What are the main air pollutants, where do they come from, and what damage do they cause?
The main air pollutants and their sources, the effects of air pollution including acid rain, smog, ozone depletion and the enhanced greenhouse effect, and methods of controlling air pollution.
A focused answer to AQA A-Level Environmental Science 3.4.2, covering the main air pollutants and their sources, the effects including acid rain, smog, ozone depletion and the enhanced greenhouse effect, and control methods.
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What this dot point is asking
AQA wants you to identify the main air pollutants and their sources, explain their effects (acid rain, smog, ozone depletion and the enhanced greenhouse effect), and describe how air pollution is controlled. The recurring command words are Explain and Describe, so you must give the chemistry and mechanisms, keeping the four major effects clearly separated.
The main air pollutants and sources
A useful distinction for exam answers is between primary pollutants (emitted directly, such as sulfur dioxide and carbon monoxide) and secondary pollutants (formed by reactions in the atmosphere, such as the ground-level ozone in photochemical smog and the acids in acid rain).
Acid rain
Sulfur dioxide and nitrogen oxides dissolve and react with water and oxygen in the atmosphere to form sulfuric and nitric acids, which fall as acid rain. This acidifies lakes and soils, killing fish and releasing toxic aluminium ions from sediments; it strips calcium and magnesium nutrients from soils and damages tree leaves, weakening forests; and it corrodes limestone and marble buildings and statues by reacting with the calcium carbonate. Because the pollutants travel in the air, acid rain can fall hundreds of kilometres downwind, sometimes across national borders, making it a transboundary problem.
Smog and the enhanced greenhouse effect
Photochemical smog forms when nitrogen oxides and unburned hydrocarbons react in strong sunlight, producing ground-level ozone and other irritants that harm lungs and reduce visibility, a problem in sunny, traffic-heavy cities. The enhanced greenhouse effect results from rising concentrations of carbon dioxide and methane, which absorb more outgoing infrared radiation and re-emit it toward the surface, raising global temperatures and driving climate change.
Ozone depletion
Controlling air pollution
- Catalytic converters on vehicles convert nitrogen oxides, carbon monoxide and unburned hydrocarbons into nitrogen, carbon dioxide and water.
- Flue-gas desulfurisation passes power-station gases through limestone or lime to remove sulfur dioxide as gypsum.
- Cleaner fuels, electric vehicles and renewables cut emissions at source.
- International agreements such as the Montreal Protocol banned CFCs, allowing the ozone layer to recover.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20186 marksExplain how acid rain forms and describe its effects on lakes, forests and buildings.Show worked answer →
A 6-mark Explain rewards the chemistry of formation plus a range of developed effects. Markers split roughly 2 for formation and 4 for effects.
Formation: sulfur dioxide (from burning sulfur-containing fossil fuels) and nitrogen oxides (from high-temperature combustion) dissolve and react with water and oxygen in the atmosphere to form sulfuric and nitric acids, which fall as acid rain, often far from the source.
Effects: lakes acidify, killing fish and invertebrates and releasing toxic aluminium ions from sediments; forests suffer as acid leaches calcium and magnesium from soils and damages leaves, weakening trees; limestone and marble buildings and statues corrode as the acid reacts with the calcium carbonate. Award the named acids, the chemical link, and at least three distinct effects.
AQA 20214 marksDescribe how a catalytic converter and flue-gas desulfurisation each reduce air pollution from different sources.Show worked answer →
Two marks each for correctly matching the control method to its source and the gas it removes.
Catalytic converter (vehicles): exhaust gases pass over a platinum and rhodium catalyst that converts nitrogen oxides, carbon monoxide and unburned hydrocarbons into nitrogen, carbon dioxide and water, reducing the pollutants from petrol engines.
Flue-gas desulfurisation (power stations): flue gases are passed through a spray of crushed limestone or lime, which reacts with sulfur dioxide to form calcium sulfate (gypsum), removing the sulfur dioxide that would otherwise cause acid rain. Award marks for naming the pollutant removed and the product formed, not just the device.
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