, be ready to name the correct device (catalytic converter, scrubber, electrostatic precipitator, or vapor recovery nozzle), state what pollutant it targets, and explain how it works rather than just naming it.
Multiple-choice and free-response items test whether you can correctly classify pollutants as primary or secondary, indoor sources as natural, human-made, or combustion, and reduction strategies as regulatory, conservation, or alternative-fuel approaches. Misclassifying ozone as primary or radon as a combustion product are common errors that cost points.
Open the individual guides for Unit 7 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 7 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAPES Unit 7: Atmospheric Pollution covers 8 topics: Introduction to Air Pollution, Photochemical Smog, Thermal Inversion, Atmospheric CO2 and Particulates, Indoor Air Pollutants, Reduction of Air Pollutants, Acid Rain, and Noise Pollution. Together they trace how air pollution forms, spreads, and can be reduced. See the full topic list and study guides at /ap-enviro/unit-7.
Unit 7 makes up 7-10% of the AP Environmental Science exam. That slice covers atmospheric pollution, including how primary pollutants form secondary ones, the causes and effects of acid rain, photochemical smog, and strategies for reducing air pollution. It's a focused unit, so strong preparation here has a clear payoff.
The APES Unit 7 progress check includes both MCQ and FRQ parts drawn from all 8 topics in the unit. MCQ questions test your ability to identify pollutant types, explain thermal inversion and photochemical smog formation, and analyze the causes and effects of acid rain. The FRQ portion typically asks you to describe a pollution scenario, explain its environmental or health impacts, and propose reduction strategies. Practice with matched questions at /ap-enviro/unit-7.
APES Unit 7 FRQs most often pull from air pollution sources and effects, acid rain chemistry, thermal inversion, and pollution reduction strategies. A typical question gives you a scenario, then asks you to identify the pollutant, explain the mechanism, and evaluate a solution. To practice, write out full responses to past prompts, check them against College Board scoring guidelines, and focus on using precise vocabulary like primary vs. secondary pollutants. Find practice FRQs for this unit at /ap-enviro/unit-7.
The best place to find APES Unit 7 practice questions, including multiple-choice and practice test sets, is /ap-enviro/unit-7. You'll find MCQs covering photochemical smog, thermal inversion, acid rain, indoor air pollutants, and noise pollution, along with FRQ practice that mirrors the real exam format. Working through a full practice test for this unit helps you spot which of the 8 topics need more attention before exam day.
Start by building a clear picture of how air pollution works: trace a pollutant from its source through its effects, using topics like photochemical smog formation and thermal inversion to understand why pollution gets trapped near the ground. Then move to acid rain, atmospheric CO2, and indoor pollutants before finishing with reduction strategies and noise pollution. A few concrete steps that work well: - Draw a cause-and-effect diagram for each major pollutant type. - Memorize the difference between primary and secondary pollutants with real examples. - Practice explaining thermal inversion and photochemical smog out loud, since these show up often on both MCQ and FRQ. - Use the progress check as a self-test after finishing all 8 topics. All study guides and practice for this unit are at /ap-enviro/unit-7.