Chemistry of the Atmosphere — AQA Combined Science: Trilogy

This unit covers how Earth's atmosphere developed, the greenhouse effect and climate change, and air pollution.

Today's atmosphere

For the last 200 million years the proportions of gases in the atmosphere have been roughly:

• Nitrogen — about 78%
• Oxygen — about 21%
• Small amounts of carbon dioxide, water vapour and noble gases (such as argon).

How the early atmosphere developed

Theories are uncertain because the evidence is from billions of years ago, but the widely accepted account is:

1. The early atmosphere was formed by intense volcanic activity, which released carbon dioxide, water vapour, nitrogen and small amounts of methane and ammonia. The early atmosphere was probably mostly carbon dioxide with little or no oxygen — similar to the atmospheres of Mars and Venus today.
2. As the Earth cooled, water vapour condensed to form the oceans.
3. Much of the carbon dioxide dissolved in the oceans, and carbonates were precipitated, forming sediments. CO₂ levels fell.
4. Algae and plants evolved and, through photosynthesis, removed CO₂ and produced oxygen. Over about two billion years, oxygen levels rose enough for animals to evolve.
5. Carbon became locked up in sedimentary rocks (limestone) and in fossil fuels (coal, crude oil, natural gas) formed from the remains of organisms.

The greenhouse effect

Some gases in the atmosphere are greenhouse gases — mainly carbon dioxide, methane and water vapour. They maintain temperatures on Earth high enough to support life.

How it works:

• Short-wavelength radiation (including visible light) from the Sun passes through the atmosphere and is absorbed by the Earth's surface.
• The Earth re-emits energy as longer-wavelength infrared radiation.
• Greenhouse gases absorb this infrared radiation and re-radiate it in all directions, including back to the surface, keeping the Earth warm.

Human activities and climate change

Human activities are increasing the amounts of some greenhouse gases:

• Carbon dioxide — from burning fossil fuels and deforestation.
• Methane — from cattle farming, rice fields and landfill waste.

Based on peer-reviewed evidence, most scientists agree that human activities are causing the global temperature to increase, leading to climate change.

It is difficult to model the climate fully, which leads to simplified models, speculation and biased opinions (e.g. in some media reports). You should be able to evaluate the quality of evidence and recognise that scientific consensus is based on peer-reviewed data.

Consequences of global climate change

• Melting ice caps and rising sea levels, causing flooding.
• More frequent and severe storms and changes in rainfall patterns affecting food production.
• Changes to the distribution of species and loss of biodiversity.

Carbon footprint

The 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. It can be reduced by using renewable energy, increasing efficiency, carbon capture and storage, taxes/quotas, and individual lifestyle choices. Reducing it is difficult because of cost, lifestyle, and lack of international cooperation.

Air pollution from burning fuels

Burning fuels releases substances into the atmosphere:

• Carbon dioxide — a greenhouse gas (from complete combustion).
• Carbon monoxide — a toxic, colourless, odourless gas from incomplete combustion; it reduces the blood's ability to carry oxygen.
• Soot (carbon particulates) — from incomplete combustion; causes respiratory problems and global dimming, and makes buildings dirty.
• Sulfur dioxide — from sulfur impurities in fuels; causes acid rain and respiratory problems.
• Oxides of nitrogen (NOₓ) — formed at the high temperatures inside engines; cause acid rain and respiratory problems.

Exam tips

• Learn the approximate composition of today's atmosphere (78% N₂, 21% O₂).
• Be able to describe how oxygen increased (photosynthesis by algae/plants) and how CO₂ decreased (oceans, photosynthesis, locked in rocks and fossil fuels).
• Explain the greenhouse effect using infrared radiation being absorbed and re-emitted.
• Learn the pollutants with their source and effect — especially the difference between CO₂, CO, SO₂ and NOₓ.
• Recognise that climate conclusions are based on peer-reviewed evidence, while media reports may be biased.