Syllabus coverage
- Define acid deposition as a mix of air pollutants that deposit from the atmosphere as acidic wet deposition (with a pH below 5.6) or acidic dry deposition
- Describe the two types of acid deposition: wet deposition (snow, rain, hail, fog) and dry deposition (dust and gases)
- Outline the formation of acid deposition from sulfur dioxide released by fossil fuel combustion and from oxides of nitrogen released in vehicle emissions
- Outline the impacts of acid deposition on aquatic environments, vegetation and crops, and stone and brick buildings
- Define photochemical smog as a mixture of air pollutants and particulates, including ground level ozone, formed when oxides of nitrogen and volatile organic compounds (VOCs) react in the presence of sunlight, and describe its impacts
- Describe strategies for managing air pollution, including reduced use of fossil fuels, flue gas desulfurisation, catalytic converters, electrostatic precipitators, VOC controls, restricting vehicle use and legislation
- Outline how ozone depletion occurs through CFCs, ultraviolet light and chlorine atoms, and state that ozone concentration is measured using the Dobson Unit
- Define the term ozone hole and explain why ozone depletion has been greatest over Antarctica, including temperature, polar vortex and polar stratospheric clouds
- Describe the impacts of ozone depletion, evaluate the international agreements used to phase out ozone depleting substances, and outline the impacts of alternatives including HCFCs and F-gases
- Outline the importance of experimental evidence to support a hypothesis, using the Rowland-Molina ozone destruction hypothesis as an example
Part of the AS Level Environmental Management syllabus 8291.
Acid deposition: wet and dry
Acid deposition is a mix of air pollutants that deposit from the atmosphere as either acidic wet deposition or acidic dry deposition. Wet deposition is acidic material carried down in precipitation, so it falls as snow, rain, hail or fog with a pH below 5.6. Dry deposition is acidic material that settles directly onto surfaces as dust or as gases, with no precipitation involved, and it is often washed off later when rain eventually does fall.
The pH 5.6 threshold is the marker, because even clean rain is slightly acidic. Carbon dioxide dissolved in rainwater forms weak carbonic acid, giving natural rain a pH of around 5.6, so only deposition below that value counts as acid deposition. The term acid rain names only the wet form, and writing acid deposition correctly shows the examiner that you are covering both types.
| Type of deposition | How it reaches the surface | Examples |
|---|---|---|
| Wet deposition | Carried down in precipitation | Snow, rain, hail, fog |
| Dry deposition | Settles directly onto surfaces | Dust and gases |
How acid deposition forms
The sulfur pathway starts underground. Fossil fuels contain sulfur compounds, and the combustion of fossil fuels releases sulfur dioxide gas when they are burned in power stations, factories and homes. Once in the air, sulfur dioxide gas reacts with water and oxygen in the atmosphere to form sulfuric acid, which then falls as acid deposition.
The nitrogen pathway starts in engines. Nitrogen from the atmosphere reacts with oxygen in the high temperatures of vehicle engines to form nitrogen monoxide gas, and this nitrogen monoxide gas is released into the atmosphere in vehicle emissions. The nitrogen monoxide gas then reacts with oxygen and water in the atmosphere to form nitric acid.
Both pathways matter because the question will usually expect you to name the starting pollutant, the reaction it undergoes and the acid it ends up as. Sulfur dioxide comes from burning fossil fuels and makes sulfuric acid, while oxides of nitrogen come from vehicle exhausts and make nitric acid.
- Sulfur pathway: fossil fuels contain sulfur compounds, combustion releases sulfur dioxide, and sulfur dioxide reacts with water and oxygen to form sulfuric acid.
- Nitrogen pathway: atmospheric nitrogen reacts with oxygen at the high temperatures of vehicle engines to form nitrogen monoxide, released in exhaust, which then reacts with oxygen and water to form nitric acid.
The impacts of acid deposition
In aquatic environments acid deposition lowers the pH of lakes and rivers and can also leach toxic aluminium from surrounding soils into the water. The acid damages fish gills, making them produce excess mucus so the fish cannot take in enough oxygen, and this kills fish and reduces fish populations, sometimes emptying a lake of fish entirely.
For vegetation and crops the damage shows as defoliation, the loss of leaves, and a reduced crop yield. Acid deposition strips nutrients such as calcium and magnesium from soils and damages leaves directly, so trees weaken and farm yields fall. Forests on thin, acidic soils are hit hardest.
On stone and brick buildings acid deposition causes enhanced chemical weathering. Sulfuric acid reacts with calcium carbonate in limestone and marble, dissolving the stone and wearing away carvings, statues and building facades, so buildings corrode far faster than they would in clean air.
- Aquatic environments: acidified water damages fish gills and kills fish, so fish populations decline or disappear from affected lakes.
- Vegetation and crops: acid deposition causes defoliation and reduced crop yield, partly by leaching soil nutrients.
- Stone and brick buildings: acid deposition speeds up chemical weathering, attacking limestone and marble.
Photochemical smog
Photochemical smog is a mixture of air pollutants and particulates, including ground level ozone, that is formed when oxides of nitrogen and volatile organic compounds (VOCs) react in the presence of sunlight. The sunlight is essential: it drives the reactions that turn the primary pollutants from vehicles and industry into a cocktail of secondary pollutants, so smog builds up most on hot, sunny days.
The impacts fall into three groups. The pollutants cause eye and respiratory irritation, aggravating asthma and other breathing conditions. They cause decreased crop yields, because ground level ozone damages plant leaves and cuts photosynthesis. And they cause deterioration of plastics and rubber, which become brittle and crack in polluted air.
Do not confuse the two ozones. Ground level ozone in photochemical smog is a harmful pollutant that damages lungs, plants and materials, while the stratospheric ozone high above is the protective layer that screens out ultraviolet radiation. The same molecule does two very different jobs depending on where it sits.
- Eye and respiratory irritation in people, especially those with existing breathing conditions.
- Decreased crop yields, because ground level ozone damages plant leaves and reduces photosynthesis.
- Deterioration of plastics and rubber, which become brittle and crack.
Managing air pollution
The cheapest pollution is the pollution never released, so the first strategy is reduced use of fossil fuels. Switching to renewable energy, improving energy efficiency and encouraging public transport all cut the fuel burned, and therefore the sulfur dioxide, oxides of nitrogen and particulates released. Restricting vehicle use in urban areas - through low emission zones, congestion charging or park-and-ride schemes - directly reduces the oxides of nitrogen and VOCs that drive photochemical smog.
Where fuel is still burned, the emissions can be cleaned before release. Sulfur dioxide is reduced by flue gas desulfurisation, scrubbing the waste gases of power stations, and by fuel desulfurisation, removing sulfur from the fuel before it is burned. Oxides of nitrogen are reduced by catalytic converters, which convert them in the exhaust into harmless nitrogen and oxygen. Particulates are removed by electrostatic precipitators, which give the particles an electric charge and pull them onto collecting plates. VOCs are controlled through the safe usage, storage and disposal of household products such as solvents and paints.
Technology is backed up by legislation at local, national and international level. Detailed knowledge of specific laws and agreements is not required, but you should understand the polluter pays principle: the cost of preventing or cleaning up pollution falls on the polluter, which gives businesses a financial reason to cut emissions.
| Pollutant | Strategy that reduces it |
|---|---|
| Sulfur dioxide | Flue gas desulfurisation and fuel desulfurisation |
| Oxides of nitrogen | Catalytic converters |
| Particulates | Electrostatic precipitators |
| Volatile organic compounds (VOCs) | Safe usage, storage and disposal of household products |
Ozone depletion and the ozone hole
Chlorofluorocarbons (CFCs) from aerosols and refrigerants are unreactive compounds and are not broken down in the troposphere, so they drift intact into the stratosphere. There, CFCs break down in the presence of ultraviolet light to release a chlorine atom, and rapid reactions between chlorine atoms and ozone break down ozone (O3) to oxygen (O2). The chlorine atom is not used up in the reaction, so it remains in the stratosphere and can continue to destroy ozone molecule after molecule.
Ozone concentration is measured using the Dobson Unit, and the ozone hole is defined as an area where the average concentration of ozone is below 100 Dobson Units. Note the wording: it is a region of severely thinned ozone, not a region with no ozone at all.
Ozone depletion has been greatest over Antarctica because of three linked conditions. The temperature there is extremely low, and the polar vortex - a ring of strong winds - isolates the cold air over the continent, stopping warmer ozone-rich air from mixing in. Inside that cold, isolated air, polar stratospheric clouds (PSCs) form, and their surfaces convert chlorine compounds into a reactive form that destroys ozone rapidly when sunlight returns in the Antarctic spring.
Impacts of ozone depletion, alternatives and the international response
A thinner ozone layer lets more ultraviolet radiation through, and every impact flows from that. For human health the extra UV raises the risk of cataracts and skin cancer. It causes decreased crop yields, damages the biodiversity of terrestrial and aquatic ecosystems - including the phytoplankton at the base of marine food webs - and speeds the degradation of materials used in clothing and construction.
The response has been international, because ozone depletion is a global problem no single country can solve alone. International agreements to reduce and phase out ozone depleting substances, led by the Montreal Protocol of 1987, have cut CFC production dramatically and the ozone layer is now beginning to recover. Detailed knowledge of specific agreements is not required, but you should be able to evaluate the approach: it worked because it was binding and global, while its limits include slow compliance and illegal production of banned substances.
The replacements bring their own costs. Hydrochlorofluorocarbons (HCFCs) still deplete some ozone, though far less than CFCs, and are being phased out in turn. Fluorinated gases (F-gases) do not deplete ozone at all but are potent greenhouse gases, so swapping CFCs for F-gases eased the ozone problem while adding to climate change - a trade-off worth a mark in an evaluate question.
The Rowland-Molina hypothesis and the Great Smog of London
The ozone destruction hypothesis proposed by Rowland and Molina shows how science tests an idea against evidence. Their 1974 proposal that CFCs break down in the stratosphere and release chlorine that destroys ozone was initially not accepted, and some of the auxiliary hypotheses made alongside it were not backed up by experimental evidence. Crucially, the hypothesis led to further research and data collection by other scientists, which confirmed that CFCs are ozone depleting. A hypothesis earns its place by predicting what later measurements will find, not by being believed.
A classic case study of a named atmospheric pollution event is the Great Smog of London in December 1952. Cold, still weather trapped a temperature inversion over the city, so smoke from domestic coal fires and industry - releasing sulfur dioxide and particulates - could not disperse and built into a thick, acidic smog for about five days. The causes were coal burning combined with the inversion, and the impacts were severe: thousands of excess deaths from respiratory illness, and major disruption to the city.
The management response shows legislation working. The UK passed the Clean Air Act of 1956, creating smoke control areas where only smokeless fuels could be burned, and later acts pushed industry to relocate and cut emissions. Evaluating the management: it was effective because it removed the cause, by cutting coal smoke at the source, and London's air has not seen a smog on that scale since.
Key terms you must be able to define
- Acid deposition
- A mix of air pollutants that deposit from the atmosphere as acidic wet deposition, with a pH below 5.6, or as acidic dry deposition.
- Wet deposition
- Acidic pollutants carried down in precipitation - snow, rain, hail or fog - with a pH below 5.6.
- Dry deposition
- Acidic pollutants that settle directly onto surfaces as dust or as gases, without any precipitation.
- Photochemical smog
- A mixture of air pollutants and particulates, including ground level ozone, formed when oxides of nitrogen and volatile organic compounds (VOCs) react in the presence of sunlight.
- Flue gas desulfurisation
- Removing sulfur dioxide from the waste gases of a power station or factory before they are released, for example by scrubbing them with limestone.
- Catalytic converter
- A device in a vehicle exhaust that converts oxides of nitrogen, carbon monoxide and unburnt hydrocarbons into less harmful gases such as nitrogen, carbon dioxide and water.
- Electrostatic precipitator
- A device that removes particulates from waste gases by giving the particles an electric charge and attracting them onto collecting plates.
- Dobson Unit
- The unit used to measure the concentration of ozone in the atmosphere.
- Ozone hole
- An area of the atmosphere where the average concentration of ozone is below 100 Dobson Units.
- Chlorofluorocarbons (CFCs)
- Unreactive compounds from aerosols and refrigerants that are not broken down in the troposphere, reach the stratosphere, and release chlorine atoms that destroy ozone.
Applied to real places
Cambridge sets 8291 in named real contexts - a river basin, a city, a coastline, a mine, a farm, a national park. Use these as models for how to apply the management ideas, then write your own.
In December 1952 a temperature inversion trapped smoke from domestic coal fires and industry over London for about five days. The causes were sulfur dioxide and particulates from coal burning held down by the inversion, and the impacts were thousands of excess deaths from respiratory illness. Management came through the Clean Air Act of 1956, which created smoke control areas and forced a switch to smokeless fuels, removing the cause at source.
The Montreal Protocol bound signatory countries to phase out CFCs and other ozone depleting substances. Global CFC production fell sharply, and the ozone layer is now beginning to recover, making it the most successful international environmental agreement. Its limits show in evaluate answers: compliance is not universal and illegal CFC production continues, and the chosen replacements include potent greenhouse gases.
Frank Sherwood Rowland and Mario Molina proposed that CFCs rise to the stratosphere, break down under ultraviolet light, and release chlorine atoms that destroy ozone. The idea was initially rejected, and some of its supporting predictions lacked experimental evidence, but it prompted other scientists to collect data - including the Antarctic measurements that later confirmed CFCs are ozone depleting. They shared the 1995 Nobel Prize in Chemistry.
Los Angeles is the classic example of photochemical smog. Its sunny climate, large car-dependent population and surrounding mountains that trap polluted air combine to give the high temperatures and sunlight that drive the reaction of oxides of nitrogen and VOCs into ground level ozone. The result is the brownish haze, eye irritation and breathing problems the syllabus describes.
During the 1980s, coniferous forests in the Black Forest region of Germany showed widespread needle loss and tree damage attributed to acid deposition. This defoliation, driven by sulfur dioxide and oxides of nitrogen from industry and vehicles dissolving into sulfuric and nitric acids, is the real-world version of the defoliation and reduced growth the syllabus asks you to describe.
Flashcards
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Exam-style questions
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Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Exam technique for this topic
- Write the full term acid deposition and split it into wet and dry. Acid rain is only the wet form, and naming both types with their examples (snow, rain, hail, fog for wet; dust and gases for dry) is where the marks sit.
- Learn the two formation pathways as separate chains: sulfur compounds in fossil fuels become sulfur dioxide then sulfuric acid, while atmospheric nitrogen becomes nitrogen monoxide in hot engines then nitric acid. Name the starting pollutant and the final acid in each.
- For the Antarctica question, all three reasons must appear - very low temperature, the polar vortex, and polar stratospheric clouds - and you must link each to how it helps destroy ozone, not just list the words.
- In an evaluate question on the international agreements, give success and a limitation. The Montreal Protocol cut CFCs and the ozone layer is recovering, but compliance is uneven and the replacements are greenhouse gases; a one-sided answer cannot reach the top band.
- Match the control technology to the pollutant, because the examiner will expect the pair: sulfur dioxide with desulfurisation, oxides of nitrogen with catalytic converters, particulates with electrostatic precipitators, and VOCs with safe storage and disposal.
- For the importance of experimental evidence, use the Rowland-Molina sequence in order: the hypothesis was proposed, initially not accepted, some auxiliary predictions lacked evidence, further research was prompted, and that research confirmed CFCs deplete ozone.
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Where this fits in AS Level Environmental Management
This page is one topic of Cambridge AS Level Environmental Management 8291. AS Level Environmental Management is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the AS Level Environmental Management syllabus page.