Air Pollution

Summary: Pollutants from fossil fuel combustion: CO (toxic, binds hemoglobin), SO₂ (acid rain), NOx (acid rain, photochemical smog), particulates (respiratory issues). Catalytic converters reduce emissions. Tags: igcse chemistry environmental-chem Created: 2026-07-14 Last Updated: 2026-07-14

Content

Sources of Air Pollution

The primary source of atmospheric pollutants is the combustion of fossil fuels — coal, petroleum, and natural gas. These fuels contain carbon, hydrogen, sulfur (as impurities), and nitrogen-containing compounds. Under different combustion conditions, different pollutants form. Additional pollutants are generated from industrial processes, vehicle engines, and agricultural activities.

The table below summarises the major air pollutants, their formation, and their effects:

Carbon Monoxide (CO)

Formation: Carbon monoxide is produced by the incomplete combustion of carbon-containing fuels. When a fuel burns in a limited supply of oxygen, carbon is not fully oxidised to CO₂:

[ 2\text{C (s)} + \text{O}_2\text{ (g)} \rightarrow 2\text{CO (g)} ]

In a car engine, incomplete combustion of petrol (approximated as octane, C₈H₁₈) produces CO:

[ 2\text{C}8\text{H}{18}\text{ (l)} + 17\text{O}_2\text{ (g)} \rightarrow 16\text{CO (g)} + 18\text{H}_2\text{O (g)} ]

(Compare with complete combustion: 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O)

Common situations producing CO include: faulty or poorly maintained gas boilers, car engines running in enclosed spaces, open fires with inadequate ventilation, and any combustion appliance operating with insufficient air supply.

Toxicity: Carbon monoxide is a colourless, odourless, and tasteless gas — it cannot be detected by human senses, making it particularly dangerous. Its toxicity arises from its ability to bind to haemoglobin in red blood cells. CO binds to the iron(II) ion in haemoglobin approximately 200–250 times more strongly than oxygen does, forming a stable compound called carboxyhaemoglobin:

[ \text{Hb} + \text{CO} \rightarrow \text{HbCO} ]

Once CO occupies the binding sites on haemoglobin, those sites are unavailable for oxygen transport. Even at low concentrations (as little as 0.1% CO in inhaled air), enough haemoglobin becomes blocked to cause oxygen starvation (hypoxia). Symptoms of CO poisoning include headache, dizziness, nausea, confusion, and — at higher levels — loss of consciousness and death.

Prevention: Proper ventilation of combustion appliances, regular servicing of gas boilers, use of CO detectors in homes, and catalytic converters in vehicle exhaust systems.

Sulfur Dioxide (SO₂)

Formation: Sulfur dioxide is produced when fossil fuels containing sulfur impurities are burned. Coal and certain grades of diesel and heavy fuel oil contain significant amounts of sulfur (typically 0.5–5% by mass). During combustion, the sulfur is oxidised:

[ \text{S (s)} + \text{O}_2\text{ (g)} \rightarrow \text{SO}_2\text{ (g)} ]

Sulfur dioxide is a colourless gas with a pungent, choking smell (often described as the smell of a burning match).

Acid rain formation: Once released into the atmosphere, SO₂ dissolves in water droplets (clouds, rain) to form sulfurous acid:

[ \text{SO}_2\text{ (g)} + \text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{SO}_3\text{ (aq)} ]

Sulfurous acid is further oxidised by dissolved oxygen or by atmospheric oxidants (such as ozone or hydrogen peroxide) to sulfuric acid:

[ 2\text{H}_2\text{SO}_3\text{ (aq)} + \text{O}_2\text{ (g)} \rightarrow 2\text{H}_2\text{SO}_4\text{ (aq)} ]

Alternatively, SO₂ itself can be oxidised in the atmosphere to SO₃, which then reacts with water to form H₂SO₄ directly:

[ 2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3 ] [ \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 ]

Sulfuric acid is a strong acid, and precipitation containing it has a pH as low as 3–4 (compared to normal rain pH of approximately 5.6, which is naturally acidic due to dissolved CO₂ forming carbonic acid).

Nitrogen Oxides (NOₓ)

Formation: Nitrogen oxides — primarily nitrogen monoxide (NO) and nitrogen dioxide (NO₂), collectively called NOₓ — are formed not from nitrogen compounds in the fuel but directly from the nitrogen and oxygen in the air. At the high temperatures reached in internal combustion engines and furnaces (above approximately 1500 °C), N₂ and O₂ react:

[ \text{N}_2\text{ (g)} + \text{O}_2\text{ (g)} \rightarrow 2\text{NO (g)} ]

This is an endothermic reaction favoured by high temperatures. Once released, NO is further oxidised in the atmosphere to NO₂:

[ 2\text{NO (g)} + \text{O}_2\text{ (g)} \rightarrow 2\text{NO}_2\text{ (g)} ]

Nitrogen dioxide is a brown, toxic gas with a sharp odour. It is a respiratory irritant that can cause inflammation of the airways and exacerbate conditions like asthma.

Acid rain from NOₓ: Nitrogen dioxide dissolves in water to form a mixture of nitrous and nitric acids:

[ 2\text{NO}_2\text{ (g)} + \text{H}_2\text{O (l)} \rightarrow \text{HNO}_2\text{ (aq)} + \text{HNO}_3\text{ (aq)} ]

Photochemical smog: In the presence of sunlight, NO₂ undergoes photolysis, releasing reactive oxygen atoms that drive a chain of reactions with unburnt hydrocarbons and oxygen to produce ground-level ozone (O₃), peroxyacetyl nitrate (PAN), and other irritating secondary pollutants. This haze of oxidising chemicals is called photochemical smog.

Particulates (Soot)

Formation: Particulates — also referred to as particulate matter (PM) or soot — are tiny solid particles of carbon (and associated hydrocarbons) produced by incomplete combustion, particularly of diesel fuel. Diesel engines tend to produce more soot than petrol engines because of the higher carbon-to-hydrogen ratio in diesel fuel and the nature of compression-ignition combustion.

Effects:

• Respiratory damage: Fine particles (especially PM2.5 — particles smaller than 2.5 micrometres in diameter) penetrate deep into the lungs, causing bronchitis, asthma, and increased risk of lung cancer.
• Global dimming: Soot particles in the atmosphere reflect sunlight back into space, reducing the amount of solar radiation reaching the Earth’s surface. This has a cooling effect that partially offsets greenhouse gas warming, but it also affects weather patterns and reduces agricultural productivity.
• Black carbon deposition: When soot settles on ice and snow, it darkens the surface, increasing absorption of solar radiation and accelerating melting.

Effects of Acid Rain

Acid rain (precipitation with a pH below that of natural rainwater, ~5.6) causes extensive environmental damage:

Catalytic Converters

Catalytic converters are fitted to the exhaust systems of petrol-powered vehicles to reduce emissions of CO, NOₓ, and unburnt hydrocarbons. They contain a ceramic honeycomb structure coated with catalysts — typically a mixture of platinum (Pt), palladium (Pd), and rhodium (Rh). The honeycomb design provides a very large surface area for the catalyst, maximising contact between the exhaust gases and the catalytic surface.

The catalyst facilitates two simultaneous processes:

Oxidation of carbon monoxide: [ 2\text{CO (g)} + \text{O}_2\text{ (g)} \xrightarrow{\text{Pt/Pd}} 2\text{CO}_2\text{ (g)} ]

Reduction of nitrogen monoxide: [ 2\text{NO (g)} + 2\text{CO (g)} \xrightarrow{\text{Rh}} \text{N}_2\text{ (g)} + 2\text{CO}_2\text{ (g)} ]

Notice that in the second reaction, CO acts as the reducing agent for NO — the two toxic pollutants eliminate each other, producing the less harmful products N₂ and CO₂. Any unburnt hydrocarbons in the exhaust are also oxidised to CO₂ and H₂O.

Limitations of catalytic converters:

• They only function effectively when hot (typically require exhaust temperatures above approximately 300 °C), meaning they are ineffective during the first few minutes of a journey (the “cold start” problem).
• They do not eliminate CO₂ emissions — in fact, they convert CO into CO₂, contributing to greenhouse gas emissions.
• They require unleaded petrol because lead compounds poison the catalyst by coating its surface and blocking active sites.
• They do not reduce sulfur dioxide emissions, as sulfur is not catalytically converted.

Reducing Air Pollution

Several strategies are employed to reduce emissions of atmospheric pollutants:

Flue gas desulfurisation (FGD): In coal-fired power stations, the exhaust gases (flue gases) are passed through a scrubber containing a slurry of limestone (CaCO₃) or quicklime (CaO). The basic oxide/hydroxide reacts with acidic SO₂:

[ \text{CaCO}_3\text{ (s)} + \text{SO}_2\text{ (g)} \rightarrow \text{CaSO}_3\text{ (s)} + \text{CO}_2\text{ (g)} ]

or

[ \text{CaO (s)} + \text{SO}_2\text{ (g)} \rightarrow \text{CaSO}_3\text{ (s)} ]

The calcium sulfite (CaSO₃) can be further oxidised to calcium sulfate (CaSO₄, gypsum), which is used to make plasterboard. FGD can remove over 90% of SO₂ from flue gases.

Low-sulfur fuels: Using natural gas (which contains negligible sulfur) instead of coal or diesel, or using petroleum that has been treated to remove sulfur compounds.

Catalytic converters: As described above, reduce CO, NOₓ, and hydrocarbon emissions from petrol vehicles.

Renewable energy sources: Wind, solar, hydroelectric, tidal, and nuclear power generate electricity without burning fossil fuels, eliminating SO₂, NOₓ, and particulate emissions at the point of generation.

Emission control technologies: Electrostatic precipitators remove particulate matter from industrial flue gases by charging particles electrically and collecting them on oppositely charged plates. Diesel particulate filters (DPFs) trap soot from diesel exhausts.

Legislation: Clean Air Acts and vehicle emission standards (such as Euro standards in Europe) set legal limits on pollutant emissions from industrial and mobile sources.

Lead Compounds (Historical Context)

From the 1920s through to the late 20th century, tetraethyllead, Pb(C₂H₅)₄, was added to petrol as an anti-knock agent. It raised the octane rating of petrol, preventing premature ignition (knocking) in engines and allowing higher compression ratios. During combustion, lead was released into the atmosphere as lead bromide and lead chloride particulates (halide scavengers were added to prevent lead oxide deposits inside the engine).

Lead is a potent neurotoxin. Even low-level exposure, particularly in children, impairs cognitive development, reduces IQ, and causes behavioural problems. Atmospheric lead from vehicle exhaust settled on soil and dust, making it a widespread public health hazard.

Leaded petrol has been phased out in virtually all countries (the last country to ban it, Algeria, did so in 2021). Its removal was, alongside the introduction of catalytic converters (which require lead-free fuel), one of the most successful public health interventions of the 20th century.

Key Concepts from Past Papers

• carbon dioxide
• mark each for any 2 of: carbon monoxide carbon water
• calcium oxide carbon dioxide
• carbon dioxide water
• mark each for any two of: plastics sewage microbes
• carbon carbon dioxide
• mark each for any two of: catalytic converters low sulfur fuels flue gas desulfurisation
• mark each for any 2 of:

Keywords from Past Papers

carbon, electrons, dioxide, bonding, between, arrangement, hydrogen, monoxide, compound, gas, only, non, pair, water, extra

Related Notes

• Composition of Air
• Climate Change and Greenhouse Gases
• Water Treatment

Sources

• OpenStax, Chemistry 2e, Chapter 18: Representative Metals, Metalloids, and Nonmetals — Section 18.11, occurrence, preparation, and properties of nitrogen and phosphorus, including NOₓ chemistry; Chapter 9: Gases — atmospheric chemistry and air pollution. https://openstax.org/books/chemistry-2e/
• BBC Bitesize, GCSE Chemistry (Single Science), “Polluting the Atmosphere” and “Atmospheric Pollutants.” https://www.bbc.co.uk/bitesize/topics/z9k3qyc
• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 11: Air and Water — pollutants from combustion of fuels, acid rain formation, catalytic converters.
• CK-12 Foundation, Chemistry, Chapter: Atmosphere — air pollution and air quality. https://www.ck12.org/chemistry/

Past Paper Sources

• 0620/31 May/June 2015: Q11(c)(i) (0m), Q33(b)(ii) (0m)
• 0620/32 Feb/March 2015: Q66(c)(i) (2m), Q11(a)(ii) (1m), Q44(b)(iii) (2m) (+2 more)
• 0620/32 Feb/March 2017: Q33(b)(iii) (1m), Q22(e)(ii) (0m), Q22(e)(ii) (0m) (+1 more)
• 0620/32 Feb/March 2018: Q11(a)(iv) (1m), Q77(b)(v) (1m), Q77(b)(v) (1m)
• 0620/32 Feb/March 2019: Q77(b)(ii) (2m), Q77(b)(i) (1m), Q77(b)(i) (1m)
• 0620/32 Feb/March 2022: Q55(b)(ii) (4m), Q55(b)(ii) (4m)
• 0620/32 Feb/March 2023: Q77(a)(iii) (2m), Q33(b)(ii) (1m), Q77(a)(iii) (1m)
• 0620/32 May/June 2018: Q11(a)(iv) (1m), Q22(b)(ii) (1m), Q22(c)(i) (0m) (+2 more)
• 0620/32 May/June 2020: Q11(a)(i) (1m), Q44(a)(ii) (1m), Q44(a)(ii) (1m)
• 0620/33 May/June 2016: Q88(c)(i) (1m), Q55(b)(ii) (0m), Q55(b)(ii) (0m)
• 0620/33 May/June 2019: Q33(b)(i) (1m)
• 0620/33 May/June 2021: Q55(c)(i) (2m), Q55(c)(i) (2m)

Common Misconceptions

• Complete combustion produces CO and incomplete combustion produces CO₂. This is reversed. Complete combustion (plenty of oxygen) produces CO₂; incomplete combustion (limited oxygen) produces CO and/or C (soot).
• CO₂ is the main cause of acid rain. Carbon dioxide dissolves in water to form the weakly acidic carbonic acid (giving natural rain a pH of ~5.6), but it is sulfur dioxide and nitrogen oxides that are responsible for the severe acidity of acid rain (pH 3–4). Calling CO₂ a cause of acid rain conflates the natural acidity of rain with the environmental problem of anthropogenic acidification.
• Acid rain is caused by acids being released directly from factories. Most acid rain precursors (SO₂, NOₓ) are not themselves acids. They are gases that undergo chemical reactions in the atmosphere to form sulfuric and nitric acids.
• Catalytic converters eliminate all pollution from vehicle exhausts. They reduce CO, NOₓ, and hydrocarbons, but do not remove CO₂ (they increase it by converting CO to CO₂) and do not address SO₂ from sulfur-containing fuels.
• Nitrogen oxides come from nitrogen impurities in the fuel. NOₓ form from the direct reaction between N₂ and O₂ in the air at high temperatures, not from nitrogen compounds in petrol or diesel. The fuel itself contains negligible nitrogen.
• Carbon monoxide can be smelled because it is a combustion product. CO is odourless — this is why it is so dangerous. The smell sometimes associated with combustion (e.g. exhaust fumes) comes from other compounds like hydrocarbons and NO₂, not CO.
• Sulfur dioxide is only produced by industrial processes. Natural sources such as volcanic eruptions and the decomposition of organic matter also release SO₂, though human activity is the dominant contributor in most regions.
• Particulates are only a nuisance, not a serious health threat. Fine particulate matter (PM2.5) is one of the most harmful air pollutants by health impact, contributing to millions of premature deaths annually through respiratory and cardiovascular disease.