AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/7/7-1)
Unit 7 · Topic 7.1
7.1 Introduction to Air Pollution
Burning fossil fuels in power plants, factories and vehicles is the biggest human source of outdoor air pollution. This topic sorts pollutants into primary (released directly) and secondary (formed in the air) and introduces the Clean Air Act, the main US law for controlling them.
Key terms
- primary pollutant
- secondary pollutant
- criteria air pollutants
- Clean Air Act
- nitrogen oxides (NOₓ)
- sulfur dioxide (SO₂)
What burning fuel puts in the air
Coal combustion releases carbon dioxide (CO₂), sulfur dioxide (SO₂) from sulfur in the coal, nitrogen oxides (NOₓ), particulate matter (tiny solid and liquid particles), and toxic metals such as mercury and lead. Burning gasoline and diesel in vehicles releases CO₂, NOₓ, carbon monoxide (CO), volatile organic compounds (VOCs, unburned or evaporated hydrocarbons) and particulates. Diesel fuel has traditionally contained more sulfur than gasoline, which is why low-sulfur diesel rules cut SO₂ from trucks and buses.
Where do nitrogen oxides come from? Air is about 78 percent nitrogen gas. At the very high temperatures inside engines and boilers, some of that nitrogen reacts with oxygen to form NO and NO₂. So NOₓ forms from the air itself, not mainly from the fuel.
Primary versus secondary pollutants
A primary pollutant is emitted directly from a source, such as CO from a tailpipe or SO₂ from a smokestack. A secondary pollutant forms in the atmosphere when primary pollutants react with each other or with sunlight, water or oxygen.
| Primary pollutants | Secondary pollutants |
|---|---|
| Carbon monoxide (CO) | Ground-level ozone (O₃), formed from NOₓ and VOCs in sunlight |
| Nitrogen oxides (NO, NO₂) | Sulfuric acid (H₂SO₄), formed from SO₂ and water |
| Sulfur dioxide (SO₂) | Nitric acid (HNO₃), formed from NOₓ and water |
| Particulate matter, soot | Photochemical smog components such as PAN |
| VOCs, lead, mercury, CO₂ | Some fine particles (sulfate and nitrate particles) |
The Clean Air Act
The Clean Air Act, passed in 1970 and strengthened in 1977 and 1990, lets the Environmental Protection Agency (EPA) set national limits on outdoor air pollutants and requires states to meet them. The EPA sets air-quality standards for six criteria air pollutants, called that because the limits are based on scientific criteria about health effects:
- Carbon monoxide (CO)
- Lead (Pb)
- Nitrogen dioxide (NO₂)
- Ground-level ozone (O₃)
- Particulate matter (PM₁₀ and PM₂.₅)
- Sulfur dioxide (SO₂)
A success story: lead
Lead was once added to gasoline to help engines run smoothly. Lead is a neurotoxin (it damages the nervous system), and children are especially vulnerable. The US phased lead out of gasoline starting in the 1970s and banned it in fuel for ordinary cars in 1996. Lead levels in the air fell by more than 90 percent, and average blood lead levels in American children dropped sharply. It is one of the clearest examples of a regulation working.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying pollutants
Classify each as a primary or secondary pollutant: (a) sulfur dioxide from a coal plant; (b) ozone near the ground on a summer afternoon; (c) carbon monoxide from a car; (d) sulfuric acid in rain.
Show the solutionHide the solution
- Step 1: Ask: did it come out of a source in this form, or did it form later in the air?
- Step 2: (a) SO₂ comes straight out of the smokestack: primary.
- Step 3: (b) Ground-level ozone isn't emitted by anything; it forms when NOₓ and VOCs react in sunlight: secondary.
- Step 4: (c) CO comes straight out of the tailpipe: primary.
- Step 5: (d) Sulfuric acid forms when SO₂ reacts with water and oxygen in the air: secondary.
Answer: (a) Primary. (b) Secondary. (c) Primary. (d) Secondary.
- Example 2Calculator allowed
Percent reduction in emissions
A city's NOₓ emissions fell from 150 metric tons per day to 45 metric tons per day after new vehicle and power plant rules. Calculate the percent reduction.
Show the solutionHide the solution
- Step 1: Percent change = (new − old) ÷ old × 100.
- Step 2: (45 − 150) ÷ 150 × 100 = −105 ÷ 150 × 100 = −70%.
- Step 3: A negative percent change means a decrease, so emissions fell by 70%.
Answer: A 70% reduction.
Common mistakes
- Calling ground-level ozone a primary pollutant. Nothing emits it directly; it forms from NOₓ and VOCs in sunlight.
- Listing CO₂ as one of the six criteria air pollutants. The six are CO, lead, NO₂, ozone, particulate matter and SO₂.
- Dividing by the new value instead of the original value when calculating percent change.
On the exam
- Know the six criteria pollutants and be ready to identify primary versus secondary examples.
- Percent change calculations appear often in this unit. Write the formula, plug in, and state whether it is an increase or decrease.
Connected topics
Videos
Check yourself
4 questions on 7.1 Introduction to Air Pollution. Pick an answer to see if you got it, and why.
Which of the following is a secondary air pollutant?
| Year | Average lead concentration in US urban air (µg/m³) |
|---|---|
| 1975 | 1.20 |
| 1985 | 0.40 |
| 1995 | 0.06 |
| 2005 | 0.03 |
Hypothetical data based on national trends
Which action best explains the trend in the data?
By what percentage did the average lead concentration decrease from 1975 to 1995?
Which of these activities releases mercury into the air, where it can later settle into lakes and rivers?
0 of 4 answered