Atmospheric Pollution
Unit 7 of AP Environmental Science, worth 7–10% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Air pollutants, photochemical smog, thermal inversion, indoor air pollution, pollution reduction, acid rain, noise pollution.
How this unit is tested
Start by sorting pollutants into two buckets: primary (emitted directly, like CO and SO2) and secondary (formed by atmospheric reactions, like ozone and PAN). Almost every AP question on smog or acid rain is really asking you to trace a chemical pathway from source gas to secondary product, so learn the reactions, not just the names.
For smog and thermal inversion, think in terms of conditions: what weather (sunlight, heat, calm air, temperature layering) makes pollution worse, and why. Draw the vertical temperature profile for a normal day versus an inversion day — the AP loves a diagram-based reasoning question here even when no diagram is given.
For acid rain, memorize the source gases (SO2 from coal, NOx from combustion generally), the two acids they form, and why buffering capacity (limestone vs. granite bedrock) determines ecosystem vulnerability. Keep in mind normal rainwater is already mildly acidic (pH ~5.6) because of dissolved CO2 — acid rain is anything measurably below that.
For pollution-reduction technology and noise pollution, know which device removes which pollutant (scrubbers for SO2 gas, electrostatic precipitators and baghouses for particulates, catalytic converters for vehicle exhaust) and remember both pH and decibels are logarithmic scales — a 2-unit pH change or 20 dB change means a 100-fold difference, not a doubling.
What you have to know
Primary vs. secondary pollutant
A primary pollutant is emitted directly from a source (e.g., CO, SO2, particulate matter). A secondary pollutant forms from chemical reactions between primary pollutants in the atmosphere (e.g., tropospheric ozone, PAN, acid rain components).
Photochemical smog formation
NOx and volatile organic compounds (VOCs) react in the presence of sunlight and heat to produce ground-level (tropospheric) ozone and other secondary pollutants; the reaction is strongest on hot, sunny, low-wind days.
Acid deposition chemistry
SO2 and NOx react with water vapor and oxygen in the atmosphere to form sulfuric acid (H2SO4) and nitric acid (HNO3), which fall as wet deposition (rain, snow, fog) or settle as dry deposition.
Thermal inversion
Normally air temperature decreases with altitude, allowing warm surface air to rise and disperse pollutants. In a thermal inversion, a layer of warm air sits above cooler surface air, blocking vertical mixing and trapping pollutants near the ground.
Logarithmic scales (pH and decibels)
Both the pH scale and the decibel scale are logarithmic: each whole-number change in pH represents a tenfold change in acidity, and each 10 dB increase represents a tenfold increase in sound intensity.
14 practice questions
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Which of the following is a secondary air pollutant formed through atmospheric chemical reactions rather than emitted directly from a source?
- Carbon monoxide (CO)
- Sulfur dioxide (SO2)
- Tropospheric ozone (O3)
- Particulate matter from diesel exhaust
Show the answer
Answer. Tropospheric (ground-level) ozone
Ozone forms when NOx and VOCs react in sunlight; it is never emitted directly. CO, SO2, and diesel particulate matter are all primary pollutants released straight from combustion sources. -
What are the two main classes of chemicals that react with sunlight to produce photochemical smog?
Show the answer
Answer. Nitrogen oxides (NOx) and volatile organic compounds (VOCs)
Sunlight drives reactions between NOx (from combustion) and VOCs (from fuel evaporation, solvents, vegetation) to generate ozone and other secondary pollutants that make up smog. -
Explain how a thermal inversion forms and why it worsens urban air pollution.
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Answer. A layer of warm air settles above a layer of cooler surface air, preventing the normal upward convection that disperses pollutants, so they accumulate near the ground.
Normally temperature falls with altitude, letting warm polluted surface air rise and mix away. In an inversion this profile flips, trapping pollutants like a lid, especially in valleys or basins. -
Which of the following is NOT one of the six criteria air pollutants regulated under the Clean Air Act's NAAQS?
- Carbon dioxide (CO2)
- Ground-level ozone (O3)
- Particulate matter (PM)
- Sulfur dioxide (SO2)
Show the answer
Answer. Carbon dioxide (CO2)
The six criteria pollutants are CO, NO2, SO2, O3, particulate matter, and lead. CO2 is a greenhouse gas regulated through separate climate policy, not NAAQS. -
Rainwater with a pH of 4 is how many times more acidic than rainwater with a pH of 6?
Show the answer
Answer. 100 times more acidic
pH is a logarithmic scale, so each whole-unit decrease represents a tenfold increase in acidity. A 2-unit difference (6 to 4) means 10 × 10 = 100 times more acidic. -
Name two indoor air pollutants and describe the primary health risk associated with each.
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Answer. Radon causes lung cancer through long-term inhalation of radioactive decay particles; carbon monoxide causes headaches, impaired cognition, and death by binding hemoglobin and blocking oxygen transport.
Radon seeps from uranium-bearing soil/rock into basements, while CO comes from incomplete combustion of fuels (furnaces, stoves, cars) in poorly ventilated spaces. -
Which technology is most effective at removing sulfur dioxide gas from coal power plant exhaust before release?
- Catalytic converter
- Wet scrubber
- Electrostatic precipitator
- Carbon capture pipeline
Show the answer
Answer. Wet scrubber
Scrubbers spray a limestone slurry through flue gas to chemically bind and remove SO2. Catalytic converters treat vehicle exhaust, and electrostatic precipitators target particulates, not gaseous SO2. -
What chemical transformations occur inside a catalytic converter?
Show the answer
Answer. CO is oxidized to CO2, NOx is reduced to N2 and O2, and unburned hydrocarbons are oxidized to CO2 and water vapor.
Metal catalysts (platinum, palladium, rhodium) in the converter speed up these reactions, converting harmful exhaust gases into less toxic products before they leave the tailpipe. -
How does an electrostatic precipitator remove particulate matter from smokestack emissions?
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Answer. It electrically charges particles in the exhaust stream, then collects them on oppositely charged plates.
This method removes fine particulates (soot, ash) without needing a liquid scrubbing agent, making it useful for controlling PM emissions from power plants and factories. -
Why are ecosystems on granite bedrock more damaged by acid rain than ecosystems on limestone bedrock?
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Answer. Limestone (calcium carbonate) can chemically neutralize incoming acid, buffering the water and soil, while granite lacks this buffering capacity, so acids accumulate and harm organisms.
Buffering capacity determines vulnerability: limestone regions resist pH drops, while granite/quartz regions in acid-rain-affected areas (like parts of the northeastern U.S. and Scandinavia) see severe lake and soil acidification. -
A sound of 70 decibels is how many times more intense than a sound of 50 decibels?
Show the answer
Answer. 100 times more intense
The decibel scale is logarithmic, with each 10 dB increase representing a tenfold rise in sound intensity. A 20 dB difference equals 10 × 10 = 100-fold increase. -
Identify two documented effects of chronic noise pollution on humans or wildlife.
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Answer. In humans: hearing loss, elevated stress hormones, and hypertension. In wildlife: disrupted communication, altered migration routes, and reduced reproductive success.
Noise pollution from traffic, industry, and shipping interferes with animal echolocation and mating calls, and chronic exposure in humans is linked to cardiovascular and psychological harm. -
Which atmospheric condition is most likely to produce a thermal inversion that traps pollutants in a valley city?
- Strong daytime heating of the ground
- A layer of warm air settling above a layer of cool surface air on a calm night
- High winds mixing the atmosphere
- Heavy rainfall washing out pollutants
Show the answer
Answer. A layer of warm air settling above a layer of cool surface air on a calm night
Calm, clear nights allow the ground to radiate heat away quickly, cooling surface air while a warmer air layer remains aloft (or descends with a high-pressure system), creating the inversion. -
Distinguish between wet deposition and dry deposition of acidic pollutants.
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Answer. Wet deposition is acid falling to earth dissolved in precipitation such as rain, snow, or fog; dry deposition is acidic gases and particles settling directly onto surfaces without moisture.
Both pathways deliver sulfuric and nitric acid derived from SO2 and NOx emissions, but dry deposition can travel and land even in the absence of rainfall, later reacting with moisture on surfaces.
What people get wrong
- Assuming ozone is emitted directly from tailpipes or smokestacks. Correct approach: ozone is always a secondary pollutant, formed when NOx and VOCs react in sunlight — never list it as a primary pollutant.
- Treating CO2 as one of the EPA's six criteria air pollutants. Correct approach: the criteria pollutants are CO, NO2, SO2, O3, particulate matter, and lead; CO2 is a greenhouse gas regulated separately, not under NAAQS.
- Describing a thermal inversion as 'cold air rising' or 'warm air rising through cold air.' Correct approach: an inversion is warm air sitting ABOVE cool surface air, which suppresses the normal upward mixing rather than promoting it.
- Calculating pH or decibel differences linearly (e.g., saying pH 4 is 'twice as acidic' as pH 6). Correct approach: treat each unit as a factor of 10, so a 2-unit pH difference is a 100-fold difference in acidity.
- Mixing up which device controls which pollutant, e.g., saying a catalytic converter is installed on a coal power plant. Correct approach: catalytic converters treat vehicle exhaust; scrubbers and electrostatic precipitators treat stationary-source emissions like power plants.
- Assuming acid rain only damages aquatic ecosystems. Correct approach: also account for its effects on forest soils (leaching nutrients, releasing toxic aluminum), and on human-made structures like limestone and marble buildings/monuments.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Environmental Science. Free, and no account needed to start.