Atmospheric Pollutants

Burning fuels powers our cars, homes and industry, but it also releases a range of harmful substances into the air. Some come from burning the fuel itself without enough oxygen; others come from impurities in the fuel or from the air being heated in an engine. These atmospheric pollutants cause problems ranging from poisoning and breathing difficulties to acid rain and global dimming. This lesson, part of Topic C6 of OCR Gateway Science A, explains the difference between complete and incomplete combustion and identifies the main pollutants — carbon monoxide, particulates, sulfur dioxide and oxides of nitrogen — where each comes from, and the problem each causes.

By the end of this lesson you should be able to compare complete and incomplete combustion, write equations for both, name the main pollutants from burning fuels, state the source and the harmful effect of each, and suggest ways to reduce them.

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Complete and Incomplete Combustion

When a hydrocarbon fuel burns in plenty of oxygen, complete combustion occurs and the only products are carbon dioxide and water:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$CH4​+2O2​→CO2​+2H2​O

But when there is a limited supply of oxygen — for example in a poorly ventilated boiler or an engine — incomplete combustion occurs. The carbon in the fuel is not fully oxidised, so as well as water the products include carbon monoxide (CO) and carbon (soot/particulates):

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$2CH4​+3O2​→2CO+4H2​O

Incomplete combustion also releases less energy than complete combustion, so it is wasteful as well as dangerous.

	Oxygen supply	Carbon-containing products
Complete combustion	Plenty	Carbon dioxide only
Incomplete combustion	Limited	Carbon monoxide and carbon (soot)

Exam Tip: Tie the products to the oxygen: plenty of oxygen → carbon dioxide (complete); limited oxygen → carbon monoxide and soot (incomplete). A common error is to say carbon monoxide comes from complete combustion — it comes from incomplete combustion.

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Carbon Monoxide

Carbon monoxide ($\text{CO}$CO) is produced by the incomplete combustion of carbon-based fuels. It is dangerous because it is:

• toxic (poisonous);
• colourless and odourless — you cannot see or smell it, so it gives no warning.

Carbon monoxide is toxic because it binds to haemoglobin in red blood cells in place of oxygen. Haemoglobin normally carries oxygen around the body, but carbon monoxide attaches to it more readily, so the blood can carry less oxygen. This can cause drowsiness, unconsciousness and, in high enough amounts, death. Because it cannot be detected by sight or smell, faulty gas heaters and boilers are a particular danger, which is why carbon monoxide alarms are used.

Exam Tip: For carbon monoxide, the marks are: toxic, colourless, odourless, and it binds to haemoglobin so the blood carries less oxygen. Source: incomplete combustion.

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Particulates (Soot)

Particulates are tiny solid particles of carbon (soot) and unburnt hydrocarbons, also produced by incomplete combustion (especially of diesel). They cause two main problems:

• Global dimming: particulates in the air reflect sunlight back into space and help clouds form, reducing the amount of sunlight reaching the Earth's surface.
• Respiratory problems: breathing in particulates can damage the lungs and worsen conditions such as asthma and bronchitis.

Exam Tip: Particulates (soot/carbon) come from incomplete combustion; their two effects are global dimming and respiratory (breathing) problems. Don't confuse global dimming with the greenhouse effect — they are different.

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Sulfur Dioxide

Sulfur dioxide ($\text{SO}_2$SO2​) is produced when fuels that contain sulfur impurities are burned. Many fossil fuels contain small amounts of sulfur; when the fuel burns, the sulfur is oxidised to sulfur dioxide:

$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$S+O2​→SO2​

The problems caused by sulfur dioxide are:

• Acid rain: sulfur dioxide dissolves in water in the atmosphere and is oxidised to form acidic solutions, which fall as acid rain. Acid rain damages buildings and statues (especially limestone), harms trees and plants, and makes lakes and rivers acidic, killing fish.
• Respiratory problems: sulfur dioxide irritates the airways and can cause breathing difficulties.

The sulfur dioxide problem is reduced by removing sulfur from fuels before they are burned, and by removing sulfur dioxide from the waste gases of power stations (flue-gas desulfurisation).

Exam Tip: Sulfur dioxide comes from sulfur impurities in the fuel (not from nitrogen in the air) and causes acid rain (and respiratory problems). Remember: it is sulfur dioxide and oxides of nitrogen — not carbon dioxide — that cause acid rain.

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Oxides of Nitrogen (NOx)

Oxides of nitrogen (NOx), such as nitrogen monoxide and nitrogen dioxide, are formed inside engines. The high temperatures in a car or aircraft engine give the nitrogen and oxygen in the air enough energy to react together:

$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$N2​+O2​→2NO

(Nitrogen is normally unreactive, but the very high temperatures in an engine make it react.) The problems caused by oxides of nitrogen are:

• Acid rain (like sulfur dioxide);
• Respiratory problems;
• Photochemical smog — a hazy, harmful smog that forms over cities in sunlight.

Oxides of nitrogen are reduced by fitting catalytic converters to vehicles, which convert them into harmless nitrogen and other gases.

Exam Tip: Oxides of nitrogen (NOx) form from nitrogen and oxygen in the air reacting at the high temperatures in engines — not from the fuel. Effects: acid rain, respiratory problems and smog.

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Summary Table of Pollutants

Pollutant	Source	Problem(s)
Carbon monoxide (CO)	Incomplete combustion	Toxic — binds to haemoglobin, so blood carries less oxygen
Particulates (soot/carbon)	Incomplete combustion	Global dimming and respiratory problems
Sulfur dioxide ($\text{SO}_2$SO2​)	Sulfur impurities in the fuel	Acid rain and respiratory problems
Oxides of nitrogen (NOx)	Nitrogen + oxygen reacting at the high temperature in engines	Acid rain, respiratory problems and smog

Worked Example — Identifying a pollutant

A power station burns a fuel that contains sulfur. Lakes downwind of the power station are becoming acidic and fish are dying. Name the pollutant responsible, explain how it forms, and state how the problem could be reduced.

Step 1 — link the source to the pollutant: the fuel contains sulfur, so burning it produces sulfur dioxide ($\text{S} + \text{O}_2 \rightarrow \text{SO}_2$S+O2​→SO2​).

Step 2 — explain the problem: sulfur dioxide dissolves in water in the atmosphere to form an acidic solution that falls as acid rain, which makes the lakes acidic and kills fish.

Step 3 — suggest a reduction: remove the sulfur from the fuel before burning, or remove the sulfur dioxide from the waste gases (flue-gas desulfurisation).

Answer: the pollutant is sulfur dioxide, formed from the sulfur impurity in the fuel; it causes acid rain, and can be reduced by removing sulfur from the fuel or scrubbing it from the flue gases.

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Reducing Atmospheric Pollutants

Because these pollutants cause real harm, a great deal of effort goes into reducing them. The method depends on the pollutant and where it comes from: