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AP Environmental Science · Unit 7 of 9
Atmospheric Pollution
7–10% of the exam4 lessons · 49 min19 terms
What this unit covers
The topics below follow the published Env. Science course framework for Unit 7. This unit is worth 7–10% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Air pollutantsSmogAcid rainIndoor air
Lessons in this unit
- Air Pollutants & the Clean Air Act12 min · 3 objectivesDistinguish primary from secondary air pollutants · Identify the criteria pollutants and their major sources · Express and interpret pollutant concentrations in parts per million
- Smog & Thermal Inversions12 min · 3 objectivesDistinguish photochemical smog from industrial (gray) smog · Trace the formation of photochemical smog from its precursors · Explain how a thermal inversion traps pollutants and worsens smog
- Acid Deposition13 min · 3 objectivesExplain how sulfur and nitrogen oxides produce acid deposition · Use the pH scale to compare the acidity of rainwater samples · Describe the ecological and structural effects of acid rain and why it is a regional problem
- Indoor Air Pollution & Control12 min · 3 objectivesIdentify the major indoor air pollutants in developed and developing nations · Describe technologies that reduce pollutant emissions from combustion sources · Calculate the mass of pollutant removed by a control device of given efficiency
Formulas in Unit 7
Parts per million
1 ppm = 1 part per 1,000,000 = 0.0001% ; ppm = (parts of pollutant / total parts) × 1,000,000
ppm is a concentration ratio, not a mass. To convert ppm to a percent, divide by 10,000 (35 ppm ÷ 10,000 = 0.0035%).
Photochemical smog formation
NOₓ + VOCs + sunlight → ground-level ozone (O₃) + other secondary pollutants
The precursors (NOₓ and VOCs) come mostly from vehicle exhaust. Sunlight is essential, which is why photochemical smog peaks on hot, sunny afternoons.
pH is logarithmic
each 1-unit drop in pH = 10× more acidic (10× higher H⁺ concentration)
A difference of n pH units equals a factor of 10ⁿ in acidity. pH 3 vs pH 6 is a difference of 3 units = 10³ = 1,000 times more acidic.
Pollutant removal by a control device
mass removed = input mass × efficiency ; mass emitted = input mass × (1 − efficiency)
Express efficiency as a decimal (98% = 0.98). A 98%-efficient device still lets 2% of the pollutant escape.
Every term in Unit 7
All 19 terms we publish for Atmospheric Pollution, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Photochemical smog
- NOx and volatile organic compounds react in sunlight to form tropospheric ozone. Worst on hot, sunny, windless days in traffic-heavy cities.
- Thermal inversion
- A warm air layer traps cooler air beneath it, so pollutants cannot disperse upward and concentrations build. Worse in valleys and basins.
- Primary vs secondary pollutants
- Primary pollutants are emitted directly (CO, SO₂, NOx, particulates); secondary form in the atmosphere from reactions, such as ozone and sulfuric acid.
- Tropospheric vs stratospheric ozone
- Ozone at ground level is a harmful pollutant that damages lungs and crops; ozone in the stratosphere shields Earth from ultraviolet radiation.
- Acid deposition
- SO₂ and NOx form sulfuric and nitric acids that fall as rain or dry particles, acidifying lakes, leaching soil nutrients and mobilizing aluminum.
- Effects of acid rain on soil
- Leaches calcium and magnesium and frees toxic aluminum, damaging tree roots. Liming can neutralize affected soils and lakes.
- Particulate matter
- PM2.5 penetrates deep into the lungs and enters the bloodstream, making it more dangerous than the larger PM10.
- Carbon monoxide
- Colorless and odorless, from incomplete combustion. Binds hemoglobin more strongly than oxygen, so it starves tissue of oxygen.
- Catalytic converter
- Converts CO and hydrocarbons to CO₂ and water and reduces NOx to N₂. Requires unleaded fuel, since lead poisons the catalyst.
- Scrubbers and electrostatic precipitators
- Scrubbers spray a slurry that neutralizes SO₂; precipitators charge particulates so they collect on oppositely charged plates.
- Indoor air pollutants
- Radon from soil, asbestos from old insulation, formaldehyde from furnishings, carbon monoxide from faulty heaters, and smoke from indoor cooking fires.
- Radon
- Naturally occurring radioactive gas seeping from bedrock into basements. The second leading cause of lung cancer; mitigated by sub-slab ventilation.
- Clean Air Act
- Sets National Ambient Air Quality Standards for six criteria pollutants and regulates emissions from vehicles and industry.
- Noise pollution
- Causes hearing loss and stress in humans and disrupts communication, navigation and mating in wildlife, especially marine mammals.
- Six criteria pollutants
- Carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter and sulfur dioxide — the pollutants the Clean Air Act sets standards for.
- Industrial vs photochemical smog
- Industrial (gray) smog comes from sulfur and particulates in coal burning; photochemical (brown) smog forms from NOx and VOCs in sunlight.
- Why smog peaks in the afternoon
- Photochemical reactions need sunlight, so ozone builds through the day after the morning traffic peak supplies NOx.
- Volatile organic compounds
- Evaporating hydrocarbons from fuels, solvents and paints that react with NOx to form ozone.
- Vapor recovery nozzles
- Capture VOCs escaping while refuelling, a targeted control on a nonpoint source.
What examiners penalize here
- Memorize the six criteria pollutants (CO, SO₂, NOₓ, PM, ozone, lead) and which is the odd one out as a *secondary* pollutant — ground-level ozone. To convert ppm to percent, divide by 10,000; the AQI scale runs 0–500 with higher = worse.
- A classic free-response asks you to explain why a valley city has persistent smog. Hit both factors: (1) the source and formation of the smog (vehicles + sunlight → photochemical ozone), and (2) the thermal inversion that traps it. Naming both earns full credit.
- Two acid-rain must-knows: the chemistry (SO₂ → sulfuric acid, NOₓ → nitric acid) and the math (each pH unit = 10× acidity, so an n-unit drop = 10ⁿ). Lakes with limestone bedrock are **buffered** and resist acidification; granite-based lakes are not.
- For removal-efficiency problems, watch whether the question asks for the amount *removed* (input × efficiency) or the amount *emitted* (input × (1 − efficiency)). They are easy to swap. A quick check: removed + emitted should equal the total input.
Practice Env. Science
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Environmental Science exam is Unit 7?
Unit 7, Atmospheric Pollution, is worth 7–10% of the Env. Science multiple-choice section according to the published course framework. Across all 9 units that makes it a middling share, roughly what an even split across units would give.
What topics are covered in Env. Science Unit 7?
Atmospheric Pollution covers Air pollutants, Smog, Acid rain and Indoor air. We publish 19 terms with definitions for this unit, all of them on this page.
How should I study Env. Science Unit 7?
Read the 4 lessons below first — about 50 minutes — then drill the 19 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 9 units of AP Environmental Science
Unit names, topics and exam weights follow the published College Board course framework for AP Environmental Science. AP® is a trademark registered by the College Board, which does not endorse this site.