What you'll learn
The chemistry of the atmosphere covers how Earth's air formed, the gases it contains today, the greenhouse effect and climate change, and the pollutants released by burning fuels. In this guide you will learn the composition of the modern atmosphere, the theories of how it evolved, how greenhouse gases trap heat, the human activities that increase them, the consequences of climate change, and the pollutants from combustion and their effects. This topic links combustion, environmental chemistry and evaluating evidence.
Key terms and definitions
Atmosphere — the layer of gases surrounding the Earth.
Greenhouse gas — a gas that absorbs and re-emits heat (infrared) radiation, warming the planet (e.g. carbon dioxide, methane, water vapour).
Greenhouse effect — the trapping of heat by greenhouse gases in the atmosphere.
Climate change — long-term changes in global weather patterns, linked to rising greenhouse gases.
Carbon footprint — the total greenhouse gases produced by an activity, product or person.
Pollutant — a harmful substance released into the environment, e.g. from burning fuels.
Core concepts
Composition of today's atmosphere
For the last 200 million years the atmosphere has been roughly stable: about 78% nitrogen, 21% oxygen, small amounts of other gases including argon (~1%), carbon dioxide (~0.04%) and varying water vapour.
How the atmosphere evolved
The early atmosphere is thought to have been formed by intense volcanic activity, releasing mainly carbon dioxide, along with water vapour, nitrogen and small amounts of methane and ammonia. As the Earth cooled, water vapour condensed to form the oceans, and much carbon dioxide dissolved in them. Algae and plants then evolved and, by photosynthesis, gradually removed carbon dioxide and released oxygen, allowing animal life to develop. Carbon dioxide was also locked away in sedimentary rocks (such as limestone) and in fossil fuels. (These theories are based on limited evidence, so there is some uncertainty.)
The greenhouse effect
Some gases — carbon dioxide, methane and water vapour — are greenhouse gases. They allow short-wavelength radiation from the Sun to pass through, but absorb the longer-wavelength (infrared) heat radiation emitted by the Earth and re-radiate some of it back, keeping the planet warm enough for life. This is the greenhouse effect.
Human activities and climate change
Human activities increase greenhouse gases: burning fossil fuels and deforestation raise carbon dioxide; agriculture (livestock and rice fields) and landfill/waste raise methane. Based on peer-reviewed evidence, most scientists agree that increasing greenhouse gases cause global climate change. Consequences may include rising sea levels (melting ice and thermal expansion), more extreme weather, changing rainfall patterns and effects on agriculture and wildlife.
Carbon footprint
The carbon footprint is the total amount of greenhouse gases (carbon dioxide and others) given off over the life of a product, service or activity. It can be reduced by using less energy, switching to renewable energy, improving efficiency, and capturing or storing carbon dioxide — though these measures can be hard to implement due to cost, lifestyle and political factors.
Pollutants from burning fuels
Burning fuels releases pollutants:
- Carbon dioxide — a greenhouse gas (from complete combustion).
- Carbon monoxide — a toxic gas from incomplete combustion; it is colourless and odourless and reduces the blood's ability to carry oxygen.
- Soot (carbon particulates) — causes breathing problems and global dimming, and makes buildings dirty.
- Sulfur dioxide and oxides of nitrogen — cause acid rain and respiratory problems; nitrogen oxides form at the high temperatures inside engines.
Worked examples
Example 1: Composition
What are the two most abundant gases in the atmosphere?
Nitrogen (~78%) and oxygen (~21%) are the two most abundant gases.
Example 2: Role of plants
How did the amount of oxygen in the atmosphere increase?
Algae and plants carried out photosynthesis, which removed carbon dioxide and released oxygen, increasing the oxygen level over time.
Example 3: Incomplete combustion
Why is carbon monoxide dangerous?
Carbon monoxide is toxic: it is colourless and odourless and reduces the blood's ability to carry oxygen, which can be fatal.
Common mistakes and how to avoid them
Getting the percentages wrong. Nitrogen ~78%, oxygen ~21%, carbon dioxide only ~0.04%.
Confusing the early and modern atmosphere. Early: mostly CO₂ from volcanoes; modern: mostly nitrogen and oxygen.
Mixing up the pollutants. Carbon monoxide (toxic, incomplete combustion) vs sulfur dioxide (acid rain) vs particulates (global dimming).
Saying greenhouse gases block sunlight. They let sunlight in but absorb the infrared heat the Earth gives off.
Treating the theories as certain. Evidence about the early atmosphere is limited, so there is genuine uncertainty.
Exam technique for the Atmosphere
Quote the composition of today's atmosphere with approximate percentages.
Explain atmospheric evolution via volcanoes, oceans forming, and photosynthesis.
Describe the greenhouse effect correctly — infrared absorption and re-radiation.
Link human activities to specific gases and consequences of climate change.
Identify pollutants and their effects, distinguishing complete from incomplete combustion.
Quick revision summary
Today's atmosphere is about 78% nitrogen, 21% oxygen, ~1% argon and ~0.04% carbon dioxide, plus water vapour. The early atmosphere came from volcanic activity (mostly carbon dioxide, with water vapour, nitrogen and some methane and ammonia); as Earth cooled, water vapour condensed into oceans, CO₂ dissolved and was locked into rocks and fossil fuels, and algae and plants used photosynthesis to remove CO₂ and release oxygen. Greenhouse gases (carbon dioxide, methane, water vapour) absorb the Earth's infrared radiation and re-emit it, causing the greenhouse effect; human activities — burning fossil fuels, deforestation, agriculture and waste — raise these gases and drive climate change (rising sea levels, extreme weather). A carbon footprint measures the greenhouse gases from an activity and can be reduced by efficiency and renewables. Burning fuels releases carbon dioxide (greenhouse gas), toxic carbon monoxide (incomplete combustion), particulates (global dimming), and sulfur dioxide and nitrogen oxides (acid rain). Quote compositions, explain the evolution and greenhouse mechanisms accurately, and identify pollutants and their effects.