Where does air pollution come from, and what is the difference between pollution made directly and pollution made in the air?
Topic 7.1 Introduction to Air Pollution: identify the major air pollutants and their sources and distinguish primary from secondary pollutants.
A focused answer to APES Topic 7.1, covering the major air pollutants, their natural and human sources, the criteria pollutants, and the distinction between primary and secondary pollutants, with a worked emissions calculation.
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What this topic is asking
The College Board (Topic 7.1) wants you to identify the major air pollutants and their sources, and distinguish primary from secondary pollutants.
The major air pollutants and their sources
Primary versus secondary pollutants
Why this matters
This classification is the backbone of Unit 7. Photochemical smog (Topic 7.2) is built on the secondary pollutant ozone; acid rain (Topic 7.7) is a secondary product of sulfur and nitrogen oxides; and controls (Topic 7.6) target primary pollutants at the source to stop secondary pollutants forming. The fossil-fuel link ties Unit 7 back to Unit 6 and forward to the climate impacts of Unit 9. On the AP exam, expect to be given a short scenario or data set and asked to classify a named pollutant as primary or secondary and to identify its source; the reliable test is to ask whether the pollutant is emitted directly from a pipe, tailpipe or stack (primary) or formed afterwards in the air from reactions among other pollutants (secondary). Keeping the six criteria pollutants and that one distinction firmly in mind makes the rest of the unit, from smog to acid rain to control devices, far easier to reason through.
Try this
Q1. Identify two primary air pollutants. [1 point]
- Cue. Any two of carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter.
Q2. Explain the difference between a primary and a secondary air pollutant. [2 points]
- Cue. A primary pollutant is released directly into the air (such as sulfur dioxide from a smokestack); a secondary pollutant forms in the air from reactions among primary pollutants (such as ground-level ozone or acid rain).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Distinguish between a primary and a secondary air pollutant. (b) Identify one example of each. (c) Identify two major human sources of air pollution. (d) Explain why burning fossil fuels is the largest source of many air pollutants.Show worked answer →
A 4-point FRQ on air pollution basics.
(a) Distinguish (1 point): a primary pollutant is emitted directly into the air (for example carbon monoxide, sulfur dioxide); a secondary pollutant forms in the air from chemical reactions among primary pollutants (for example ground-level ozone).
(b) Identify (1 point): a primary pollutant such as carbon monoxide or nitrogen oxide; a secondary pollutant such as ground-level ozone or acid rain.
(c) Identify (1 point): any two of vehicle exhaust, power plants burning fossil fuels, industry, or biomass burning.
(d) Explain (1 point): burning fossil fuels in vehicles, power plants and industry releases carbon monoxide, nitrogen oxides, sulfur dioxide and particulates, so combustion is the dominant source.
Markers reward the directly-emitted-versus-formed-in-air distinction, a valid example of each, two valid human sources, and the fossil-fuel combustion explanation.
AP 2019 (style)1 marksSection I (multiple choice). Ground-level ozone is best described as a secondary pollutant because it: (A) is emitted directly from vehicle tailpipes (B) forms in the air from reactions among other pollutants and sunlight (C) comes only from natural sources (D) settles out of the air quickly. Justify your choice.Show worked answer →
A 1-point MCQ on pollutant types. The answer is (B).
Ground-level ozone is a secondary pollutant: it is not emitted directly but forms in the air when nitrogen oxides and volatile organic compounds react in sunlight. It is not tailpipe-emitted (A), it has human sources (C), and it does not simply settle out quickly (D). The trap is confusing ozone, which forms in the atmosphere, with primary pollutants that are emitted directly.
Related dot points
- Topic 7.2 Photochemical Smog: explain how photochemical smog forms from nitrogen oxides, volatile organic compounds and sunlight, and describe its impacts.
A focused answer to APES Topic 7.2, covering how photochemical smog forms from nitrogen oxides, volatile organic compounds and sunlight, the role of ground-level ozone, the conditions that worsen it, its health and environmental impacts, with a worked ozone-formation reasoning example.
- Topic 7.4 Atmospheric CO2 and Particulates: describe the natural and human sources of atmospheric carbon dioxide and particulate matter and their effects.
A focused answer to APES Topic 7.4, covering the natural and human sources of atmospheric carbon dioxide and particulate matter, the difference between PM10 and PM2.5, why fine particles are most dangerous, the health and environmental effects, with a worked particulate exposure calculation.
- Topic 7.7 Acid Rain: explain how acid rain forms from sulfur and nitrogen oxides and describe its environmental impacts.
A focused answer to APES Topic 7.7, covering how acid deposition forms from sulfur dioxide and nitrogen oxides, the pH scale, the impacts on lakes, forests, soils and buildings, the transboundary nature of the problem, and how to reduce it, with a worked pH calculation.
- Topic 7.6 Reduction of Air Pollutants: describe methods used to reduce air pollution, including regulation, scrubbers, catalytic converters and cleaner fuels.
A focused answer to APES Topic 7.6, covering methods to reduce air pollution including the Clean Air Act and regulation, scrubbers and electrostatic precipitators, catalytic converters, vapor recovery, cleaner fuels and renewable energy, with a worked scrubber efficiency calculation.
- Topic 6.5 Fossil Fuels: explain how fossil fuels form and are used to generate electricity, and describe their environmental impacts, including cogeneration.
A focused answer to APES Topic 6.5, covering how fossil fuels form, how a fossil-fuel power plant generates electricity, fracking, cogeneration, and the environmental impacts of coal, oil and gas, with a worked power plant efficiency calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)