Atmospheric pollutants and photochemical smog (SACE Stage 2 Chemistry)

Syllabus dot point

“Identify the major atmospheric pollutants, their sources, and the chemistry of photochemical smog formation.”

8 min readUpdated 2 Jun 2026

Quick answer

Atmospheric pollutants are split into primary pollutants, emitted directly (e.g. $\text{CO}$ , $\text{NO}$ , $\text{SO}_2$ , particulates), and secondary pollutants, formed in the air from primary ones (e.g. ozone, $\text{NO}_2$ from $\text{NO}$ , peroxyacetyl nitrate). Photochemical smog is a secondary haze produced when sunlight drives reactions between $\text{NO}_x$ and unburnt hydrocarbons from vehicle exhaust, generating ground-level ozone. You must give sources, balanced equations and the role of sunlight.

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What this dot point is asking
Lead worked calculation
Primary pollutants and their sources
The photochemical smog cycle
Acid rain chemistry
Why it matters for monitoring
Exam-style questions

What this dot point is asking

SACE expects you to classify pollutants as primary or secondary, write balanced equations for their formation, and explain the photochemical cycle that builds up ozone and smog.

Lead worked calculation

Mass of NO from incomplete combustion conditions

A car engine produces nitrogen monoxide when the high combustion temperature makes atmospheric nitrogen and oxygen react: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ . If $0.28\ \text{g}$ of $\text{N}_2$ reacts completely, what mass of $\text{NO}$ forms? ( $M(\text{N}_2) = 28.0$ , $M(\text{NO}) = 30.0\ \text{g mol}^{-1}$ .)

This is a three-step stoichiometry calculation: mass to moles, apply the mole ratio, then moles back to mass.

Step 1: Convert the mass of nitrogen to moles

Divide the given mass by the molar mass of $\text{N}_2$ to get the amount in moles:

n(\text{N}_2) = \frac{m}{M} = \frac{0.28}{28.0} = 0.010\ \text{mol.}

Step 2: Apply the mole ratio from the balanced equation

The equation $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ shows that one mole of $\text{N}_2$ produces two moles of $\text{NO}$ , giving a $1:2$ ratio:

n(\text{NO}) = 2 \times 0.010 = 0.020\ \text{mol.}

Step 3: Convert moles of NO to mass

Multiply the moles by the molar mass of $\text{NO}$ :

m(\text{NO}) = nM = 0.020 \times 30.0 = 0.60\ \text{g.}

Final answer: $0.60\ \text{g}$ of $\text{NO}$ is emitted. This $\text{NO}$ then oxidises to $\text{NO}_2$ in air, feeding the photochemical smog cycle.

Primary pollutants and their sources

Primary versus secondary pollutants

A primary pollutant is released directly into the atmosphere from a source. A secondary pollutant is not emitted directly but forms in the atmosphere from chemical reactions of primary pollutants, often driven by sunlight.

Carbon monoxide ( $\text{CO}$ ): from incomplete combustion of fuels when oxygen is limited: $2\text{C} + \text{O}_2 \rightarrow 2\text{CO}$ . Toxic because it binds haemoglobin more strongly than oxygen.
Nitrogen oxides ( $\text{NO}_x$ ): $\text{N}_2$ and $\text{O}_2$ combine at the high temperatures inside engines: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ , and $\text{NO}$ oxidises to $\text{NO}_2$ in air.
Sulfur dioxide ( $\text{SO}_2$ ): from burning sulfur-containing fuels: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$ . A cause of acid rain.
Particulates: tiny solid/liquid particles (soot, ash) from incomplete combustion, harmful to lungs.
Volatile organic compounds (VOCs): unburnt hydrocarbons from fuel and solvents, feeding smog.

The photochemical smog cycle

Photochemical smog needs three ingredients: $\text{NO}_x$ , VOCs and sunlight. The key starter is the photodissociation of $\text{NO}_2$ :

\text{NO}_2 \xrightarrow{\text{UV}} \text{NO} + \text{O}

\text{O} + \text{O}_2 \rightarrow \text{O}_3

Ground-level ozone is the hallmark secondary pollutant. The cycle is self-sustaining because $\text{NO}$ is regenerated and re-oxidised, and reactive radicals from VOCs convert more $\text{NO}$ back to $\text{NO}_2$ , allowing ozone to accumulate during sunny, still, traffic-heavy conditions. Other secondary pollutants such as peroxyacetyl nitrate (PAN) also form, irritating eyes and lungs.

Good ozone versus bad ozone

The same molecule, $\text{O}_3$ , is beneficial in the stratosphere, where it absorbs harmful UV radiation, but harmful at ground level, where it is a toxic secondary pollutant and a key marker of photochemical smog. Location, not chemistry, determines whether ozone is protective or damaging.

Acid rain chemistry

Both $\text{SO}_2$ and $\text{NO}_x$ produce acids in the atmosphere. Sulfur dioxide is oxidised then hydrated: $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$ , then $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$ . Nitrogen dioxide gives nitric acid: $3\text{NO}_2 + \text{H}_2\text{O} \rightarrow 2\text{HNO}_3 + \text{NO}$ . These acids lower rainfall pH, damaging vegetation, leaching nutrients from soil, acidifying waterways and corroding limestone buildings.

Mislabelling primary and secondary pollutants

Students often call ozone a primary pollutant because it is harmful; the classification depends on how it forms, not how dangerous it is. Ozone and $\text{NO}_2$ (when made from $\text{NO}$ ) are secondary, formed in the atmosphere. $\text{NO}$ , $\text{CO}$ , $\text{SO}_2$ and particulates emitted from the source are primary. Also write combustion equations as balanced reactions, and remember sunlight (UV) is essential to start the smog cycle.

Why it matters for monitoring

Air-quality networks track $\text{CO}$ , $\text{NO}_x$ , $\text{SO}_2$ , ozone and particulates because each signals a different pollution source and health risk. Understanding which are primary and which are secondary tells regulators whether to control emissions at source or the conditions that drive secondary formation.

Exam-style questions

Questions in the style of SACE Board exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

SACE 2022-style4 marks

Photochemical smog forms through reactions initiated by sunlight. (a) Write the equation for the photodissociation of nitrogen dioxide that starts the cycle. (b) Explain, using equations, how ground-level ozone is produced from this step. (c) State why ozone is described as a secondary pollutant.

Show worked answer →

(a) Sunlight splits $\text{NO}_2$ : $\text{NO}_2 \xrightarrow{\text{UV}} \text{NO} + \text{O}$ . (1 mark)

(b) The oxygen atom produced reacts with molecular oxygen to form ozone: $\text{O} + \text{O}_2 \rightarrow \text{O}_3$ . (2 marks)

(c) Ozone is a secondary pollutant because it is not emitted directly; it is formed in the atmosphere from primary pollutants ( $\text{NO}_2$ and $\text{O}_2$ ) through reactions driven by sunlight. (1 mark)

SACE 2020-style3 marks

A power station emits sulfur dioxide. (a) Write the equation for the formation of

\text{SO}_2

when sulfur-containing fuel is burned. (b) Explain, with an equation, how this contributes to acid rain. (c) Classify

\text{SO}_2

as a primary or secondary pollutant and justify your choice.

Show worked answer →

(a) $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$ (the sulfur impurity in the fuel is oxidised on combustion). (1 mark)

(b) In the atmosphere $\text{SO}_2$ is oxidised and dissolves to form sulfuric acid, lowering rain pH: $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$ then $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$ . (1 mark)

(c) $\text{SO}_2$ is a primary pollutant because it is released directly into the atmosphere from the source (the burning fuel), rather than being formed there from other pollutants. (1 mark)

Practise this

Sources & how we know this

Stage 2 Chemistry subject outline (2025 onwards) — SACE Board
Stage 2 Chemistry: external assessment — SACE Board