Inside This Unit: The Full Breakdown
Atmospheric Pollution covers the sources, effects, and solutions for air pollution at local, regional, and global scales. This unit examines smog, acid deposition, ozone depletion, and indoor air quality.
Why it matters
Air pollution is tested extensively on the AP Environmental Science exam. You must know the sources and effects of major air pollutants, understand the difference between primary and secondary pollutants, and explain policy solutions like the Clean Air Act and the Montreal Protocol.
Key concepts
- Primary pollutants (CO, SO₂, NOₓ, particulates, VOCs) are emitted directly from sources. Secondary pollutants (ozone, acid rain) form from chemical reactions in the atmosphere.
- Photochemical smog (brown smog) forms from NOₓ and VOCs reacting with sunlight. Industrial smog (gray smog) comes from burning coal and contains SO₂ and particulates.
- Stratospheric ozone depletion is caused by CFCs, which release chlorine atoms that catalytically destroy ozone molecules. The Montreal Protocol successfully reduced CFC use.
- Acid deposition (acid rain) results from SO₂ and NOₓ emissions reacting with water to form sulfuric and nitric acids.
Air Pollutants and Smog
The Clean Air Act regulates six criteria air pollutants: carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO₂), ozone (O₃, ground-level), particulate matter (PM), and sulfur dioxide (SO₂). Primary pollutants are emitted directly from sources like vehicle exhaust, power plants, and industrial processes. Secondary pollutants form through atmospheric chemical reactions — ground-level ozone is a secondary pollutant formed when NOₓ and volatile organic compounds (VOCs) react in the presence of sunlight. Photochemical smog (common in sunny cities like Los Angeles) contains ozone, peroxyacyl nitrates (PANs), and other irritants. Industrial smog (common in cities burning coal) contains SO₂ and particulate matter. Temperature inversions trap pollutants near the ground by preventing vertical air mixing, worsening smog episodes.
Acid Deposition and Indoor Air
Acid deposition occurs when SO₂ and NOₓ (primarily from coal-burning power plants and vehicle exhaust) react with water vapor to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These acids fall as wet deposition (acid rain, snow, fog) or dry deposition (acidic particles). Acid rain damages lakes and streams (killing aquatic life when pH drops below 5), leaches nutrients from forest soils, corrodes buildings and monuments, and mobilizes toxic metals like aluminum. Scrubbers in power plant smokestacks and catalytic converters in vehicles reduce emissions. Indoor air pollution is actually a larger health threat globally than outdoor pollution. Major indoor pollutants include radon (a radioactive gas seeping from soil), carbon monoxide (from incomplete combustion), formaldehyde (from building materials), asbestos, and particulates from cooking with biomass fuels in developing nations.
Stratospheric Ozone Depletion
The ozone layer in the stratosphere absorbs harmful UV-B and UV-C radiation, protecting life on Earth. Chlorofluorocarbons (CFCs), once widely used in refrigerants, aerosol propellants, and foam insulation, are the primary cause of ozone depletion. CFC molecules are stable in the troposphere but break down in the stratosphere, releasing chlorine atoms. Each chlorine atom can destroy thousands of ozone molecules through a catalytic cycle. The Antarctic ozone hole was discovered in the 1980s and grows seasonally. The 1987 Montreal Protocol, widely considered the most successful international environmental agreement, phased out CFC production globally. The ozone layer is slowly recovering, expected to return to 1980 levels by mid-century. This success story demonstrates that international cooperation on environmental problems is possible.
AP exam tip
On the AP exam, distinguish clearly between stratospheric ozone (good — protects from UV) and ground-level ozone (bad — a component of smog that damages lungs and plants). Students frequently confuse these two, and the distinction is commonly tested.
Connections to other units
- Unit 6 (Energy Resources): Fossil fuel combustion is the primary source of SO₂, NOₓ, CO, and particulate emissions.
- Unit 9 (Global Change): While ozone depletion and climate change are separate problems, some greenhouse gases also affect stratospheric chemistry.
- Unit 1 (Ecosystems): Acid deposition and air pollution damage ecosystems by altering soil and water chemistry.
