Atmospheric Pollutants
Dr. Davinder Bhachu
Teacher
Air Pollution
What Is Released When Fuels Burn?
When fossil fuels such as petrol, diesel, coal and natural gas burn, they do more than produce energy - they release a range of gases and particles that affect human health and the environment.
Hydrocarbon fuels are made only of carbon (C) and hydrogen (H). When they react with oxygen during combustion, a variety of products can form depending on how much oxygen is available.
Products of Combustion
1. Complete Combustion (Enough Oxygen)
When plenty of oxygen is present, hydrocarbons burn “cleanly.”
Main products:
- Carbon dioxide (CO₂)
- Water vapour (H₂O)
These products form because all carbon and hydrogen atoms are fully oxidised.
Example:
Burning methane (a hydrocarbon) in excess oxygen gives:
methane + oxygen → carbon dioxide + water
No answer provided.
2. Incomplete Combustion (Limited Oxygen)
If oxygen supply is restricted, combustion is incomplete and harmful substances are produced.
Possible products:
- Carbon monoxide (CO) – a poisonous gas
- Soot / carbon particles – tiny solid carbon pieces
- Unburned hydrocarbons – fuel molecules that didn’t react fully
- Carbon dioxide and water – still formed, but in lower amounts
Why does this happen?
Not all carbon gets fully oxidised, so some escapes as soot or becomes carbon monoxide rather than carbon dioxide.
3. Other Pollutants from Fossil Fuels
Sulfur dioxide (SO₂)
Produced if the fuel contains sulfur impurities (common in coal and some oils).
Nitrogen oxides (NOₓ)
Formed when nitrogen and oxygen in air react at very high temperatures, such as inside car engines or power-station furnaces.
NOₓ includes:
- NO (nitrogen monoxide)
- NO₂ (nitrogen dioxide)
Problems Caused By Combustion Products
Carbon Particulates (Soot)
Effects on humans
- Can become lodged in the lungs
- Linked to breathing difficulties and long-term respiratory disease
Effects on the environment
- Reflect sunlight away from Earth
- Contribute to reduced global sunlight levels, a process often referred to as global dimming
Carbon Monoxide (CO)
Carbon monoxide is especially dangerous because:
- It attaches to haemoglobin more strongly than oxygen does
- This means the blood can’t carry enough oxygen around the body
- This may lead to fainting, organ damage, coma, or death
- It is colourless and odourless, so it is extremely hard to detect without specialised equipment
Acid Rain
Acid rain forms when:
- Sulfur dioxide (SO₂) reacts with water in clouds → sulfuric acid
- Nitrogen oxides (NOₓ) react with water in clouds → nitric acid
These acids fall to the ground as acid rain.
Environmental consequences
- Makes lakes and streams acidic → kills fish and aquatic organisms
- Damages trees by weakening leaf surfaces and removing nutrients from soil
- Erodes buildings made from limestone or marble
- Can degrade outdoor sculptures
Health Effects of SO₂ and NOₓ
Even aside from acid rain, breathing in sulfur dioxide or nitrogen oxides can:
- Trigger asthma attacks
- Irritate airways
- Cause long-term respiratory issues
These gases dissolve in the moisture lining the respiratory system, forming mild acids that irritate tissues.
Practice Questions
Question 1:
A). What two products are always formed when a hydrocarbon burns completely?
B). Name two substances that may be produced during incomplete combustion.
Answer 1:
(A)
- Carbon dioxide (CO₂)
- Water (H₂O)
Why?
Hydrocarbons only contain carbon and hydrogen. With plenty of oxygen, all carbon becomes CO₂ and all hydrogen becomes H₂O.
(B)
- Carbon monoxide (CO)
- Soot / carbon particles
- Unburned hydrocarbons
Why?
If oxygen is limited, not all carbon atoms can fully oxidise to CO₂, so CO and solid carbon are formed, and some fuel can escape unreacted.
Question 2:
A fuel contains sulfur impurities. When it burns with plenty of oxygen, which extra gas—other than CO₂ and water-will be released?
Answer 2:
Sulfur dioxide (SO₂)
Why?
Sulfur in the fuel reacts with oxygen during combustion to make SO₂:
S + O₂ → SO₂
Question 3:
Under what conditions do nitrogen oxides form during the combustion of fuels?
Answer 3:
They form when air is heated to very high temperatures, such as inside car engines or power-station furnaces, where nitrogen and oxygen from the air react together.
Why?
Normally nitrogen is unreactive, but at very high temperatures its molecules have enough energy to react with oxygen to form NO and NO₂.
Question 4:
A). Identify two gases that contribute to acid rain.
B). Describe two ways acid rain harms the environment.
Answer 4:
(A)
Sulfur dioxide (SO₂)
Nitrogen oxides (NO and NO₂ – collectively called NOₓ)
(B)
- Damages forests and other plants – acid rain can damage leaves and needles and wash away important minerals from the soil.
- Makes lakes and rivers too acidic for aquatic life – fish eggs may not hatch and adult fish can die.
- Erodes buildings and statues made from carbonate rocks – materials such as limestone and marble react with the acids and wear away.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!