On a hot, still, sunny afternoon over a big city, a brownish haze builds — and it wasn't emitted that way. Sunlight cooked car exhaust into an entirely new pollutant, ground-level ozone, that stings eyes and inflames lungs. That's the crucial idea of Unit 7: some pollutants come straight out of the tailpipe or smokestack (primary), but others form in the atmosphere from chemical reactions (secondary). Add in the geography of a valley and a temperature inversion that traps the bad air near the ground, and you get the smog episodes that once killed thousands. This lesson maps air pollution's sources, chemistry, and the law that cleaned much of it up.
[DIAGRAM: Primary → secondary flow. Sources (cars, power plants) emit primary pollutants (NOₓ, VOCs, SO₂). Sunlight + NOₓ + VOCs → secondary pollutant ground-level ozone (O₃) and photochemical smog. SO₂/NOₓ + water → secondary acids.]
The US EPA regulates six criteria pollutants under the Clean Air Act: 1. Carbon monoxide (CO) — colorless, odorless; from incomplete combustion (vehicles); binds hemoglobin, reducing oxygen delivery. 2. Nitrogen oxides (NOₓ) — from high-temperature combustion; contribute to smog and acid rain; reddish-brown. 3. Sulfur dioxide (SO₂) — from burning sulfur-containing coal; causes acid rain and respiratory harm. 4. Particulate matter (PM₁₀, PM₂.₅) — tiny particles/droplets; penetrate deep into lungs; from combustion, dust, fires. Fine particulates are among the deadliest pollutants. 5. Ground-level ozone (O₃) — a secondary pollutant; main component of photochemical smog; damages lungs and plants. 6. Lead (Pb) — from leaded gasoline (now phased out in most countries), smelters; a neurotoxin. Removing lead from gasoline dramatically cut blood-lead levels.
Photochemical smog (brown smog) forms on hot, sunny days when sunlight drives reactions among NOₓ and VOCs (from vehicle exhaust) to produce ground-level ozone and other oxidants. It peaks in the afternoon as sunlight intensity peaks, and is worst in sunny, car-dependent cities (Los Angeles, Mexico City).
Industrial (gray/sulfurous) smog is different: it comes from burning coal (SO₂ + particulates) in cool, humid conditions — the historic "London smog" type (the deadly 1952 Great Smog of London).
Normally air temperature decreases with altitude, so warm surface air rises and disperses pollutants. In a thermal inversion, a layer of warm air sits on top of cooler surface air, acting like a lid that traps pollutants near the ground, allowing them to accumulate to dangerous levels — especially in valleys.
[DIAGRAM: Thermal inversion cross-section. Normal: warm air near ground rises (pollution disperses upward). Inversion: a warm-air layer aloft caps cool air below in a valley, trapping smog near the surface. Show temperature increasing with height in the inversion layer.]
The US Clean Air Act (major amendments 1970, 1990) authorizes the EPA to set National Ambient Air Quality Standards (NAAQS) for the criteria pollutants, regulate emissions from vehicles and industry, and it established a cap-and-trade program for SO₂ that successfully reduced acid rain. It's credited with major reductions in US air pollution. (Cite it by name and general purpose; don't invent specific numeric limits.)
Control technologies: catalytic converters (cars — convert CO, NOₓ, VOCs to less harmful gases), scrubbers (remove SO₂ from smokestack gases), electrostatic precipitators and baghouses (capture particulates).
Distinguishing primary vs. secondary pollutants, identifying the criteria pollutants and their sources, explaining photochemical smog and inversions, and citing the Clean Air Act and control technologies are core Unit 7 skills tested every year.
Classify: (i) SO₂ from a coal plant, (ii) ground-level ozone, (iii) CO from a car.
Solution: (i) SO₂ = primary (emitted directly); (ii) ground-level ozone = secondary (forms in the atmosphere); (iii) CO = primary.
Interpretation: Emitted directly = primary; formed by atmospheric reactions = secondary (ozone is the classic secondary pollutant).
Explain why photochemical smog (ozone) levels are highest on sunny afternoons.
Solution: Photochemical smog forms when sunlight drives reactions between NOₓ and VOCs from morning traffic to produce ground-level ozone. Ozone accumulates as sunlight intensifies, so concentrations peak in the afternoon on hot, sunny days.
Interpretation: Sunlight is the reactant/energy source, so ozone tracks the sun.
A valley city has severe smog during a thermal inversion. Explain how the inversion worsens air quality.
Solution: In a thermal inversion, a warm-air layer sits above cooler surface air, preventing the surface air (and its pollutants) from rising and dispersing. Acting as a lid, it traps pollutants near the ground, so they accumulate to high, harmful concentrations — worsened by the valley walls that also block horizontal dispersal.
Interpretation: Inversion = warm lid on cool air = pollutants trapped and concentrated.
Explain how a catalytic converter reduces vehicle air pollution and which pollutants it targets.
Solution: A catalytic converter uses catalysts to chemically convert vehicle exhaust pollutants — CO, NOₓ, and unburned hydrocarbons (VOCs) — into less harmful CO₂, N₂, and water before they exit the tailpipe, reducing the ingredients of photochemical smog.
Interpretation: Catalytic converters cut the smog precursors (CO, NOₓ, VOCs) at the source.
(B) Penetrates deep into the lungs.
Photochemical smog forms when sunlight drives reactions between NOₓ and VOCs (from vehicle exhaust) to produce ground-level ozone. It's worst on hot, sunny afternoons because sunlight (the energy source) is most intense then, maximizing ozone formation. A thermal inversion puts a warm-air lid over the cool surface air, trapping pollutants near the ground so ozone and precursors accumulate to high levels; valley walls block dispersal, compounding it.
(a) Ozone forms via sunlight-driven reactions, so it's high on sunny days and low when clouds block the sunlight. (b) Target vehicle exhaust (NOₓ, VOCs) and industrial/power-plant emissions.
FRQ rubric (10 pts): - (a) 1 pt primary = emitted directly (e.g., NOₓ, VOCs, CO from cars); 1 pt secondary = forms in atmosphere (ground-level ozone). (2) - (b) 1 pt sunlight drives reactions among NOₓ and VOCs; 1 pt producing ground-level ozone (peaks midday/afternoon). (2) - (c) 1 pt inversion = warm air over cool surface air acts as a lid; 1 pt traps pollutants near the ground so they accumulate. (2) - (d) For each of two solutions: 1 pt name + 1 pt justification, with a relevant technology/law (catalytic converters; public transit/EVs to cut NOₓ/VOCs; Clean Air Act standards; reformulated fuels; driving restrictions on high-ozone days). (4)
A sunny, car-dependent city in a valley suffers frequent summer smog episodes with high ground-level ozone, worsened by thermal inversions.
(a) Distinguish primary and secondary pollutants, giving an example of each relevant to this smog. (2 pts) (b) Explain the role of sunlight in forming the city's ozone problem. (2 pts) (c) Explain how the valley's thermal inversions worsen the episodes. (2 pts) (d) Propose two solutions to reduce the ozone problem and justify each, naming one relevant technology or law. (4 pts)
MC: 1. (C) Ground-level ozone (secondary). 2. (B) NOₓ and VOCs reacting in sunlight. 3. (B) Warm air aloft traps cool polluted air. 4. (C) Carbon dioxide is not a criteria pollutant. 5. (B) CO, NOₓ, and VOCs from vehicles. 6. (B) Burning coal in cool, humid conditions. 7. (B) SO₂. 8. (B) Carbon monoxide. 9. (B) Set NAAQS for criteria pollutants. 10. (B) Penetrates deep into the lungs.
Photochemical smog forms when sunlight drives reactions between NOₓ and VOCs (from vehicle exhaust) to produce ground-level ozone. It's worst on hot, sunny afternoons because sunlight (the energy source) is most intense then, maximizing ozone formation. A thermal inversion puts a warm-air lid over the cool surface air, trapping pollutants near the ground so ozone and precursors accumulate to high levels; valley walls block dispersal, compounding it.
(a) Ozone forms via sunlight-driven reactions, so it's high on sunny days and low when clouds block the sunlight. (b) Target vehicle exhaust (NOₓ, VOCs) and industrial/power-plant emissions.
FRQ rubric (10 pts): - (a) 1 pt primary = emitted directly (e.g., NOₓ, VOCs, CO from cars); 1 pt secondary = forms in atmosphere (ground-level ozone). (2) - (b) 1 pt sunlight drives reactions among NOₓ and VOCs; 1 pt producing ground-level ozone (peaks midday/afternoon). (2) - (c) 1 pt inversion = warm air over cool surface air acts as a lid; 1 pt traps pollutants near the ground so they accumulate. (2) - (d) For each of two solutions: 1 pt name + 1 pt justification, with a relevant technology/law (catalytic converters; public transit/EVs to cut NOₓ/VOCs; Clean Air Act standards; reformulated fuels; driving restrictions on high-ozone days). (4)
⭐ Exam strategy: Anchor Unit 7 on two dichotomies — primary (emitted) vs. secondary (formed, e.g., ozone), and photochemical (hot/sunny/cars/ozone) vs. industrial (cool/humid/coal/SO₂). Inversions trap pollution because temperature rises with height (a warm lid).
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