A Level · Cambridge (CIE) · 9701

Explain PAN formation in photochemical smog

Connect sunlight, atmospheric NOₓ and unburned hydrocarbons to peroxyacetyl nitrate as a smog component.

Nitrogen and sulfur · Cambridge 9701 AS Chemistry · 12.1

AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Explain the stated molecular, acid–base and atmospheric reaction pathways; balance equations and distinguish direct reactions from catalytic cycles. Written practice is self-checked

Before you begin

Use covalent bond order, lone pairs, Brønsted–Lowry proton transfer, oxidation numbers and balanced equations.

Work on paper. Try each question before revealing help, and explain why your method works.

Quick readiness check

What does PAN stand for?

What you will learn

Expand PAN
Identify the atmospheric reactant families
Explain the role of sunlight and photochemical reactions

Schematic nitrogen and sulfur atmospheric pathways. High-temperature nitrogen and oxygen form NO, which can oxidise to NO₂. In sunlight, NOₓ and unburned hydrocarbons can form PAN, a component of photochemical smog. NO₂ and water can form nitric acid directly. Separately, NO₂ oxidises SO₂ to SO₃ and becomes NO; oxygen regenerates NO₂, and SO₃ plus water forms sulfuric acid. Arrows show simplified syllabus pathways, not a complete atmospheric model.

Photochemical smog forms through sunlight-driven reactions

In polluted air, sunlight initiates photochemical reactions involving nitrogen dioxide and unburned hydrocarbons. A sequence of reactions can form secondary pollutants rather than simply releasing one pollutant directly from a source.

Atmospheric NO and NO₂ can react with unburned hydrocarbons to form peroxyacetyl nitrate, abbreviated PAN. PAN is one component of photochemical smog.

Track the inputs and avoid overclaiming

The syllabus asks students to know that NOₓ and unburned hydrocarbons can form PAN; a detailed mechanism is not needed here.

Smog composition depends on sunlight, precursor concentrations and atmospheric conditions. Do not describe PAN as a nitrogen oxide or claim it is the only component of smog.

Pause and explain

Which precursor combination is linked to PAN formation in the atmosphere?

Put the idea to work

Worked example

State how PAN can form and why it is relevant to photochemical smog.

Show the worked solution

Sunlight drives atmospheric photochemical reactions.
NO and NO₂ can react with unburned hydrocarbons in these reaction sequences.
One product is peroxyacetyl nitrate (PAN), a component of photochemical smog.

Answer Sunlight-driven reactions involving atmospheric NOₓ and unburned hydrocarbons can form peroxyacetyl nitrate (PAN), which is one component of photochemical smog.

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From guided practice to a new situation

Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.

1 · Guided

Expand PAN and state its role in photochemical smog.

Give me a hint

Write the full name and distinguish a component from the whole mixture.

Compare my reasoning

PAN is peroxyacetyl nitrate.
It is formed in atmospheric reaction sequences.
It is one component of photochemical smog.

PAN is peroxyacetyl nitrate, a component of photochemical smog.

Look for these in your work

Name expanded
Atmospheric context stated
Component distinction made

2 · Independent

Describe the precursor pathway that can form PAN.

Give me a hint

Include sunlight, NOₓ and unburned hydrocarbons.

Compare my reasoning

Sunlight drives atmospheric photochemical reactions.
NO and NO₂ react in sequences with unburned hydrocarbons.
These reactions can form PAN.

Sunlight-driven reactions between atmospheric NOₓ and unburned hydrocarbons can form PAN.

Look for these in your work

Sunlight included
Both precursor families included
Product named

3 · Transfer

A response says 'NO₂ is PAN and causes all photochemical smog.' Correct it.

Give me a hint

Distinguish a precursor, a product and the wider pollutant mixture.

Compare my reasoning

NO₂ is a nitrogen oxide precursor, not PAN.
NOₓ and unburned hydrocarbons can react in sunlight to form PAN.
PAN is one component of photochemical smog, not the entire mixture.

NO₂ is a precursor; photochemical reactions with unburned hydrocarbons can form PAN, which is one component of smog.

Look for these in your work

Species distinguished
Formation explained
Scope corrected

Common mistakes

Expanding PAN incorrectly
Calling PAN a primary exhaust emission in every case
Saying sunlight is a product or that PAN is the only smog component

Recall without your notes

What can you explain now?

What does PAN stand for?

Compare with the explanation

Peroxyacetyl nitrate

PAN is the abbreviation for peroxyacetyl nitrate.

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Make it stick

Which two precursor families and environmental condition are linked to PAN formation?

If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.

Keep learning Chemistry

Check the official syllabus 2028–2030

Course scope and assessment

Four hundred and forty-two original lessons map the published learning outcomes across AS Chemistry Topics 1–22 and A Level Topics 23–37, with guided, independent and transfer practice and explained answers. Every numbered subject-content topic and listed outcome tag now has a mapped lesson sequence. Topic 32 maps all 12 hydroxy-compound outcome tags in 11 lessons; Topic 33 maps all 15 carboxylic-acid and derivative outcome tags in 16 lessons; Topic 34 maps all 22 nitrogen-compound outcome tags in 20 lessons; Topic 35 maps all 12 polymerisation outcome tags in 16 lessons; Topic 36 maps all four organic-synthesis outcome tags in 14 lessons; Topic 37 maps all 22 chromatography and NMR outcome tags in 18 lessons. Cambridge's 2028–2030 Version 1 syllabus states that there are no significant changes which affect teaching from the preceding edition; the routes are aligned to that edition. This is 37 of the 37 numbered subject-content topics. Full cumulative exam assessment, Paper 3 laboratory competence, Paper 5 investigation practice and human subject-teacher review remain outstanding. Written practice is self-checked and does not certify exam readiness.