Introduction to
Air Pollution
Explore the sources, types, and impacts of atmospheric contaminants that threaten human health and the environment.
AP Environmental Science
What Is Air Pollution?
Air pollution is the introduction of harmful substances into the atmosphere in quantities that damage human health, ecosystems, and the built environment. These substances can be gases, particulates, or biological molecules that alter the natural composition of the air.
Primary vs Secondary Pollutants
Primary Pollutants
Emitted directly from a source into the atmosphere. Examples: \(\text{CO}\), \(\text{SO}_2\), \(\text{NO}_x\), particulate matter (PM).
Secondary Pollutants
Formed in the atmosphere through chemical reactions involving primary pollutants. Examples: \(\text{O}_3\), \(\text{H}_2\text{SO}_4\), \(\text{HNO}_3\).
Sources of Air Pollution
The Six Criteria Pollutants
Under the Clean Air Act, the EPA monitors six pollutants using the National Ambient Air Quality Standards (NAAQS). These are the most widespread threats to air quality.
\(\text{CO}\)
Carbon monoxide: colorless, odorless gas from incomplete combustion
\(\text{SO}_2\)
Sulfur dioxide: from burning coal and oil; causes acid rain
\(\text{NO}_2\)
Nitrogen dioxide: from vehicles and power plants; smog precursor
\(\text{O}_3\)
Ground-level ozone: a secondary pollutant formed by sunlight + VOCs + \(\text{NO}_x\)
\(\text{PM}\)
Particulate matter (\(\text{PM}_{2.5}\) and \(\text{PM}_{10}\)): tiny particles that penetrate lungs
\(\text{Pb}\)
Lead: historically from leaded gasoline; now from smelters and batteries
How Ground-Level Ozone Forms
Indoor vs Outdoor Pollution
| Indoor | Outdoor | |
|---|---|---|
| Key Sources | Cooking fuels, tobacco, radon, mold, asbestos, VOCs from paint | Vehicles, power plants, factories, wildfires |
| Major Pollutants | \(\text{CO}\), radon, formaldehyde, \(\text{PM}\) | \(\text{SO}_2\), \(\text{NO}_x\), \(\text{O}_3\), \(\text{PM}\) |
| Health Impact | Kills ~3.8 million/year globally (WHO) | Kills ~4.2 million/year globally (WHO) |
| Worst In | Developing nations (biomass stoves) | Industrial cities, traffic corridors |
The Air Quality Index (AQI)
The AQI translates pollutant concentrations into a standardized scale from 0 to 500. Higher values mean greater health concern. The EPA calculates AQI for each of the six criteria pollutants; the highest value determines the reported AQI.
Acid Deposition and Its Chemistry
Acid deposition (acid rain) occurs when \(\text{SO}_2\) and \(\text{NO}_x\) react with water vapor in the atmosphere to form sulfuric acid and nitric acid:
1
Wet deposition: acids dissolved in rain, snow, or fog (pH < 5.6)
2
Dry deposition: acidic particles and gases settle directly on surfaces
3
Effects: damages forests, acidifies lakes, erodes buildings, harms aquatic life
How to Identify Pollutant Types
1
Identify the source. Is the substance released directly from a smokestack, tailpipe, or natural event? If yes, it is a primary pollutant.
2
Check for atmospheric reactions. Does the substance form through chemical reactions in the air (e.g., sunlight, moisture)? If yes, it is a secondary pollutant.
3
Watch for dual roles. Some pollutants like \(\text{NO}_2\) are both primary (emitted) AND contribute to forming secondary pollutants (\(\text{O}_3\)).
4
Apply the AQI context. Determine whether the question asks about concentration, health effects, or regulatory category.
Example: Classifying Pollutants
A coal-fired power plant emits \(\text{SO}_2\) into the atmosphere. Days later, rain downwind has a pH of 4.3. Identify the primary and secondary pollutants involved.
Example: Thermal Inversions
Industrial vs Photochemical Smog
| Industrial (London) Smog | Photochemical (LA) Smog | |
|---|---|---|
| Primary Cause | Burning coal (\(\text{SO}_2\) + \(\text{PM}\)) | Vehicle exhaust (\(\text{NO}_x\) + \(\text{VOCs}\)) |
| Key Pollutant | \(\text{SO}_2\), sulfate particles | \(\text{O}_3\) (secondary) |
| Color | Grey / black (soot) | Brown / hazy |
| Season | Winter (cold, damp) | Summer (hot, sunny) |
| Climate | Cool, humid cities | Warm, sunny cities |
Health and Environmental Effects
Human Health
Respiratory disease, cardiovascular damage, cancer risk, neurological harm from lead and mercury exposure
Ecosystems
Acid rain damages forests and acidifies waterways; ground-level ozone reduces crop yields by up to 15%
Built Environment
Acid deposition corrodes stone, metal, and paint; PM deposits reduce visibility and solar panel efficiency
Legislation and Solutions
1
Clean Air Act (1970): Established NAAQS, required EPA to regulate criteria pollutants; amended in 1990 to address acid rain and ozone depletion.
2
Cap-and-Trade: Market-based approach where polluters receive emission allowances they can trade; successfully reduced \(\text{SO}_2\) by over 80%.
3
Catalytic Converters: Required on all vehicles since 1975; convert \(\text{CO}\), \(\text{NO}_x\), and hydrocarbons into less harmful \(\text{CO}_2\), \(\text{N}_2\), and \(\text{H}_2\text{O}\).
4
Scrubbers and Filters: Electrostatic precipitators and wet scrubbers remove \(\text{PM}\) and \(\text{SO}_2\) from smokestack emissions at power plants.
Control devices remove different pollutants through distinct mechanisms
Electrostatic collection transfers particles to collected ash. Alkaline scrubbing transfers sulfur species into a managed liquid or solid waste stream. Different pollutants require different removal mechanisms.Control devices remove different pollutants through distinct mechanisms
Electrostatic collection transfers particles to collected ash. Alkaline scrubbing transfers sulfur species into a managed liquid or solid waste stream. Different pollutants require different removal mechanisms.Fabric filtration
Fabric filtration captures particles in a dust layer while cleaned gas passes through. Collected solids leave as a separate waste stream.Vehicle catalyst honeycomb
A vehicle catalytic converter promotes oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides. These reactions target different pollutants through different catalytic pathways.Common Mistakes
✕ Ozone Confusion
Mixing up stratospheric ozone (protective layer) with ground-level ozone (harmful smog). They are the same molecule but in different locations.
✕ Primary vs Secondary
Calling \(\text{O}_3\) a primary pollutant. Ground-level ozone is always secondary: it forms from reactions, never from direct emission.
✕ Ignoring Indoor Pollution
Forgetting that indoor air can be 2-5x worse than outdoor air. Radon, CO from stoves, and VOCs from household products are major indoor threats.
Question 1 of 5
Which of the following is a secondary air pollutant?
Ground-level ozone is not emitted directly: it forms when \(\text{NO}_x\) and VOCs react in the presence of sunlight, making it a secondary pollutant. \(\text{SO}_2\), \(\text{CO}\), and \(\text{PM}\) are all emitted directly from sources.
Question 2 of 5
A thermal inversion traps pollutants near the ground because:
In a thermal inversion, a warm air layer acts as a lid above cooler surface air, preventing normal convective mixing. Pollutants accumulate near the ground because the cool air cannot rise through the warm layer above it.
Question 3 of 5
Photochemical smog is most likely to form under which conditions?
Photochemical smog requires sunlight (UV radiation) to drive the reactions between \(\text{NO}_x\) and VOCs that produce \(\text{O}_3\). Hot, sunny, stagnant conditions maximize smog formation. Cold/humid conditions favor industrial smog instead.
Question 4 of 5
Which legislation established the cap-and-trade system that successfully reduced \(\text{SO}_2\) emissions in the United States?
The 1990 Amendments to the Clean Air Act introduced the Acid Rain Program, which used a cap-and-trade system for \(\text{SO}_2\). The original 1970 Act set up NAAQS but did not include cap-and-trade. NEPA and the Clean Water Act address different environmental areas.
Question 5 of 5
An AQI reading of 175 would fall into which category, and what does it mean?
An AQI of 175 falls in the 151-200 range, which is classified as "Unhealthy": everyone may begin to experience adverse health effects, and members of sensitive groups may experience more serious effects. "Hazardous" starts at 301+.