GCSE — chemistryFoundation7 min read

Chemistry of the Atmosphere

Understand Earth's atmosphere evolution, greenhouse gases, climate change, and air pollutants for GCSE Chemistry.

Tutor AI Team2026-02-25T12:40:06.724Z

# Chemistry of the Atmosphere

Earth's atmosphere is a thin layer of gases that makes life possible. But our atmosphere hasn't always been the way it is today — it has changed dramatically over billions of years. Understanding how the atmosphere evolved, what greenhouse gases are, and how human activities affect air quality is an important part of GCSE Chemistry.

1. Today's Atmosphere

The current composition of Earth's atmosphere:

Gas	Approximate %
Nitrogen ( $\text{N}_2$ )	78%
Oxygen ( $\text{O}_2$ )	21%
Argon (Ar)	0.93%
Carbon dioxide ( $\text{CO}_2$ )	0.04%
Water vapour	Variable (0–4%)
Other gases	Trace amounts

This composition has been relatively stable for the last 200 million years.

2. Evolution of Earth's Atmosphere

Phase 1: Early Atmosphere (~4.6–3.5 billion years ago)

Earth's early atmosphere was very different:

Mainly carbon dioxide (like Mars and Venus today)
Little or no oxygen
Water vapour, methane, ammonia, and nitrogen present
Volcanic activity released these gases (degassing)

As Earth cooled, water vapour condensed to form the oceans.

Phase 2: Oxygen Appears (~2.7–1.5 billion years ago)

Algae and cyanobacteria evolved in the oceans
They carried out photosynthesis, producing oxygen:

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Oxygen levels gradually increased
CO₂ levels gradually decreased because:
1. Photosynthesis used up CO₂
2. CO₂ dissolved in the oceans
3. Marine organisms used CO₂ to make shells and skeletons (calcium carbonate)
4. Dead organisms formed sedimentary rocks (limestone) and fossil fuels

Phase 3: Modern Atmosphere (~200 million years ago – present)

Plants evolved and increased oxygen further
The ozone layer ( $\text{O}_3$ ) formed, protecting life from UV radiation
This allowed life to move onto land
The atmosphere stabilised to today's composition

3. Greenhouse Effect and Climate Change

The Greenhouse Effect

The greenhouse effect is a natural process that warms Earth's surface:

The Sun emits short-wavelength radiation (visible light)
Earth's surface absorbs this energy and re-emits long-wavelength infrared radiation
Greenhouse gases in the atmosphere absorb some of this infrared radiation
They re-emit it in all directions, including back towards Earth
This keeps Earth warm enough to support life (~15°C average)

Greenhouse Gases

Gas	Sources
Carbon dioxide ( $\text{CO}_2$ )	Burning fossil fuels, deforestation
Methane ( $\text{CH}_4$ )	Agriculture (cattle, rice), landfill, natural gas
Water vapour ( $\text{H}_2\text{O}$ )	Evaporation (natural)

Enhanced Greenhouse Effect

Human activities are increasing greenhouse gas concentrations, causing global warming:

Burning fossil fuels → more CO₂
Deforestation → less CO₂ absorbed by trees
Agriculture → more methane
Landfill → methane from decomposition

Evidence for Climate Change

Global temperature records show warming
Ice cores contain trapped air bubbles showing past CO₂ levels
Correlation between CO₂ levels and temperature
Sea level rise, melting ice caps, extreme weather events

Consequences of Climate Change

Sea level rise (thermal expansion + melting ice) → flooding
More frequent extreme weather (storms, droughts, floods)
Loss of habitats and biodiversity
Changes in food production patterns
Ocean acidification (CO₂ dissolves in seawater)

4. Carbon Footprint

A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted by an activity, product, or person over its lifetime.

Reducing Carbon Footprint

Strategy	How
Use renewable energy	Solar, wind, hydro instead of fossil fuels
Improve energy efficiency	Better insulation, LED lighting, efficient appliances
Reduce, reuse, recycle	Less manufacturing = fewer emissions
Carbon capture and storage (CCS)	Capture CO₂ from power stations and store underground
Plant more trees	Trees absorb CO₂ through photosynthesis
Use public transport	Less fuel per person than individual cars

Limitations

International agreements are hard to enforce
Developing countries need cheap energy for growth
Some technologies are expensive or not yet proven at scale
Individual actions are small compared to industrial emissions

5. Air Pollutants

Common Pollutants from Burning Fuels

Pollutant	Source	Effects
CO₂	Complete combustion of any fuel	Greenhouse gas → climate change
CO	Incomplete combustion	Toxic — binds to haemoglobin
Particulates (soot)	Incomplete combustion	Respiratory disease; global dimming
$\text{SO}_2$	Burning fuels containing sulfur	Acid rain → kills fish, damages buildings
$\text{NO}_x$	High-temperature combustion (N₂ + O₂ in engines)	Acid rain; smog; respiratory problems

Acid Rain

$\text{SO}_2$ and $\text{NO}_x$ dissolve in rainwater to form sulfuric acid and nitric acid:

$\text{SO}_2 + \text{H}_2\text{O} + \frac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{SO}_4$

Acid rain effects:

Kills trees and aquatic life (acidifies lakes)
Corrodes limestone buildings and metal structures
Damages ecosystems

Reducing Pollution

Catalytic converters in cars: convert CO and NOₓ to CO₂ and N₂
Flue gas desulfurisation in power stations: removes SO₂
Using cleaner fuels (natural gas instead of coal)
Electric vehicles produce no direct exhaust emissions

Worked Example: Explaining Atmosphere Changes

Problem

Question: Explain how photosynthesis changed Earth's atmosphere.

Solution

Early photosynthetic organisms (algae and cyanobacteria) used carbon dioxide and water to produce glucose and oxygen. Over billions of years, this gradually increased oxygen levels and decreased CO₂ levels in the atmosphere, eventually leading to the composition we have today.

Worked Example: Greenhouse Effect

Problem

Question: Explain how burning fossil fuels contributes to climate change.

Solution

Burning fossil fuels releases carbon dioxide into the atmosphere. CO₂ is a greenhouse gas that absorbs and re-emits infrared radiation. Increased CO₂ enhances the greenhouse effect, causing more heat to be trapped in the atmosphere, leading to global warming and associated climate change.

7. Practice Questions

1. State the approximate percentage of nitrogen, oxygen, and CO₂ in today's atmosphere.
1. Describe how Earth's early atmosphere was different from today's.
1. Explain how CO₂ levels decreased over geological time. Give three reasons.
1. Describe the greenhouse effect in your own words.
1. Name three greenhouse gases and state one human activity that increases each.

Want to check your answers and get step-by-step solutions?

8. Common Misconceptions

Misconception	Reality
The greenhouse effect is bad	The natural greenhouse effect is essential for life; the enhanced effect is the problem
CO₂ is the only greenhouse gas	Methane and water vapour are also greenhouse gases
The ozone hole causes global warming	The ozone hole and global warming are different issues (though related)
We know exactly what the early atmosphere was like	Our knowledge is based on evidence and modelling — there is some uncertainty

9. Exam Tips

Know the approximate percentages: N₂ (78%), O₂ (21%), Ar (~1%), CO₂ (0.04%)
When explaining CO₂ decrease, give multiple reasons (photosynthesis, dissolving in oceans, forming rocks/fossil fuels)
Distinguish between the natural greenhouse effect and the enhanced greenhouse effect
For pollution questions, match each pollutant to its source and effects

Summary

Today's atmosphere: 78% N₂, 21% O₂, ~1% Ar, 0.04% CO₂
Early atmosphere was mostly CO₂; photosynthesis added O₂ and removed CO₂
Greenhouse effect: greenhouse gases trap infrared radiation → warms Earth
Human activities enhance the greenhouse effect → climate change
Pollutants: CO₂, CO, SO₂, NOₓ, particulates → various health and environmental problems
Reducing emissions: renewables, CCS, catalytic converters, efficiency improvements

Tags:

#gcse#chemistry#atmosphere#climate-change#greenhouse

Ready to Ace Your GCSE chemistry?

Get instant step-by-step solutions to any problem. Snap a photo and learn with Tutor AI — your personal exam prep companion.