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SAChemistrySyllabus dot point

How do we monitor and explain the chemistry of air pollution?

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

Sources and chemistry of CO, NOx, SO2, VOCs and particulates, plus how sunlight drives photochemical smog and ground-level ozone formation.

Generated by Claude Opus 4.77 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Primary pollutants and their sources
  3. Photochemical smog
  4. Why monitoring matters

What this dot point is asking

You need to name the principal primary pollutants, give a realistic source for each, and explain how secondary pollutants - especially ground-level ozone - are produced by sunlight-driven reactions. This dot point links combustion chemistry to environmental monitoring.

Primary pollutants and their sources

A primary pollutant is emitted directly into the atmosphere. A secondary pollutant is formed by chemical reactions once the primary species are in the air.

Carbon monoxide

Incomplete combustion occurs when oxygen supply is limited:

2 C8H18(g)+17 O2(g)→16 CO(g)+18 H2O(g)2\,\text{C}_8\text{H}_{18}(g) + 17\,\text{O}_2(g) \rightarrow 16\,\text{CO}(g) + 18\,\text{H}_2\text{O}(g)

CO\text{CO} is toxic because it binds to haemoglobin more strongly than O2\text{O}_2, reducing oxygen transport in blood.

Oxides of nitrogen

In a hot engine cylinder, nitrogen and oxygen from the air combine:

N2(g)+O2(g)→2 NO(g)\text{N}_2(g) + \text{O}_2(g) \rightarrow 2\,\text{NO}(g)

Nitrogen monoxide then oxidises in air to nitrogen dioxide:

2 NO(g)+O2(g)→2 NO2(g)2\,\text{NO}(g) + \text{O}_2(g) \rightarrow 2\,\text{NO}_2(g)

NO2\text{NO}_2 is a brown gas and is the key player in smog and acid rain.

Photochemical smog

Photochemical smog is a brownish haze that forms over cities on warm, sunny, still days. It needs three ingredients: NOx\text{NO}_x, VOCs, and sunlight (UV).

The ozone cycle

Sunlight splits nitrogen dioxide:

NO2(g)→  UV  NO(g)+O(g)\text{NO}_2(g) \xrightarrow{\;UV\;} \text{NO}(g) + \text{O}(g)

The free oxygen atom reacts with molecular oxygen to make ground-level ozone:

O(g)+O2(g)→O3(g)\text{O}(g) + \text{O}_2(g) \rightarrow \text{O}_3(g)

Ozone is a secondary pollutant. At ground level it is a respiratory irritant and damages plants, even though stratospheric ozone is beneficial (it absorbs UV). VOCs feed the cycle by converting NO\text{NO} back to NO2\text{NO}_2, allowing ozone to accumulate rather than being destroyed.

Why monitoring matters

Air-quality monitoring measures these pollutants so health and environmental impacts can be managed. Catalytic converters in cars reduce CO\text{CO}, NOx\text{NO}_x and unburnt hydrocarbons, while low-sulfur fuels cut SO2\text{SO}_2.

Remember that NOx\text{NO}_x forms from atmospheric nitrogen and oxygen at high engine temperatures - not from nitrogen in the fuel. Linking the right source to each pollutant is exactly what markers reward.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2022 SACE Stage 26 marksExplain why it is desirable to reduce the quantities of nitrogen oxides generated by motor vehicles. Include at least one equation to illustrate your answer.
Show worked answer →

A full 6-mark answer needs several linked environmental effects, each tied to chemistry, plus at least one equation.

  1. Nitrogen oxides contribute to photochemical smog. In sunlight, NO2 photodissociates to give NO and an oxygen atom, which forms ground-level ozone: NO2 + sunlight -> NO + O, then O + O2 -> O3. Ground-level ozone is a respiratory irritant and damages plants and materials.

  2. Nitrogen oxides cause acid rain. NO is oxidised to NO2, which reacts with water to form nitric acid: 3NO2 + H2O -> 2HNO3 + NO. Acid rain lowers the pH of waterways and soils, harming aquatic life and corroding buildings.

  3. Health effects. NO2 is a toxic gas that irritates the lungs and aggravates asthma.

Markers award marks for distinct, correctly explained effects and for at least one balanced, relevant equation.

2024 SACE Stage 23 marksExplain why it is desirable to reduce the concentration of NO in exhaust emissions from diesel engines.
Show worked answer →

For 3 marks, give distinct reasons, each grounded in chemistry.

  1. NO is oxidised in air to NO2, a brown toxic gas that irritates the respiratory system.

  2. NO2 reacts with water to form nitric acid (3NO2 + H2O -> 2HNO3 + NO), a major cause of acid rain that lowers the pH of soils and waterways and corrodes structures.

  3. Nitrogen oxides drive the formation of photochemical smog and ground-level ozone in the presence of sunlight, which damages health, vegetation, and materials.

Any three correctly explained points earn full marks.

2024 SACE Stage 22 marksDescribe one undesirable consequence of the presence of CO in exhaust gases.
Show worked answer →

Carbon monoxide is a toxic gas. It binds to haemoglobin in the blood much more strongly than oxygen does, forming carboxyhaemoglobin. This reduces the bloods capacity to carry oxygen to body tissues, causing oxygen deprivation that can lead to headaches, unconsciousness, and at high concentrations, death.

One mark for identifying the toxic effect, one for explaining the mechanism (competing with oxygen for haemoglobin and reducing oxygen transport).

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