Topic 9: Chemistry of the atmosphere

Earth's atmosphere has changed dramatically over billions of years, shaped by volcanic activity, oceans and living things. Today the proportions of gases are roughly stable, but human activity is increasing greenhouse gases and releasing harmful pollutants from burning fuels. This topic explains how the atmosphere formed, why the climate is changing, and how we can reduce our impact.

Today's atmosphere

For about the last 200 million years the proportions of gases in the atmosphere have stayed roughly the same. Dry air is approximately four-fifths (about 78 percent) nitrogen and one-fifth (about 21 percent) oxygen. The remaining 1 percent is made up of small amounts of other gases, mainly argon (a noble gas, around 0.9 percent), carbon dioxide (around 0.04 percent) and varying amounts of water vapour. Knowing these rough proportions is a common exam requirement.

The early atmosphere

Theories about the early atmosphere are uncertain because the evidence is limited and comes from billions of years ago. One widely accepted idea is that during the first billion years the Earth's surface was covered with intense volcanic activity. Volcanoes released gases that formed the early atmosphere and also released water vapour and carbon dioxide. The early atmosphere is thought to have been mostly carbon dioxide, with little or no oxygen, similar in some ways to the atmospheres of Mars and Venus today. Volcanoes also released nitrogen, which gradually built up, plus small amounts of methane and ammonia.

How the oceans and carbon dioxide changed

As the Earth cooled, water vapour condensed to form the oceans. Large amounts of carbon dioxide then dissolved into the oceans, removing it from the air. Dissolved carbon dioxide produced carbonate precipitates that formed sediments on the sea bed. Over time, the carbon from carbon dioxide also became locked away (sequestered) in sedimentary rocks such as limestone and in fossil fuels like coal, crude oil and natural gas, which formed from the remains of dead organisms. These processes greatly reduced the level of carbon dioxide in the atmosphere.

How oxygen increased

About 2.7 billion years ago algae first appeared in the oceans, followed later by green plants on land. These organisms carry out photosynthesis, which uses carbon dioxide and water and releases oxygen. The word equation is: carbon dioxide plus water gives glucose plus oxygen. Over the next billion years and beyond, algae and plants steadily removed carbon dioxide and added oxygen, until the level of oxygen rose high enough for animals to evolve.

Greenhouse gases and the greenhouse effect

Greenhouse gases in the atmosphere keep the Earth warm enough to support life. The main greenhouse gases are carbon dioxide, methane and water vapour. They work by allowing short-wavelength radiation from the Sun to pass through to the Earth's surface, which warms up and re-emits energy as longer-wavelength infrared radiation. Greenhouse gases absorb this outgoing infrared radiation and re-radiate some of it back towards the surface, trapping heat. Without this natural greenhouse effect the Earth would be too cold for life.

Human activities and climate change

Human activities increase the amounts of greenhouse gases in the atmosphere. Burning fossil fuels and deforestation raise carbon dioxide levels, while farming livestock and growing rice in flooded fields, along with rotting waste in landfill, raise methane levels. Based on peer-reviewed evidence, most scientists agree that extra carbon dioxide and methane are causing global warming, leading to climate change. Possible effects include rising sea levels from melting ice and thermal expansion, more frequent and severe storms, changes to rainfall patterns affecting food production, and shifts in the distribution of wildlife. Predictions are difficult because the Earth's systems are complex, and the topic is sometimes oversimplified, biased or based on incomplete data in the media and on opinion rather than evidence.

Carbon footprint and how to reduce it

The carbon footprint is the total amount of greenhouse gases, especially carbon dioxide and methane, released over the full life cycle of a product, service or event. It can be reduced by using renewable or nuclear energy instead of fossil fuels, increasing energy efficiency, capturing and storing carbon dioxide, and taxing or trading carbon emissions. In practice this is hard to achieve because of cost, the slow pace of technological development, a lack of international agreements, public reluctance to change lifestyles, and incomplete education and information about the issue.

Pollutants from burning fuels

Most fuels are hydrocarbons that also contain some impurities such as sulfur. Complete combustion, with plenty of oxygen, releases energy and produces carbon dioxide and water. Incomplete combustion, when there is not enough oxygen, produces carbon monoxide and solid particles of carbon (soot, also called particulates) along with unburnt hydrocarbons. Carbon monoxide is a toxic gas that is colourless and odourless, so it cannot be detected easily; it reduces the amount of oxygen the blood can carry. Soot and particulates cause respiratory problems in humans and form global dimming, where smoke and dust reflect sunlight. The sulfur impurities burn to form sulfur dioxide (SO2), and the high temperatures of combustion in engines make nitrogen and oxygen from the air react to form nitrogen oxides (NOx). Sulfur dioxide and nitrogen oxides cause acid rain, which damages buildings, plants and aquatic life, and they also cause respiratory problems.