Atmospheric Pollution

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.

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
• Primary and Secondary Pollutants, and Reading a Smog Curve17 min · 3 objectivesDistinguish primary from secondary pollutants and classify examples · Explain photochemical smog formation and the daily concentration curve · Compare control technologies and the pollutants each addresses

Formulas in Unit 7

Every term in Unit 7

All 31 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 layer of warm air sits above cooler surface air, reversing the normal temperature profile. Because the cool surface air cannot rise, pollutants are trapped near the ground and accumulate. Common in valleys and basins such as Los Angeles, Mexico City, and Denver.

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.

The role of NOx in ozone formation

NO₂ splits in sunlight to NO plus a free oxygen atom, which joins O₂ to make ozone. VOCs sustain the cycle by consuming the NO that would otherwise destroy the ozone again — so cutting VOCs alone can fail if NOx is high, and vice versa.

Sulfur dioxide: sources and effects

Chiefly coal combustion and metal smelting. It irritates airways, forms sulfate aerosols that scatter light and reduce visibility, and oxidizes to sulfuric acid, making it the main driver of acid deposition downwind of coal regions.

Nitrogen oxides: sources

Formed whenever combustion is hot enough to make atmospheric nitrogen and oxygen react — vehicles, power plants, industrial boilers. Because the source is the temperature rather than the fuel's composition, cleaner fuel alone does not solve it.

PM2.5 vs PM10

PM10 lodges in the upper airway; PM2.5 is fine enough to reach the alveoli and cross into the bloodstream, which is why it dominates the mortality statistics. PM2.5 comes mostly from combustion, PM10 more from dust and mechanical processes.

Ozone health effects

Ground-level ozone inflames airway tissue, reduces lung function and triggers asthma attacks, with damage rising during outdoor exertion on hot afternoons. It also reduces crop yields — an economic cost usually omitted from air-quality arguments.

Lead as an air pollutant

Once emitted from leaded gasoline, now mainly from smelting and older industry. It is a neurotoxin with no safe exposure level in children, and removing it from gasoline produced one of the largest measured public-health gains of any environmental regulation.

Radiation vs subsidence inversion

A radiation inversion forms overnight as the ground cools rapidly, trapping pollutants until morning heating breaks it. A subsidence inversion forms when descending high-pressure air warms aloft, and can persist for days — the Los Angeles pattern.

Buffering capacity

Soils and lakes over limestone neutralize incoming acid; those over granite cannot. This is why identical acid deposition devastates lakes in the Adirondacks and Scandinavia while leaving limestone regions nearly unaffected.

Liming

Adding powdered limestone to an acidified lake or soil raises pH directly. It treats the symptom, must be repeated, and does nothing about the emissions upwind — a useful example of remediation versus prevention.

Wet vs dry deposition

Wet deposition arrives as acidic rain, snow or fog; dry deposition arrives as acidic particles and gases that become acidic on contact with moist surfaces. Both damage leaves and stone, and dry deposition dominates close to the source.

Sick building syndrome

Occupants report headaches, irritation and fatigue that improve on leaving the building. It follows from tightly sealed construction with low air exchange, which raises indoor concentrations of VOCs, mold and CO₂ that used to leak away.

Air Quality Index

A 0–500 scale converting measured concentrations of the criteria pollutants into a single health-based number, reported for whichever pollutant is worst that day. Above 100 is unhealthy for sensitive groups — the threshold that triggers public advisories.

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.
• Never propose controlling ozone at the tailpipe or smokestack. It is a **secondary** pollutant and does not exist at the source. The only available lever is reducing its precursors — nitrogen oxides and volatile organic compounds — and saying so is the point the question is testing.

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

All 9 units of AP Environmental Science

1. Unit 1 · The Living World: Ecosystems
2. Unit 2 · The Living World: Biodiversity
3. Unit 3 · Populations
4. Unit 4 · Earth Systems & Resources
5. Unit 5 · Land & Water Use
6. Unit 6 · Energy Resources & Consumption
7. Unit 7 · Atmospheric Pollution
8. Unit 8 · Aquatic & Terrestrial Pollution
9. Unit 9 · Global Change