Why does a brown haze form over cities on hot, sunny days?
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.
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What this topic is asking
The College Board (Topic 7.2) wants you to explain how photochemical smog forms from nitrogen oxides, volatile organic compounds and sunlight, and describe its impacts.
How photochemical smog forms
Conditions that worsen smog
Impacts
Why this matters
Photochemical smog is the headline secondary-pollutant problem of Unit 7, building directly on the primary-versus-secondary distinction of Topic 7.1. It connects to thermal inversions (which trap it), to urbanization (traffic and population), and to the control strategies of Topic 7.6 (catalytic converters, cleaner fuels) that cut its precursors.
Try this
Q1. Identify the harmful secondary pollutant at the heart of photochemical smog. [1 point]
- Cue. Ground-level ozone (O3).
Q2. Explain why photochemical smog is worst on hot, sunny days. [2 points]
- Cue. Sunlight provides the energy that drives the reactions between nitrogen oxides and volatile organic compounds to form ground-level ozone, so the more intense the sunlight and warmth, the more smog forms.
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 2022 (style)4 marksSection II (FRQ). (a) Identify the two main precursor pollutants that lead to photochemical smog. (b) Explain the role of sunlight in forming photochemical smog. (c) Identify the harmful secondary pollutant at the heart of photochemical smog. (d) Describe one human health effect of photochemical smog.Show worked answer →
A 4-point FRQ on photochemical smog.
(a) Identify (1 point): nitrogen oxides (NOx) and volatile organic compounds (VOCs), mostly from vehicle exhaust.
(b) Explain (1 point): sunlight provides the energy that drives the reactions among nitrogen oxides and volatile organic compounds, producing ground-level ozone, which is why smog peaks on sunny days.
(c) Identify (1 point): ground-level ozone (O3).
(d) Describe (1 point): ground-level ozone irritates the eyes, throat and lungs, worsens asthma and other respiratory conditions, and reduces lung function.
Markers reward nitrogen oxides and volatile organic compounds as precursors, sunlight as the energy source, ground-level ozone as the key product, and a valid respiratory health effect.
AP 2018 (style)1 marksSection I (multiple choice). Photochemical smog is worst on days that are: (A) cold and dark (B) hot, sunny and with heavy traffic (C) rainy and windy (D) cool and overcast. Justify your choice.Show worked answer →
A 1-point MCQ on smog conditions. The answer is (B).
Photochemical smog forms when nitrogen oxides and volatile organic compounds from traffic react in strong sunlight, so it peaks on hot, sunny days with heavy traffic. Cold and dark (A), rainy and windy (C, which disperses pollutants), and cool overcast conditions (D) all reduce smog formation. The trap is forgetting that sunlight and warmth drive the reactions, so smog is a warm-weather, daytime problem.
Related dot points
- 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.
- Topic 7.3 Thermal Inversion: explain how a thermal inversion forms and why it traps air pollution near the ground.
A focused answer to APES Topic 7.3, covering how a thermal inversion forms, why it reverses the normal temperature profile, how it traps pollutants near the surface, the role of topography, and its link to severe smog events, with a worked reasoning example.
- 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 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 5.10 Urbanization: explain the environmental effects of urbanization, including impervious surfaces, runoff, the urban heat island, sprawl and saltwater intrusion.
A focused answer to APES Topic 5.10, covering urbanization, impervious surfaces and increased runoff, the urban heat island effect, urban sprawl, depletion and saltwater intrusion, and the benefits of smart growth, with a worked impervious-surface calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)