← AQA GCSE Chemistry/Topic 9

UNDERSTAND THE AIR AROUND YOU.

Chemistry of the atmosphere

Trace our changing atmosphere and explore the chemistry of climate and pollution.

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Quick reference: key definitions
Key Definitions

Topic Core Terms

Greenhouse effect
The natural process where greenhouse gases in the atmosphere absorb and re-radiate infrared radiation, keeping the Earth warm enough to support life.
Enhanced greenhouse effect
The additional warming caused by increased concentrations of greenhouse gases due to human activities.
Carbon footprint
The total amount of CO₂ and other greenhouse gases emitted over the full life cycle of a product, service, or event.
Climate change
A long-term shift in global or regional climate patterns, largely attributed to increased levels of greenhouse gases since the Industrial Revolution.
Fossil fuel
A fuel formed over millions of years from the remains of ancient organisms (coal, oil, natural gas). Burning them releases CO₂.
Global dimming
The reduction in sunlight reaching the Earth’s surface, caused by particulates in the atmosphere reflecting and absorbing sunlight.
CHAPTER 01 · 3 sectionsOur changing atmosphereThe air today, the early Earth and rising oxygen

The Current Atmosphere

The proportions of gases in the Earth’s atmosphere today have been relatively stable for about 200 million years:

Gas Formula Proportion
NitrogenN₂~78%
OxygenO₂~21%
ArgonAr~0.9%
Carbon dioxideCO₂~0.04%
Water vapour + trace gases - Variable, very small
Composition of the Earth's atmosphere dry air by volume nitrogen, N₂ about 78% about four-fifths oxygen, O₂ about 21% about one-fifth argon and other gases about 1% including carbon dioxide, about 0.04%

The proportions of gases in the Earth's atmosphere have been stable for around 200 million years, overwhelmingly dominated by nitrogen and oxygen.

Key Concept

Key Fact

These percentages have been broadly stable for 200 million years. However, CO₂ levels have been rising significantly since the Industrial Revolution.

The Early Atmosphere

For the first billion years of Earth's existence, the atmosphere was very different. Intense volcanic activity released gases that formed the early atmosphere.

Composition of the Early Atmosphere

  • Mainly carbon dioxide (CO₂) - similar to Mars and Venus today.
  • Very little or no oxygen.
  • Water vapour (from volcanic eruptions), which later condensed to form the oceans.
  • Smaller amounts of methane and ammonia.
Exam Tip

Exam Technique

We can't be 100% certain about the early atmosphere because there were no direct measurements. Scientists use evidence from rocks, fossils, and comparing with other planets to estimate.

How Oxygen Increased

How the Earth's atmosphere evolved CO₂ H₂O N₂, CH₄, NH₃ O₂ O₂ CO₂ ocean sedimentary rock N₂ 78% O₂ 21% CO₂ about 0.04% 1. Early atmosphere 2. Oceans and algae 3. Today volcanoes released mainly CO₂, with little or no O₂; water vapour formed oceans CO₂ dissolved in the oceans and was locked up in rocks; algae and plants made O₂ mostly nitrogen and oxygen, very little CO₂; stable for about 200 million years

The Earth's atmosphere evolved from a volcanic, CO₂-rich environment into today's oxygen-rich, life-supporting atmosphere over 4.6 billion years.

Stage 1: Photosynthesis by Early Life Forms

Around 2.7 billion years ago, simple organisms like cyanobacteria (algae) evolved. They photosynthesised, absorbing CO₂ and releasing O₂.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Over hundreds of millions of years, this gradually increased oxygen levels.

Stage 2: Reducing CO₂

Carbon dioxide was removed from the atmosphere by:

  • Dissolving in the oceans - forming carbonate sedimentary rocks (limestone) and by marine organisms incorporating it into shells.
  • Photosynthesis - locked up carbon in biomass.
  • Formation of fossil fuels - dead organisms were buried, compressed, and transformed into coal, oil, and gas over millions of years.
CHAPTER 02 · 2 sectionsGreenhouse gases and climateEnergy transfer and a changing climate

Greenhouse Gases & the Greenhouse Effect

Some gases in the atmosphere absorb heat radiation (infrared) emitted by the Earth's surface and re-radiate it in all directions, including back towards the surface. This is the greenhouse effect, and it keeps the Earth warm enough to support life.

The greenhouse effect atmosphere space Earth's surface CO₂ H₂O CH₄ 1 2 3 4 1 Short-wavelength radiation fromthe Sun passes through the air. 2 The Earth warms and emitslong-wavelength infrared (IR). 3 Greenhouse gases absorb the IRand re-emit it, some back down. 4 Some IR escapes to space. Moregreenhouse gas: less escapes.

The greenhouse effect occurs when short-wavelength solar radiation passes through the atmosphere, but the long-wavelength infrared radiation emitted by the Earth is absorbed and re-emitted by greenhouse gases like CO₂, CH₄, and H₂O.

The Main Greenhouse Gases

Gas Formula Main human sources
Carbon dioxideCO₂Burning fossil fuels, deforestation
MethaneCH₄Cattle farming, rice paddies, landfill, natural gas leaks
Water vapourH₂ONatural evaporation (not directly from human activity)
Key Concept

Key Fact

Without the greenhouse effect, Earth would be too cold to support life. The problem is the enhanced greenhouse effect - extra greenhouse gases trap more heat, causing global temperatures to rise.

Climate Change

Since the Industrial Revolution (mid-1800s), human activities have significantly increased the concentration of greenhouse gases. This enhanced greenhouse effect is causing global temperatures to rise - known as global warming.

Consequences of Climate Change

  • Polar ice caps melting → sea level rise → flooding of low-lying areas
  • More extreme weather events (droughts, floods, storms)
  • Changes to ecosystems and habitats
  • Impact on agriculture and food production
  • Migration and redistribution of species

Why is There a Debate?

While the vast majority of scientists agree that human activity is the main cause, the evidence involves very complex climate models and long-term data. Media, politics, and economics also influence public perception.

Human Activities Driving Climate Change

  • Burning fossil fuels (coal, oil, gas) for energy, transport, and industry → releases CO₂.
  • Deforestation → fewer trees to absorb CO₂ by photosynthesis, and burning trees releases stored carbon.
  • Agriculture → cattle produce methane; rice paddies release methane; fertilisers release nitrous oxide.
  • Landfill → decomposing waste releases methane.
Exam Tip

Exam Technique

In the exam, acknowledge the scientific consensus but mention that climate models have limitations and that peer review is vital for assessing climate data.

CHAPTER 03 · 1 sectionCarbon footprintsMeasuring and reducing our impact

Carbon Footprint

A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event.

Ways to Reduce Carbon Footprint

  • Using renewable energy sources (solar, wind, tidal) instead of fossil fuels.
  • Improving energy efficiency of buildings and vehicles.
  • Carbon capture and storage (CCS) technology.
  • Reducing waste, reusing materials, and recycling.
  • Planting trees to absorb CO₂.
  • Using public transport or cycling instead of driving.
Exam Tip

Exam Technique

Be able to discuss why complete reduction of carbon footprint is difficult - it requires lifestyle changes, is economically challenging, and some processes currently have no alternative to fossil fuels.

Worked Example

Comparing carbon footprints

A school is comparing two options for heating: natural gas boiler or a ground-source heat pump powered by renewable electricity. Discuss which has the lower carbon footprint.

Gas boiler versus heat pump: carbon footprint CO₂ natural gas heat pump Gas boiler burns natural gas (a fossil fuel), so it releases CO₂ every time it runs Heat pump (renewable electricity) no CO₂ while it runs, but making and installing it still releases some CO₂ carbon footprint high carbon footprint lower, but not zero

Gas boiler: Burns natural gas (fossil fuel) → releases CO₂ directly. Manufacturing and transporting gas also contributes. Carbon footprint = high.

Heat pump (renewable): Uses electricity from renewable sources → no CO₂ during operation. However, manufacturing the pump, drilling boreholes, and transporting materials all produce some CO₂. Carbon footprint = lower, but not zero.

Conclusion: The heat pump has a significantly lower carbon footprint over its lifetime, but the full life cycle (manufacturing + installation + disposal) must be considered.

CHAPTER 04 · 1 sectionAtmospheric pollutantsSources and effects of air pollution

Atmospheric Pollutants

Burning fossil fuels releases several harmful pollutants:

Pollutants from burning fossil fuels and their effects burning fossil fuels sulfur dioxide, SO₂ from sulfur impurities in the fuel acid rain and breathing (respiratory) problems oxides of nitrogen, NOₓ N₂ and O₂ from air react in hot engines acid rain and breathing (respiratory) problems particulates (soot) from incomplete combustion global dimming and health problems in the lungs carbon monoxide, CO from incomplete combustion toxic gas colourless and odourless: hard to detect !

Burning fossil fuels releases a variety of atmospheric pollutants, each with distinct negative consequences for human health and the environment.

Pollutant Source Harm caused Solution
Carbon monoxide (CO) Incomplete combustion Toxic - binds to haemoglobin, prevents O₂ transport. Colourless/odourless. Catalytic converters (CO → CO₂)
Sulfur dioxide (SO₂) Burning fuels with sulfur impurities Acid rain → damages buildings (limestone), kills aquatic life, harms plants Flue gas desulfurisation; use low-sulfur fuels
Nitrogen oxides (NOₓ) N₂ + O₂ at high engine temps Acid rain, photochemical smog, respiratory problems Catalytic converters (NOₓ → N₂)
Particulates (soot) Incomplete combustion Respiratory problems, global dimming, darkens buildings Particulate filters in vehicles
Key Concept

Key Fact

Catalytic converters in car exhausts reduce emissions of CO and NOₓ by converting them into less harmful gases: CO → CO₂ and NOₓ → N₂.

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