Air Quality, Climate and Fertilisers

Q: What is the percentage composition of clean dry air?

Clean dry air is 78% nitrogen, 21% oxygen, <1% argon, and 0.04% carbon dioxide.

Q: What is the difference between the natural and enhanced greenhouse effect?

The natural greenhouse effect is essential to keep Earth at a habitable 15°C average temperature. The enhanced greenhouse effect is human-caused, driven by excess greenhouse gas emissions, leading to rapid global warming and climate change.

Q: Why are nitrogen fertilisers used in agriculture?

Nitrogen is an essential element for plants to make proteins and grow. Fertilisers supplement soil nitrogen levels to increase crop yields.

Chemistry· 10.2, 10.3· 25 min read

1. Composition of Air and Common Air Pollutants (Core)★★☆☆☆⏱ 5 min

Clean dry air has a consistent percentage composition: 78% nitrogen ( $N_{2}$ ), 21% oxygen ( $O_{2}$ ), ~1% argon (Ar), and ~0.04% carbon dioxide ( $C O_{2}$ ). Air pollutants are substances present at harmful concentrations, almost all generated by human industrial or domestic activity.

📘 Definition

Air Pollutant

A harmful substance released into the atmosphere that negatively impacts human health, ecosystems, or the climate

📐 Worked Example

List three common air pollutants, their main source, and one negative effect of each.

1
1. Pollutant: Carbon monoxide (CO). Source: Incomplete combustion of carbon-containing fuels (e.g., car engines, faulty gas boilers). Effect: Binds irreversibly to haemoglobin in blood, reducing oxygen transport, which can cause unconsciousness or death.
2
1. Pollutant: Sulfur dioxide ( $S O_{2}$ ). Source: Combustion of fossil fuels with sulfur impurities (e.g., coal in power stations). Effect: Dissolves in rainwater to form acid rain, which erodes buildings, kills aquatic life, and damages plant leaves.
3
1. Pollutant: Nitrogen oxides ( $N O_{x}$ ). Source: High-temperature combustion in car engines where atmospheric nitrogen and oxygen react. Effect: Causes respiratory irritation in humans, and contributes to acid rain and photochemical smog.

Exam tip:

Always link pollutant sources to specific human activities for full marks; avoid vague answers like 'factories' without explaining the associated process.

2. Greenhouse Effect and Climate Change (Core)★★☆☆☆⏱ 6 min

The natural greenhouse effect keeps Earth at a habitable average temperature of ~15°C. Radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, which re-emits it as longer-wavelength thermal (infrared) energy. The greenhouse gases carbon dioxide and methane absorb, reflect and re-emit this thermal energy, reducing the loss of thermal energy to space and so warming the lower atmosphere.

📘 Definition

Greenhouse Gas

An atmospheric gas that absorbs infrared radiation, contributing to the greenhouse effect. Key examples include carbon dioxide, methane, water vapour, and nitrous oxide.

📐 Worked Example

Explain how rising atmospheric carbon dioxide levels contribute to climate change.

1
1. Human activities including fossil fuel combustion and deforestation release excess $C O_{2}$ into the atmosphere, raising concentrations above natural levels.
2
1. Higher $C O_{2}$ levels mean more infrared radiation emitted from the Earth's surface is absorbed, rather than escaping into space.
3
1. This enhanced greenhouse effect raises average global surface temperatures, leading to impacts including melting polar ice, sea level rise, more frequent extreme weather, and disrupted food production.

3. Fertilisers (Core)★★☆☆☆⏱ 5 min

Ammonium salts and nitrates are used as fertilisers. NPK fertilisers supply the three elements nitrogen (N), phosphorus (P) and potassium (K), which plants need for improved growth (for example, nitrogen is used to make proteins).

📘 Definition

NPK fertiliser

A fertiliser that provides the elements nitrogen, phosphorus and potassium for improved plant growth

Exam tip:

For fertilisers you only need to state that ammonium salts and nitrates are used, and that NPK fertilisers supply nitrogen, phosphorus and potassium. You do not need the step-by-step eutrophication mechanism.

4. Photosynthesis★★☆☆☆⏱ 5 min

Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll (the green pigment in plants) and using energy from light. Because it removes carbon dioxide from the air, photosynthesis is an important part of the carbon cycle.

📘 Definition

Photosynthesis

The reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll and using energy from light

\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}

📐 Worked Example

State the word equation for photosynthesis and name the two conditions required.

1
1. Word equation: carbon dioxide + water → glucose + oxygen.
2
1. Conditions: chlorophyll must be present, and energy from light is required.

Exam tip:

Learn the word equation for Core; for Extended also learn the balanced symbol equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Do not forget the two conditions: chlorophyll and light energy.

5. Common Pitfalls

Wrong move:

Stating air is ~70% oxygen

Why:

Learners mix up nitrogen and oxygen percentages in clean air

Correct move:

Recall clean dry air is 78% nitrogen, 21% oxygen, <1% other gases

Wrong move:

Claiming carbon monoxide causes acid rain

Why:

Confusing CO effects with sulfur dioxide and nitrogen oxides

Correct move:

Link acid rain only to $S O_{2}$ and $N O_{x}$ ; CO reduces oxygen transport in blood

Wrong move:

Describing the greenhouse effect as entirely harmful

Why:

Forgetting the natural greenhouse effect is essential to keep Earth habitable

Correct move:

Explicitly distinguish the natural beneficial effect from the human-caused enhanced effect driving climate change

Wrong move:

Writing the full step-by-step eutrophication mechanism in a 0620 answer

Why:

The 0620 syllabus states that details of the eutrophication process are not required

Correct move:

State only that nitrates and phosphates lead to deoxygenation of water and damage to aquatic life

Wrong move:

Forgetting the conditions in the photosynthesis equation

Why:

Photosynthesis only occurs in the presence of chlorophyll and using energy from light

Correct move:

Include the conditions; for Extended, balance the symbol equation as $6 C O_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$

6. Quick Reference Cheatsheet

Concept	Core Key Fact	Extended Key Fact
Clean Air Composition	78% N₂, 21% O₂, remainder noble gases and CO₂	Same as Core
Acid Rain Causes	Sulfur dioxide ( $S O_{2}$ ) and oxides of nitrogen ( $N O_{x}$ )	Same as Core
Greenhouse Effect	Natural = essential for life; enhanced = drives climate change	CO₂ and CH₄ absorb, reflect and re-emit thermal energy, reducing loss to space
Catalytic converters	Reduce CO and oxides of nitrogen from car exhausts	$2 C O + 2 N O \to 2 C O_{2} + N_{2}$
Photosynthesis	carbon dioxide + water → glucose + oxygen (chlorophyll, light)	$6 C O_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$
Fertilisers	Ammonium salts and nitrates used; NPK provides N, P and K for growth	Same as Core
Fertiliser runoff	Nitrates/phosphates cause deoxygenation of water and harm aquatic life (Water topic)	Same as Core

7. Frequently Asked

What is the percentage composition of clean dry air?

Clean dry air is ~78% nitrogen, 21% oxygen, <1% argon, and ~0.04% carbon dioxide.

What is the difference between the natural and enhanced greenhouse effect?

The natural greenhouse effect is essential to keep Earth at a habitable ~15°C average temperature. The enhanced greenhouse effect is human-caused, driven by excess greenhouse gas emissions, leading to rapid global warming and climate change.

Why are nitrogen fertilisers used in agriculture?

Nitrogen is an essential element for plants to make proteins and grow. Fertilisers supplement soil nitrogen levels to increase crop yields.

What's Next

You have now covered the Core and Extended content for CIE IGCSE Chemistry 0620 Unit 10.2 and 10.3 on air quality, climate, photosynthesis, and fertilisers. Next, practice structured exam questions on this topic to reinforce your knowledge, particularly air pollutant sources and effects, the greenhouse effect, and the photosynthesis equations. You should also revise the Water subtopic in the Chemistry of the Environment unit, before completing full unit revision for your mock and final exams. Use the included cheatsheet for quick, last-minute revision in the days before your exam.