Notes
Major air pollutants
| Pollutant | Main sources | Effects |
|---|---|---|
| Nitrogen oxides ($NO_x$) | Vehicle exhaust, power plants | Photochemical smog, acid rain, respiratory problems |
| Sulfur dioxide ($\mathrm{SO_2}$) | Burning coal | Acid rain, respiratory irritation |
| Particulate matter (PM) | Burning fuel, dust, wildfires | Lung and heart disease, haze |
| Carbon monoxide (CO) | Incomplete combustion | Reduces oxygen delivery in blood |
| Tropospheric ozone ($\mathrm{O_3}$) | Formed from $NO_x$ + VOCs + sunlight | Smog, lung damage, harms plants |
| Volatile organic compounds (VOCs) | Gasoline, paints, solvents | Help form ozone and smog |
- Primary pollutants are released directly. Secondary pollutants form in the atmosphere, like tropospheric ozone.
- The Clean Air Act regulates criteria air pollutants.
Photochemical smog
Sunlight breaks $\mathrm{NO_2}$ into NO and a free O atom, which joins $\mathrm{O_2}$ to make ozone. VOCs keep ozone from breaking down. Smog peaks on hot, sunny afternoons in cities with lots of traffic.
Thermal inversions
Normally air cools with height, so polluted air rises and disperses. In a thermal inversion, a warm layer sits above cooler air at the surface, trapping pollutants near the ground.
Acid deposition
$\mathrm{SO_2}$ and $NO_x$ react with water to form sulfuric and nitric acid. Acid rain lowers the pH of lakes and soils, harms aquatic life, leaches nutrients, and damages limestone buildings. Limestone bedrock can neutralize acid (buffering).
Indoor air pollution and noise
- Indoor pollutants: radon (radioactive gas from uranium in rock, linked to lung cancer), carbon monoxide from faulty heaters, asbestos, formaldehyde, and smoke from burning wood or dung indoors in developing countries.
- Noise pollution disrupts animal communication and migration.
Reducing air pollution
Catalytic converters reduce CO, $NO_x$ and VOCs from cars. Scrubbers remove $\mathrm{SO_2}$ from smokestacks. Electrostatic precipitators remove particulates. Vapor recovery nozzles at gas pumps cut VOCs.
Practice
Ozone near the ground forms from reactions of $NO_x$ and VOCs in sunlight.
Coal contains sulfur that forms $\mathrm{SO_2}$ when burned.
Sunlight drives the reactions that make ozone.
The warm layer acts like a lid.
Calcium carbonate buffers the acid.
Radon comes from uranium decay in rock and soil.
It converts these gases into $\mathrm{CO_2}$, $\mathrm{N_2}$ and water.
CO binds to hemoglobin more strongly than oxygen.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A city has frequent smog on summer afternoons.
(a) Explain how vehicle exhaust leads to the formation of tropospheric ozone.
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Cars release $NO_x$ and VOCs. Sunlight splits $\mathrm{NO_2}$ into NO and a single oxygen atom, which combines with $\mathrm{O_2}$ to form $\mathrm{O_3}$. VOCs react with NO so it can't break the ozone down, so ozone builds up.
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(b) Describe one human health effect of tropospheric ozone.
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It irritates the lungs and airways, worsening asthma and reducing lung function.
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(c) Propose one policy that would reduce smog in the city and explain how it works.
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Expand public transportation (or set stricter emissions standards). Fewer cars or cleaner cars release less $NO_x$ and VOCs, so less ozone forms.
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