C32: Air Pollution
Sulfur dioxide, nitrogen oxides, particulates and carbon monoxide as air pollutants, acid rain formation and effects, photochemical smog, catalytic converters, and the health and environmental impacts of pollution.
Sources of Air Pollution
Air pollution is caused by the release of harmful substances into the atmosphere. The main pollutants come from combustion of fossil fuels in power stations, vehicles and industry. Some pollutants are also produced by natural processes such as volcanic eruptions and wildfires, but human activities are the dominant source.
Sulfur Dioxide (SO₂)
Sulfur dioxide is produced when fossil fuels containing sulfur impurities are burned. Coal and some oils contain significant amounts of sulfur. When these fuels are combusted in power stations and industrial processes, SO₂ is released into the atmosphere.
Formation of sulfur dioxide:
S + O₂ → SO₂
Effects of sulfur dioxide:
- Dissolves in water vapour in the atmosphere to form sulfurous acid (H₂SO₃), which contributes to acid rain
- Further oxidised to sulfur trioxide (SO₃), which dissolves to form sulfuric acid (H₂SO₄) — a stronger acid
- Causes respiratory problems in humans, particularly asthma and bronchitis
- Damage to buildings and stonework (especially limestone and marble, which react with acid)
- Harmful to aquatic life when acid rain enters lakes and rivers
- Damage to plants and trees — acid damages leaves and roots
Oxides of Nitrogen (NOₓ)
Nitrogen oxides (mainly NO and NO₂, collectively called NOₓ) are formed when nitrogen and oxygen in the air react at the high temperatures found in car engines and power stations. The high temperature provides enough energy to break the strong N≡N triple bond in nitrogen gas.
Formation of nitrogen monoxide:
N₂ + O₂ → 2NO (at high temperatures)
Oxidation to nitrogen dioxide:
2NO + O₂ → 2NO₂
Effects of nitrogen oxides:
- Dissolve in water vapour to form nitric acid, contributing to acid rain
- Cause respiratory problems — irritate and inflame the airways
- React with sunlight and hydrocarbons to form photochemical smog
- Contribute to the formation of ground-level ozone, which is harmful to breathe
- Damage plants — nitrogen dioxide can cause leaf damage and reduced growth
Carbon Monoxide (CO)
Carbon monoxide is produced by the incomplete combustion of carbon-containing fuels. This happens when there is insufficient oxygen for complete combustion. It is mainly produced by petrol engines in cars and by faulty gas appliances.
Incomplete combustion:
2C + O₂ → 2CO (limited oxygen)
2CH₄ + 3O₂ → 2CO + 4H₂O (limited oxygen)
Effects of carbon monoxide:
- Toxic — CO binds to haemoglobin in red blood cells much more strongly than oxygen (about 200–300 times more strongly)
- Reduces the blood's ability to carry oxygen around the body
- Causes dizziness, headaches, fatigue and at high concentrations, death
- CO is colourless and odourless, making it particularly dangerous — people cannot detect it
Make sure you understand the difference between complete and incomplete combustion. Complete combustion produces CO₂ and H₂O. Incomplete combustion produces CO (and/or carbon particulates) and H₂O. Both release energy, but incomplete combustion releases less energy per mole of fuel.
Particulates (Particulate Matter)
Particulates are tiny solid particles (such as soot / unburnt carbon) and liquid droplets suspended in the air. They are produced by:
- Incomplete combustion of fossil fuels (especially diesel engines)
- Wood burning and forest fires
- Industrial processes (cement manufacture, mining, construction)
- Agricultural dust
Effects of particulates:
- Worsen asthma and other respiratory conditions
- Link to lung cancer and heart disease with long-term exposure
- Reduce visibility (contribute to smog and haze)
- Deposit on buildings, causing blackening and damage
- May affect climate by reflecting or absorbing sunlight
Particulates are classified by size: PM10 (particles with diameter less than 10 micrometres) and PM2.5 (less than 2.5 micrometres). Smaller particles are more dangerous because they can penetrate deeper into the lungs and even enter the bloodstream.
Acid Rain
Acid rain is rain that has a pH lower than about 5.2 (normal rain has a pH of about 5.6 due to dissolved CO₂ forming weak carbonic acid). Acid rain is caused by sulfur dioxide and nitrogen oxides dissolving in atmospheric moisture.
Formation of acid rain from SO₂:
SO₂ + H₂O → H₂SO₃ (sulfurous acid)
2SO₂ + O₂ → 2SO₃
SO₃ + H₂O → H₂SO₄ (sulfuric acid)
Formation of acid rain from NO₂:
2NO₂ + H₂O → HNO₃ + HNO₂ (nitric acid + nitrous acid)
Effects of acid rain:
- Lakes and rivers — acidification kills fish and other aquatic organisms. Aluminium ions leached from soil into water are toxic to fish gills.
- Trees and plants — acid damages leaves (causing yellowing and dropping), leaches nutrients from soil, and releases toxic aluminium ions from soil.
- Buildings — acid rain reacts with limestone (calcium carbonate) and marble, dissolving the stone and causing structural damage.
- Metals — accelerates corrosion of iron and steel structures such as bridges and railings.
Acid rain reacting with limestone:
CaCO₃(s) + H₂SO₄(aq) → CaSO₄(aq) + H₂O(l) + CO₂(g)
Photochemical Smog
Photochemical smog is a type of air pollution formed when nitrogen oxides and unburnt hydrocarbons (from vehicle exhausts) react in the presence of sunlight. It is most common in cities with heavy traffic and sunny, warm climates (e.g. Los Angeles, Mexico City).
Formation of photochemical smog:
- Nitrogen oxides and unburnt hydrocarbons are released from vehicle exhausts.
- Sunlight provides energy for chemical reactions between these pollutants.
- These reactions produce ground-level ozone (O₃) and other harmful oxidising chemicals.
- The mixture of pollutants, ozone and fine particles forms a brownish haze — photochemical smog.
Effects of photochemical smog:
- Ground-level ozone irritates eyes, nose and throat and causes breathing difficulties
- Aggravates asthma and other lung conditions
- Reduces crop yields — ozone damages plant tissues
- Reduces visibility
Do not confuse ground-level ozone (a harmful pollutant in photochemical smog) with the ozone layer in the upper atmosphere (which protects us from UV radiation). Ground-level ozone is bad; stratospheric ozone is good.
Catalytic Converters
Catalytic converters are devices fitted in car exhaust systems to reduce the emissions of harmful pollutants. They contain a catalyst (typically a mix of platinum, palladium and rhodium) coated on a ceramic honeycomb structure that provides a large surface area.
Reactions in a catalytic converter:
2CO + 2NO → 2CO₂ + N₂ (carbon monoxide + nitrogen monoxide → carbon dioxide + nitrogen)
CₓHᵧ + (4x+y)/4 NOₓ → xCO₂ + y/2 H₂O + (4x+y)/4 N₂ (unburnt hydrocarbons + nitrogen oxides → carbon dioxide + water + nitrogen)
Catalytic converters work by converting harmful exhaust gases into less harmful products:
- Carbon monoxide is oxidised to carbon dioxide
- Nitrogen oxides are reduced to nitrogen gas
- Unburnt hydrocarbons are oxidised to carbon dioxide and water
Limitations of catalytic converters:
- They only work when hot — they are ineffective during the first few minutes after a cold engine start.
- They do not remove CO₂, which is a greenhouse gas.
- They do not reduce particulate emissions (diesel particulate filters are needed for this).
- They contain expensive rare metals (platinum, palladium, rhodium).
Summary of Air Pollutants
| Pollutant | Source | Health Effects | Environmental Effects |
|---|---|---|---|
| Sulfur dioxide (SO₂) | Burning sulfur-containing fossil fuels (coal, oil) | Respiratory problems, asthma, bronchitis | Acid rain, damage to buildings, plants and aquatic life |
| Nitrogen oxides (NOₓ) | High-temperature combustion in engines and power stations | Respiratory irritation, worsens asthma | Acid rain, photochemical smog, ground-level ozone |
| Carbon monoxide (CO) | Incomplete combustion of fuels (petrol engines, faulty gas appliances) | Toxic — binds to haemoglobin, reduces oxygen transport; can be fatal | Minimal direct environmental effect |
| Particulates (PM) | Diesel engines, wood burning, industrial processes | Respiratory and cardiovascular disease, lung cancer | Reduced visibility, building damage, climate effects |
| Carbon dioxide (CO₂) | Complete combustion of fossil fuels | Not directly toxic at normal levels | Greenhouse gas — contributes to climate change |
| Unburnt hydrocarbons | Incomplete combustion in vehicle engines | Some are carcinogenic | Contribute to photochemical smog formation |
Reducing Air Pollution
Strategies to reduce air pollution include:
- Catalytic converters on vehicles to reduce CO, NOₓ and unburnt hydrocarbon emissions
- Diesel particulate filters to trap particulate matter from diesel engines
- Flue gas desulfurisation (scrubbers) in power stations — SO₂ is removed by reacting with calcium oxide or calcium carbonate
- Using low-sulfur fuels — reduces SO₂ emissions directly
- Switching to renewable energy — solar, wind and hydroelectric power produce no direct air pollutants
- Electric vehicles — produce zero exhaust emissions (though power station emissions depend on the electricity source)
- Improved engine design — lean-burn engines and better combustion efficiency reduce pollutant formation
- Traffic management — congestion charges, low-emission zones and public transport investment reduce vehicle emissions in cities
Flue gas desulfurisation:
CaO(s) + SO₂(g) → CaSO₃(s) (calcium oxide + sulfur dioxide → calcium sulfite)
CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g) (limestone + sulfur dioxide → calcium sulfite + carbon dioxide)
When asked about reducing air pollution, consider the source of the pollutant and match the solution. For SO₂: use low-sulfur fuels or flue gas desulfurisation. For NOₓ: catalytic converters or lower combustion temperatures. For particulates: diesel particulate filters or switch to cleaner fuels. For CO₂: switch to renewable energy.
Practice Questions
1. Explain how sulfur dioxide is produced and how it leads to the formation of acid rain.
Sulfur dioxide is produced when fossil fuels containing sulfur impurities (such as coal and oil) are burned. The sulfur reacts with oxygen during combustion to form SO₂. In the atmosphere, SO₂ dissolves in water vapour to form sulfurous acid (H₂SO₃). It can also be further oxidised to SO₃, which dissolves in water to form sulfuric acid (H₂SO₄). These acids fall as acid rain, which has a pH below 5.2.
2. Describe how a catalytic converter reduces the emissions of harmful gases from a car exhaust.
A catalytic converter contains a catalyst (platinum, palladium and rhodium) on a ceramic honeycomb. It converts carbon monoxide to carbon dioxide by oxidation, nitrogen oxides to nitrogen gas by reduction, and unburnt hydrocarbons to carbon dioxide and water by oxidation. These reactions happen on the catalyst surface when the exhaust gases pass through the hot converter.
3. Explain why carbon monoxide is dangerous to humans.
Carbon monoxide is toxic because it binds to haemoglobin in red blood cells about 200–300 times more strongly than oxygen. This reduces the amount of oxygen the blood can carry around the body. Tissues and organs are deprived of oxygen, causing dizziness, headaches, fatigue and, at high concentrations, death. CO is also colourless and odourless, so people cannot detect it.
4. State two effects of acid rain on the environment and explain the chemistry behind one of them.
Acid rain damages limestone buildings and kills aquatic life in lakes. The chemistry behind building damage: sulfuric acid in acid rain reacts with calcium carbonate in limestone to form calcium sulfate, water and carbon dioxide. The calcium sulfate is soluble and washes away, gradually dissolving the stone and causing structural damage. CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂.
5. Explain how photochemical smog is formed and why it is more common in warm, sunny cities.
Photochemical smog is formed when nitrogen oxides and unburnt hydrocarbons from vehicle exhausts react in the presence of sunlight. The UV radiation in sunlight provides energy for chemical reactions that produce ground-level ozone and other harmful oxidants. This is why it is more common in warm, sunny cities — there is more sunlight to drive the photochemical reactions. Heavy traffic provides the NOₓ and hydrocarbons, and warm still weather prevents dispersal of the pollutants.
Maths Skills
Interpreting Pollution Data
Pollution data is often presented as tables, bar charts or line graphs showing concentration of pollutants over time or in different locations.
Comparing data: When comparing pollution levels between cities or years, calculate percentage differences. For example, if City A has 40 micrograms per cubic metre of NO₂ and City B has 15, City A has (40-15)/15 x 100 = 167% higher NO₂ levels.
Correlation: Look for relationships between variables. For example, a graph showing traffic volume and NO₂ concentration in a city over 24 hours would show a positive correlation — both peak during rush hours.
Concentration units: Pollutant concentrations are often given in micrograms per cubic metre or parts per billion (ppb). Conversions may be needed: 1 ppm = 1000 ppb.
Example: If a power station emits 5 tonnes of SO₂ per day and operates 365 days per year, annual SO₂ emissions = 5 x 365 = 1,825 tonnes per year. If flue gas desulfurization removes 90% of SO₂, emissions fall to 182.5 tonnes per year.
Common Misconceptions
Misconceptions About Air Pollution
Catalytic converters eliminate all pollution from car exhausts.
Catalytic converters reduce harmful emissions by converting CO to CO₂, NOₓ to N₂ and O₂, and unburnt hydrocarbons to CO₂ and H₂O. However, they do not eliminate all pollution — they still produce CO₂ (a greenhouse gas), and they do not work effectively when cold (e.g. on short journeys). They also do not remove particulates from diesel engines.
Acid rain is caused only by sulfur dioxide.
Acid rain is caused by both sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). SO₂ dissolves in water to form sulfurous and sulfuric acid, while NOₓ dissolves to form nitric acid. Both contribute to the acidity of rain.
If we stop burning fossil fuels, air pollution will immediately disappear.
While stopping fossil fuel combustion would dramatically reduce SO₂, NOₓ and CO emissions, some pollutants persist in the environment and other sources (agriculture, industry, natural processes) would still produce some air pollution. Additionally, existing pollution damage (e.g. acidified lakes) takes years to recover.
6-Mark Question
Explain how sulfur dioxide and nitrogen oxides cause acid rain. Discuss the effects and methods of reducing emissions.
Sulfur dioxide is produced when fossil fuels containing sulfur impurities are burned. SO₂ reacts with water and oxygen in the atmosphere to form sulfuric acid. Nitrogen oxides form when nitrogen and oxygen in the air react at the high temperatures inside car engines and power stations. NOₓ dissolves in water to form nitric acid. These acids fall as acid rain, which damages limestone buildings and statues by dissolving the calcium carbonate. Acid rain lowers the pH of lakes and rivers, killing fish and aquatic organisms. It damages tree leaves and roots, making forests more vulnerable to disease. It also leaches aluminium from soil into waterways, which is toxic to fish. Methods to reduce emissions include: using flue gas desulfurization (scrubbers) in power stations to remove SO₂ before it enters the atmosphere; burning low-sulfur fuels; using catalytic converters in cars to convert NOₓ to nitrogen and oxygen; switching to renewable energy sources; and using public transport or electric vehicles to reduce car emissions.
AO3: Analysis and Evaluation
Evaluating Methods to Reduce Air Pollution
A city centre has high levels of NO₂ and particulates, mainly from diesel vehicles. The council proposes three options:
- Option A: Ban all diesel vehicles from the city centre
- Option B: Introduce a clean air zone charge for older, more polluting vehicles
- Option C: Provide free public transport and cycle lanes
Evaluate each option and justify which would be most effective.
Answer: Option A would be most effective at reducing NO₂ and particulates quickly, as it removes the main source entirely. However, it would disproportionately affect those who rely on diesel vehicles (delivery drivers, low-income residents) and may be difficult to enforce. Option B is a market-based approach — charging for polluting vehicles incentivises upgrade to cleaner ones while allowing essential diesel vehicles to still operate if the charge is paid. This generates revenue for the council but may be seen as unfair and may not reduce pollution enough. Option C addresses the root cause by providing alternatives, but requires significant investment and would take time to change travel behaviour. It would have the widest co-benefits (reduced congestion, improved health from cycling). The most effective approach is likely a combination of B and C — charge polluting vehicles to fund improved public transport and cycling infrastructure.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Chemistry exams?
Get the best revision books and guides to boost your grades.