What you'll learn
Chemistry of the atmosphere is the unit of AQA GCSE Combined Science: Trilogy that traces the Earth's atmosphere from its violent beginnings to the composition we breathe today, and then examines how human activity is changing it again. It is unusual among chemistry units in that much of the evidence is indirect: there are no direct measurements of the atmosphere from billions of years ago, so theories about the early atmosphere are based on evidence that is limited and open to revision. By the end of this unit you should be able to state the present composition of dry air, describe how the proportion of carbon dioxide fell and oxygen rose, explain the greenhouse effect in terms of the behaviour of radiation, describe the evidence and the uncertainties around human-caused climate change, define and evaluate a carbon footprint, and name the atmospheric pollutants produced by burning fuels together with their effects. This unit is assessed on Chemistry Paper 2.
Key terms and definitions
Atmosphere — the layer of gases surrounding the Earth, roughly four-fifths nitrogen and one-fifth oxygen for the last 200 million years
Greenhouse gas — a gas that absorbs outgoing long wavelength radiation from the Earth's surface and re-radiates some of it back, keeping the planet warmer
Greenhouse effect — the natural process by which greenhouse gases maintain temperatures on Earth high enough to support life
Global climate change — changes in the Earth's climate caused by the increased levels of greenhouse gases in the atmosphere
Carbon footprint — the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event
Particulate — a small solid particle of carbon or unburnt hydrocarbon released by incomplete combustion
Acid rain — rain made acidic by dissolved sulfur dioxide and oxides of nitrogen, which damages buildings, statues and plants
Peer review — the process by which scientific work is checked by other scientists before publication
Core concepts
The atmosphere today
For the last 200 million years the proportions of gases in the atmosphere have been much as they are now. Dry air is approximately four-fifths nitrogen and one-fifth oxygen — around 80 per cent and 20 per cent respectively. Small proportions of other gases are present, including carbon dioxide, water vapour and noble gases.
The early atmosphere
Theories about the Earth's early atmosphere have changed over time, and continue to change, because the evidence is limited. The Earth is about 4.6 billion years old, and there is very little direct evidence of conditions during the first billion years.
The current best theory is that during the first billion years there was intense volcanic activity which released gases that formed the early atmosphere, together with water vapour that condensed to form the oceans. The early atmosphere is thought to have consisted mainly of carbon dioxide, with little or no oxygen — much like the atmospheres of Mars and Venus today. Volcanoes also produced nitrogen, which gradually built up, and small proportions of methane and ammonia.
Because the evidence is incomplete, these ideas should be presented as the current best theory rather than as established fact, and questions often reward exactly that caution.
How oxygen increased
When algae first produced oxygen, about 2.7 billion years ago, the proportion of oxygen in the atmosphere began to rise. Algae and plants produce oxygen by photosynthesis, in which carbon dioxide and water are converted into glucose and oxygen using light.
Algae came first, and over the following billion years plants evolved. As the amount of photosynthesising life increased, so did the level of oxygen, eventually reaching a proportion sufficient for animals to evolve.
How carbon dioxide decreased
The decrease in carbon dioxide has several causes, and a full-mark answer usually needs more than one.
Photosynthesis removed carbon dioxide from the atmosphere and locked the carbon into the bodies of plants and algae.
Carbon dioxide also dissolved in the oceans as they formed. Some of the dissolved carbon dioxide produced carbonate precipitates, which formed sediments on the sea bed and eventually sedimentary rock.
Finally, carbon became locked away in fossil fuels and in sedimentary rocks. Coal is a sedimentary rock formed from thick plant deposits that were buried and compressed. Crude oil and natural gas formed from the remains of plankton deposited in mud on the sea floor. Limestone is a sedimentary rock formed largely from the shells and skeletons of marine organisms. In every case, carbon that was once atmospheric carbon dioxide ended up in the ground.
Greenhouse gases and the greenhouse effect
Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. The main ones are carbon dioxide, methane and water vapour.
The mechanism is worth stating precisely, because vague versions score poorly. Short wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it. The Earth then emits radiation of a longer wavelength. Greenhouse gases absorb this outgoing long wavelength radiation and re-radiate some of it back towards the surface, so less energy escapes to space and the Earth stays warmer than it otherwise would.
The greenhouse effect itself is natural and essential. The concern is that human activity has increased the concentration of greenhouse gases, enhancing the effect.
Human activities and climate change
Human activities increase the amounts of greenhouse gases in the atmosphere. Carbon dioxide levels rise through the burning of fossil fuels and through deforestation, which both releases carbon and removes trees that would have absorbed it. Methane levels rise through livestock farming, particularly cattle, and through decomposition in landfill sites.
Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth's atmosphere to increase, resulting in global climate change.
It is difficult to model such complex systems, which leads to simplified models, speculation and opinions presented in the media that may be biased or incomplete. A good exam answer acknowledges this while being clear that the peer-reviewed scientific consensus supports human-caused warming.
The potential effects include rising sea levels as ice melts and water expands, causing flooding and coastal erosion; more frequent and more severe storms; changes in the amount, timing and distribution of rainfall; changes to the distribution of species; and changes to crop yields and the patterns of food production.
Carbon footprint
The carbon footprint of a product, service or event is the total amount of carbon dioxide and other greenhouse gases emitted over its whole life cycle — from obtaining the raw materials, through manufacture and use, to disposal.
It can be reduced by using less fossil fuel and more renewable energy, by increasing efficiency so less energy is needed, by capturing and storing carbon dioxide, and by using carbon taxes, licences and carbon offsetting schemes such as tree planting.
Reduction is not simple in practice, and the reasons are examinable: the necessary changes may be expensive, there may be disagreement over the scientific evidence, individuals may be unwilling to change their lifestyles, there may be no straightforward alternative technology available, and governments may be reluctant to make changes that harm their economies or that require international agreement to work.
Atmospheric pollutants
Burning fuels releases pollutants as well as energy, and each has a specific effect.
Carbon dioxide is released by complete combustion and contributes to global climate change.
Carbon monoxide is produced by incomplete combustion when the air supply is limited. It is toxic, and because it is colourless and odourless it cannot be detected by the senses.
Particulates, which are solid particles of carbon and unburnt hydrocarbons, are also produced by incomplete combustion. They cause global dimming, which is a reduction in the sunlight reaching the Earth's surface, and they damage human health by harming the lungs.
Sulfur dioxide is produced when fuels containing sulfur impurities are burned. Oxides of nitrogen form when the high temperatures inside an engine cause nitrogen and oxygen from the air to react together. Both dissolve in water in the atmosphere to produce acid rain, which damages buildings and statues, harms plants and makes lakes acidic. Both also cause respiratory problems in humans.
Worked examples
Example 1: Explaining the decrease in carbon dioxide (4 marks)
Explain two ways in which the proportion of carbon dioxide in the atmosphere decreased over billions of years.
Algae and then plants carried out photosynthesis, absorbing carbon dioxide from the atmosphere and converting the carbon into glucose and then into their own tissues. In addition, carbon dioxide dissolved in the oceans that formed as water vapour condensed, and some of the dissolved carbon dioxide formed carbonate precipitates that became sedimentary rock. The carbon in both cases was locked away rather than remaining in the atmosphere.
Example 2: Explaining the greenhouse effect (4 marks)
Explain how greenhouse gases keep the Earth warm.
Short wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, which warms and then emits radiation of a longer wavelength. Greenhouse gases such as carbon dioxide, methane and water vapour absorb this outgoing long wavelength radiation. They re-radiate some of it back towards the Earth's surface, so less energy is lost to space and the temperature at the surface is higher than it would otherwise be.
Example 3: Evaluating a claim (3 marks)
A newspaper article claims that climate change is not caused by human activity. Suggest why such claims appear despite the scientific consensus.
Climate systems are very complex and difficult to model, so simplified models are used and there is genuine uncertainty in the detail of predictions. Media reports may present speculation or opinion rather than peer-reviewed evidence, and may be biased or incomplete. However, the peer-reviewed evidence considered by many scientists supports the conclusion that human activities are causing the Earth's temperature to increase.
Common mistakes and how to avoid them
The most frequent error in this unit is describing greenhouse gases as trapping heat or as forming a blanket. The specification wants absorption of long wavelength radiation and re-radiation back towards the Earth, and vague metaphors do not earn the marks.
Students often confuse the greenhouse effect with the hole in the ozone layer. They are separate phenomena, and referring to ozone in a greenhouse effect answer suggests a misunderstanding.
Another regular slip is stating that the early atmosphere definitely consisted of certain gases. Evidence for the early atmosphere is limited, so answers should refer to what the current best theory suggests.
In pollutant questions, many answers name a pollutant without giving its effect. The marks are usually split between the two, so always pair them.
Finally, sulfur dioxide and oxides of nitrogen have different origins that students frequently merge. Sulfur dioxide comes from sulfur impurities in the fuel; oxides of nitrogen form from nitrogen and oxygen in the air at the high temperatures inside an engine.
Exam technique for "Chemistry: Chemistry of the Atmosphere"
This unit has more extended-writing questions than most, so structure matters. For a six-mark question on climate change, plan three points with a consequence for each rather than listing everything you know.
Where a question asks for evaluation, give both sides and then a judgement. Questions on reducing the carbon footprint almost always want the practical and political obstacles as well as the methods.
Use the phrase long wavelength radiation rather than heat when describing the greenhouse effect. It is the specific wording the mark scheme uses.
For pollutants, learn them as pairs of cause and effect: incomplete combustion gives carbon monoxide which is toxic, and particulates which cause global dimming and lung damage; sulfur impurities give sulfur dioxide which causes acid rain; high engine temperatures give oxides of nitrogen which cause acid rain and respiratory problems.
Quick revision summary
Dry air today is about four-fifths nitrogen and one-fifth oxygen, with small amounts of carbon dioxide, water vapour and noble gases. The early atmosphere is thought to have come from intense volcanic activity, rich in carbon dioxide with little oxygen, with water vapour condensing to form the oceans. Oxygen rose once algae began photosynthesising about 2.7 billion years ago. Carbon dioxide fell through photosynthesis, dissolving in the oceans, carbonate precipitation, and burial as fossil fuels and sedimentary rocks. Greenhouse gases absorb outgoing long wavelength radiation and re-radiate it back, keeping the Earth warm; human activity has increased them, and peer-reviewed evidence links this to global climate change with effects on sea level, storms, rainfall, species and food production. A carbon footprint covers the whole life cycle and is hard to reduce for cost, political and lifestyle reasons. Combustion pollutants are carbon dioxide, carbon monoxide, particulates, sulfur dioxide and oxides of nitrogen.