Cambridge O Level Environmental Management (5014): The atmosphere and human activities – Study Guide

Every breath you take draws on a thin shell of gas wrapped around the planet, and human activities now alter its chemistry. This topic covers what the atmosphere contains, how extra greenhouse gases warm the planet, how air pollution becomes acid rain, and why a chemical used in everyday products once threatened the ozone layer. To answer well, link each problem to its specific gases, its sources and the people and ecosystems that suffer.

Where this fits in the course

In Cambridge O Level Environmental Management (5014) for 2027 to 2029, Topic 4 has four sub-topics: the atmosphere, climate change, acid rain and ozone depletion. It builds on earlier topics about energy and land, because the fuels we burn and the forests we clear determine what enters the air. Examination questions often combine a calculation or graph with an explanation, so expect to move between numbers and words.

The atmosphere

Air is a mixture. By volume it is about 78 per cent nitrogen and 21 per cent oxygen, with small amounts of argon, carbon dioxide (about 0.04 per cent) and variable water vapour.

The atmosphere has layers. Weather and most of the air are in the troposphere, the layer we live in. Above it, the stratosphere holds the ozone layer, which filters ultraviolet radiation from the Sun. Beyond this are further layers, but the first two matter most for this topic.

The natural greenhouse effect occurs because gases such as carbon dioxide, methane and water vapour hold in some of the heat leaving the surface. Without it, the Earth would be too cold for most life.

Climate change

Climate change is a long-term shift in temperature and weather patterns. The present rapid warming is linked to more greenhouse gases from human activity, an effect called the enhanced greenhouse effect.

Sources of greenhouse gases

• Carbon dioxide: burning fossil fuels in power stations, vehicles and industry; burning and clearing forests.
• Methane: livestock, flooded rice fields, rubbish tips and fossil fuel extraction.
• Nitrous oxide: nitrogen fertilisers and some industrial processes.

Consequences

• Sea level rises as the oceans warm and expand and land ice melts, threatening coasts and small islands and spoiling fresh water with salt.
• Rainfall patterns change, so some places get more floods and others more droughts.
• Farming is affected, with pests, heat stress and water shortage.
• Coral reefs suffer bleaching when seas become too warm.
• Extreme heat and storms become more frequent in many regions.

Responses

To reduce emissions, countries can use solar, wind, hydroelectric and other low-carbon energy, improve efficiency in homes and transport, protect and plant forests, and reduce waste. To cope with change, communities can build sea defences, grow drought-tolerant crops and improve early warnings. An international agreement works only when many countries take part, and richer countries have more money to act, so cooperation and fairness are part of any evaluation.

Acid rain

Rain is naturally slightly acidic, at about pH 5.6. Acid rain is lower than this. It forms when sulfur dioxide and nitrogen oxides react with water in the air to produce acids. Sulfur dioxide comes mostly from burning coal and oil containing sulfur and from metal smelting; nitrogen oxides come from vehicle engines and high-temperature combustion.

Acid deposition can also arrive as dry particles or fog. Because the gases travel with the wind, damage often happens in a different country from the source.

Effects

• Freshwater becomes acidic, so fish eggs and aquatic insects die, reducing biodiversity.
• Soils lose nutrients and trees weaken, with yellowing and loss of leaves.
• Buildings, statues and bridges corrode.
• Human health can be affected by the fine particles formed from the gases.

Solutions

Reduce the gases at source with cleaner fuels, scrubbers in power-station chimneys, catalytic converters on vehicles, renewable energy and public transport. Lime can be spread on lakes to neutralise acidity, though this is temporary and costly.

Ozone depletion

The ozone layer is a region of the stratosphere with a higher concentration of ozone. It absorbs much of the Sun’s harmful ultraviolet rays. Chlorofluorocarbons, known as CFCs, used in old refrigerators, air conditioners, aerosol sprays and foam, drift up to the stratosphere, where ultraviolet light frees chlorine atoms that break down ozone. One chlorine atom can destroy many ozone molecules.

More ultraviolet radiation reaching the surface raises the risk of skin cancer and eye cataracts and can damage crops and plankton. The Montreal Protocol, signed in 1987, phased out CFCs, and the ozone layer has been slowly recovering. Remember that the thinning of the ozone layer is a separate problem from climate change, although some replacement chemicals are also greenhouse gases.

Worked example 1: emissions per person

A fictional city of 650,000 people releases 3.9 million tonnes of carbon dioxide a year.

1. Per person = 3,900,000 / 650,000 = 6.0 tonnes per year.
2. A programme cuts total emissions by 18 per cent. Reduction = 3,900,000 x 0.18 = 702,000 tonnes.
3. New total = 3,900,000 - 702,000 = 3,198,000 tonnes.
4. Per person = 3,198,000 / 650,000 = 4.92 tonnes, to three significant figures.

Worked example 2: reading rainfall acidity

Three rain samples have pH 5.6, 4.6 and 3.6.

1. The pH 4.6 sample is one unit lower than normal rain, so it is ten times more acidic.
2. The pH 3.6 sample is two units lower, so it is 10 x 10 = 100 times more acidic than normal rain.
3. Compared with the pH 4.6 sample, the pH 3.6 sample is ten times more acidic.

A vehicle fitted with a catalytic converter reduces nitrogen oxide output from 1.2 grams per kilometre to 0.3 grams per kilometre. On a 40 km journey this is a saving of (1.2 - 0.3) x 40 = 36 grams, a 75 per cent reduction.

Common mistakes

• Writing that carbon dioxide causes acid rain; the main gases are sulfur dioxide and nitrogen oxides.
• Saying that holes in the ozone layer let heat in and cause global warming.
• Describing the greenhouse effect as always harmful.
• Answering “stop using cars” without explaining why it reduces a specific gas.
• Treating pH 7 as the pH of normal rain.

Exam technique

Match your answer to the command word. For explain questions, use linking words such as “because”, “so” and “which causes”. For suggest questions, use the information in the stimulus, such as a graph or a described country. When asked to compare two solutions, give one strength and one limit of each and then state which you prefer. In calculations, write the working line by line, include units and round sensibly.

Check your understanding

1. Which layer of the atmosphere contains the ozone layer? (The stratosphere.)
2. Name two greenhouse gases and one human source of each. (For example carbon dioxide from burning fuels; methane from rice fields or livestock.)
3. Give two consequences of rising sea level. (For example coastal flooding and salt contamination of fresh water.)
4. Explain how sulfur dioxide leads to acid rain. (It dissolves in water droplets in the air to form acid that falls as rain.)
5. Why can acid rain damage forests in another country? (Winds carry the gases far from their source.)
6. Give one use of CFCs and the harm they cause. (Refrigerants or aerosols; they destroy stratospheric ozone.)
7. A town releases 800,000 tonnes of carbon dioxide a year and has 200,000 people. What is the amount per person? (4 tonnes.)
8. How many times more acidic is rain at pH 3.6 than rain at pH 5.6? (100 times.)

Frequently asked questions

What are the main gases in the atmosphere? By volume, air is about 78 per cent nitrogen and 21 per cent oxygen, with small amounts of other gases, including argon and carbon dioxide.

Does carbon dioxide cause acid rain? No. Acid rain forms mainly from sulfur dioxide and nitrogen oxides. Writing that carbon dioxide causes acid rain is a common mistake.

What is the pH of normal rain? Rain is naturally slightly acidic, at about pH 5.6. Treating pH 7 as the pH of normal rain is a common error, and acid rain has a pH lower than 5.6.

Why is the pH scale important in acid rain calculations? The pH scale is logarithmic, so each unit lower is ten times more acidic. Rain at pH 3.6 is 100 times more acidic than normal rain at pH 5.6.

Do holes in the ozone layer cause global warming? No. The ozone layer absorbs ultraviolet radiation, so thinning lets more ultraviolet reach the surface, raising the risk of skin cancer and eye cataracts. Saying ozone holes let heat in is a common mistake.