The atmosphere: its composition and layered structure, its role in weather and climate, the greenhouse effect, and the causes and effects of atmospheric pollution including acid rain and ozone depletion.

What this dot point is asking

The SQA wants you to describe the atmosphere: its composition and layered structure, its role in weather and climate, the greenhouse effect, and the causes and effects of atmospheric pollution, including acid rain and ozone depletion. This is the gaseous sphere of the Earth's Resources area and the springboard to climate change in Sustainability.

Composition and structure

The atmosphere is layered by temperature:

• the troposphere, the lowest layer, where almost all weather occurs and temperature falls with height;
• the stratosphere, above it, which contains the ozone layer that absorbs harmful ultraviolet radiation;
• the mesosphere and thermosphere above, increasingly thin.

Weather and climate

The atmosphere drives weather (the short-term state of the atmosphere) and climate (the long-term average pattern). Uneven heating of the Earth by the Sun, the rotation of the planet, and the transfer of heat and moisture by air masses and ocean currents create winds, pressure systems, clouds and precipitation. The atmosphere therefore distributes energy and water around the globe, shaping the biomes studied in the biosphere topic.

The greenhouse effect

The natural greenhouse effect is essential: short-wave solar radiation passes through the atmosphere and warms the surface; the warmed surface re-emits energy as longer-wave infrared; and greenhouse gases (carbon dioxide, methane, water vapour, nitrous oxide) absorb some of this infrared and re-radiate it in all directions, including back to the surface. Without it, the Earth would be far too cold for life. Enhancing this effect by adding more greenhouse gases is the basis of anthropogenic climate change, covered in Sustainability.

Atmospheric pollution

Atmospheric pollution releases harmful substances into the air, with several distinct problems:

• Acid rain. Burning fossil fuels releases sulfur dioxide and nitrogen oxides, which react with water vapour to form sulfuric and nitric acids. Acid rain acidifies lakes and rivers (harming fish), damages trees and leaches nutrients from soils, and corrodes buildings.
• Ozone depletion. CFCs (chlorofluorocarbons), once used in aerosols and refrigerants, break down ozone in the stratosphere, thinning the ozone layer and letting more harmful ultraviolet radiation reach the surface, which increases skin cancer and harms ecosystems.
• Particulates and smog. Soot and fine particles from combustion and traffic cause respiratory illness and reduce air quality, and can form photochemical smog.

It is important to keep these separate: acid rain, ozone depletion and the greenhouse effect have different causes, pollutants and effects.

Examples in context

Example 1. The Montreal Protocol and the ozone layer. After CFCs were found to thin the stratospheric ozone layer, the 1987 Montreal Protocol phased them out worldwide, and the ozone hole is now slowly recovering. It is a clear case of identifying an atmospheric pollutant, its effect (more ultraviolet reaching the surface) and a successful international response.

Example 2. Acid rain and Scandinavian lakes. Sulfur dioxide carried on the wind from industrial Europe acidified thousands of lakes in Scandinavia, killing fish, until controls on power-station emissions reduced the pollutant. It shows acid rain's cause (fossil-fuel sulfur and nitrogen oxides) and its effect on aquatic ecosystems far from the source.

Try this

Q1. State the approximate percentages of nitrogen and oxygen in the atmosphere. [1 mark]

• Cue. About 78 percent nitrogen and about 21 percent oxygen.

Q2. Name the two pollutant gases mainly responsible for acid rain. [2 marks]

• Cue. Sulfur dioxide and nitrogen oxides.