The big picture

Have you ever experienced being in an urban environment where you can taste the air or where the air is so polluted it restricts visibility, makes you cough or causes your eyes to sting? This is a common problem in many densely populated areas around the world.

Figure 1. People wearing face mask to provide some protection against air pollutants in Nizhny, Novgorod.

The increasing use of fossil fuels has lead to an increase in urban air pollution and incidents of photochemical smog. Contributing factors to this include:

• Industrial growth, the use of fossil fuels rose dramatically during the industrial revolution and continues to rise.

• Population growth, more people means that overall more energy is required to produce greater resources e.g. food, water and clothes. 

• Increase in affluence which results in more energy use e.g. through greater use of technologies that need energy such as cars and heating systems.

In this subtopic we will be looking at:

1. The main urban pollutants that contribute to air pollution and the production of photochemical smog.

2. Ozone the main component of photochemical smog.

Air pollution

Air pollution is linked to health problems which can result in premature death (death that occurs at an earlier age than expected and is often preventable). With reference to Figure 2, suggest reasons why there are more deaths from air pollution projected for BRIICS countries (Brazil, Russia, India, Indonesia, China and South Africa) and the rest of the world rather than for OECD (Organisation for Economic Cooperation) countries?

Figure 2. Premature deaths due to particulate matter (smoke and soot) and ozone pollution (main component of photochemical smog).

To reduce air pollution, nations often set emission standards and monitor levels of pollution in the atmosphere. Below in Table 1 are the World Health Organisation (WHO), EU and USA standards for some of the main air pollutants. The air quality standards here are measured in micrograms per cubic meter either per 24 hour mean or annual mean. The 24 hour mean is usually calculated from the hourly reading over a 24 hour period, whereas the annual mean is calculated from the hourly average concentration over a year. Consider why air pollution standards are sometimes not as stringent as recommended by the WHO?

International Mindedness

The WHO provide guidelines for air pollution standards. However, air pollution standards vary from one country to another and are often not as stringent as recommended by the WHO.

Urban air pollution

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Figure 1. A main source of air pollution is vehicle exhaust fumes.

The combustion of fossil fuels can lead to emissions containing carbon dioxide, water vapour, sulphur dioxide, nitrogen oxides and particulates such as smoke and soot. If combustion is incomplete there may also be emissions of carbon monoxide and hydrocarbons. These air pollutants which are directly emitted from their source are referred to as primary pollutants. Whereas, pollutants formed when primary pollutants react in the atmosphere are referred to as secondary pollutants. These include:

• Acid rain, formed when nitrogen oxide and sulphur dioxide react with water.

• Ozone, formed when nitrogen dioxide and hydrocarbons react in the presence of sunlight energy.

Nitrogen dioxide can be either a primary pollutant or secondary pollutant:

• If formed during combustion of fossil fuel and emitted into the atmosphere, it is a primary pollutant.

• If formed from nitrogen oxide in the air reacting with oxygen, it is a secondary pollutant.

Figure 2. Primary and secondary air pollutants.

Impacts of primary pollutants

The impacts of the main primary pollutants emitted into the atmosphere from the combustion of fossil fuels are discussed below.

Carbon dioxide and water vapour

Both carbon dioxide and water vapour are greenhouse gases resulting in an increase in the average global temperature and contributing to climate change. 

Sulphur dioxide

Coal and oil contain sulphur, hence when these fossil fuels are combusted they produce sulphur dioxide gas. Sulphur dioxide is toxic and can act as:

• A potent respiratory irritant which causes inflammation of the lungs. It can trigger asthma attacks, chronic bronchitis and also increase the risk of lung infection.

• An eye irritant.

• A principal component of acid rain.

Sulphur dioxide is also linked to an increase risk of cardiac disease and death.

Figure 3. Sulphur dioxide levels in the USA are concentrated around urban areas.
Source: Environmental Protection Agency

Nitrogen oxides (NOx)

Nitrogen oxides comprises of nitrogen oxide and nitrogen dioxide. They are formed during combustion of fossil fuels. Levels tend to be high where there is a large concentration of motor vehicles being used. 

Nitrogen dioxide is a yellow brown gas that can reduce visibility and appears as a haze over urban areas. Nitrogen oxides are:

• Respiratory irritants causing lung inflammation and triggering asthma.

• A principal component of acid rain.

• A precursor of photochemical smog and the formation of ozone.

Figure 4. Global nitrogen dioxide levels.

Can you make any deductions from the information in Figure 4 and your knowledge of Environmental Systems and Societies? In order to address this question, you could ask yourself a series of other questions, for example:

• Where are the highest levels of nitrogen dioxide? Using your wider knowledge of ESS can you think of possible reasons for this?

• Where are the areas with the lowest level of nitrogen dioxide? What are the possible reasons for this?

Exam tip

Ensure you can apply your wider ESS knowledge and understanding to different scenarios. Your should be able to interpret information presented in many different ways e.g maps and graphs.

Particulates

Particulates are produced during combustion of fossil fuel and emitted into the atmosphere. Suspended particulates such as smoke and soot are often categorised according to size. Of particular concern are:

• PM₁₀ - particulate matter with a diameter of 10µm or less. Exposure to PM₁₀ has also been associated with an increased risk of cardiovascular disease.

• PM_2.5 - particulate matter with a diameter of 2.5µm or less. PM_2.5 which can travel deep into the lungs, increasing the risk of respiratory diseases and cancer.

The World Health Organisation (WHO) estimated that in 2012 outdoor air pollution resulted in 3.7 million premature deaths.

 (Based on data from WHO)

Photochemical smog

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Photochemical smog is a major problem around the world. In this section, we look at the formation of photochemical smog and the impacts of tropospheric ozone.

Formation of photochemical smog 

Photochemical smog occurs when sunlight activates reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) resulting in the formation of ozone and peroxyacyl nitrates (PAN). VOCs are carbon based compounds with a low boiling point such as propane, butane and formaldehydes.

The main sources of NOx are emissions from industry, power stations and vehicles.

The main sources of volatile organic compounds (VOCs) include industry, vehicles and solvents (e.g. used in paints and adhesives).

Forest fires, whether accidental or intentional (e.g. slash and burn), also increase levels of VOCs and particulate matter (e.g. PM10). In addition forest fires leads to deforestation and loss of habitats.

Figure 1. Main sources of NOX and VOC.

The formation of photochemical smog involves a series of complex reactions. The mixture of over 100 different chemicals formed is dominated by ozone but also include other oxidants such as peroxyacyl nitrates (PAN) and aldehydes. 

Figure 2. Photochemical smog.

Factors influencing production of photochemical smog

Photochemical smog is mostly likely to be formed under the following conditions:

• High emissions of pollutants from combustion of fossil fuels e.g. through industrial activity and vehicle use. This is often influenced by:

  • The amount of industry.

  • Population size.

  • Mode of transport adopted in an area. Use of private cars will produce more air pollution than the use of public transport, which in turn produces more pollution than cycling or walking.

• High levels of sunlight, hence the highest levels of ozone occur during the sunniest part of the day.

• Calm or light winds which reduces dispersion and dilution and allows pollutants to accumulate at ground level.

• Dry weather conditions in which rain does not wash the pollutants out of the air.

• Where the topography allows pollutants to accumulated such as a valley surrounded by hills. The hills reduce the flow of air and allow the pollutants to concentrate within the valley. Tall buildings can also reduce air flow and allows pollution levels to increase.

• When a thermal inversion occurs:

  • Normally air at ground level warms and rises which also dissipates the air pollutants. This air expands and cools resulting in a temperature gradient.

    

    Figure 3. Normal conditions vs thermal inversion.

• Under a thermal inversion, cold air is trapped below a warm layer of air. This can occur during cold winter nights when the earth surface cools and chills the layer of air next to it. This temperature inversion is broken when the sun comes out in the morning and warms up the air. Therefore, often temperature inversions last only a few hours unless meteorological factors (high pressure) result in clear, windless conditions that allow pollutants to accumulate at ground level. Although thermal inversions are usually shallow, deep inversions can occur in a valley and be slow to dissipate. 

Lots of large cities around the world experience photochemical smog. However, it tends to be more common where the city is located in a valley, the weather is sunny, dry with little wind movement and where there are high emissions from cars or industry such as in Los Angeles, Rio de Janeiro and Mexico City.

Important

Ensure you understand that formation of smog is affected by climate, topography, population density and amount of industry and vehicles.

Impact of tropospheric ozone

Ozone formed at ground level:

• Is a highly reactive gas.

• Causes inflammation of the lungs causing coughing, wheezing and contributing to asthma.

• Reduces lung function, contributing to lung disease and premature death.

• Irritates the eyes and nose.

• Damages cells in the leaves disrupting photosynthesis and reducing plant growth which affects crops and forest. In crops such as wheat, soya beans, tomatoes and cotton, smog has also been found to increase risk of infection.

• Damages fabrics such as rubber and plastics.

Figure 4. Ozone damage.
Attribution: David B. Langston, University of Georgia, Bugwood.org

The impacts of ozone on health, reduced crop production and material degradation also contribute to a significant economic loss.

Air pollution can be blown downwind from its source. This can result in movement of pollution from urban to rural areas. With this in mind, air pollution can impact wide areas.

Exam tip

Ensure you do not confuse the effects of ozone in the tropopshere with ozone in the stratosphere.

Urban Air Pollution Management I

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Policies on air pollution are used to define the specific course of action being taken to reduce primary pollutants that contribute to air pollution and the formation of photochemical smog.

Policies to reduce air pollution

Policies can occur at international level, right down to local level. For example, the European Union (EU) has adopted policies on air pollution that involve action at national governmental level down though to local regional governmental level to be effective. Pollution reduction policies can employ the following approaches:

• Altering human activity that produces air pollution. This can be achieved by changing human behaviour and may:

  • Involve education and campaigns to inform the public of the effects of air pollution and ways in which they can reduce emissions.

  • Be supported by economic instruments.

  • Involve the use of alternative technologies.

• Legislation to prevent or regulate the release of air pollutants.

• Clean up and restoration of damaged systems.

Figure 1. Labels used on appliances help to inform the public about the energy efficiency levels - here 'A' indicates high energy efficiency.

International-mindedness

There are many different factors that influence the approach a country takes to cut urban air pollution. Therefore, approaches to a common problem can vary significantly between nations.

Changing human behaviour

Changing human behaviour and practices can be very difficult. Educational campaigns are often used to inform the public about:

• The damage caused by air pollution.

• How they can take appropriate action to reduce emissions.

This can involve adopting practices at an individual level that reduce energy use, such as:

• Adopting use of more energy efficient devices such as refrigerators and washing machines.

• Turning appliances off when not in use, rather than standby mode.

• Making home modifications to reduce loss of heat to the outside through windows, doors, roof and flooring, such as efficient windows (e.g. triple layered windows), wall cavity insulation and ceiling and floor insulation.

Figure 2. Typical areas of heat loss from a house.

• Decrease consumption of non-essential goods that utilise energy during production e.g. following the latest fashion can encourage over-consumption and waste.

• Reduce consumption of non-local goods that need to be transported long distances (i.e. with high road and air miles).

• Decrease individual car use, instead walk, cycle, use public transport or share vehicles (i.e. share transit or car pool).

• Adopt use of hybrid electric cars (which use both petrol and electricity) or electric car (only powered by electricity). This is particularly effective at reducing air pollution when renewable sources of energy are used to generate the electricity. Alternatively if fossil fuels are used to generate electricity, emission control technologies should be employed to reduce the levels of pollutants emitted. 

Figure 3. Charging station for electric and hybrid cars.

In addition to reducing the amount of energy consumed, renewable energy sources such as solar and wind can be used to generate electricity at a domestic level.

Economic instruments

A variety of different economic tools can be used that include:

• Charging pollution emitters to encourage reduction in emission levels e.g. pollution tax, in which tax is charged according to the amount of pollution produced.

• Use of subsidies to encourage new technologies and support research into more efficient methods that reduce pollution from combustion of fossil fuels.

• Subsidies or tax credits to promote renewable sources of energy.

• Subsidising public transport.

• Use of road tolls and parking charges to discourage use of cars.

Figure 4. Road toll payment stations.

Technological changes

A wide variety of different technologies can be used to reduce pollution production.

Catalytic converters

Catalytic converters could be used on all motor vehicles. Catalytic converters reduce the amount of potential pollutants emitted by:          

• Reducing NOx to form nitrogen gas and oxygen gas (e.g. 2NO = N₂ + O₂).

• Oxidising carbon monoxide to form carbon dioxide.

• Oxidising VOCs to carbon dioxide and water.

Figure 5. Catalytic converter used to reduce pollution emissions from vehicles.

Increase energy efficiency

Increase efficiency of processes that utilise energy, such as more energy efficient industrial processes, more efficient cars and home appliances.

Alternative energy sources

There are a range of technologies available that harness renewable energy sources (e.g. solar panels and wind turbine) that could reduce air emissions. 

Continuing research

Research is on-coming to:

• Improve energy efficiency of processes and products.

• Further improve the efficiency of renewable source of energy.

• Further develop low emission vehicles.

• Develop low emission fuels.

Technological changes often require time and financial investment to be developed. It may also require economic support to be competitive in the market place.

Theory of Knowledge

Urban air pollution is damaging to human health and the environment - is it a moral responsibility for individuals to take action to reduce emissions?

Urban Air Pollution Management II

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In the previous section we considered how human activity can be altered to reduce the production of urban air pollutants. Here we consider:

• Use of legislation to control release of pollutants.

• Clean up and restoration.

• Industrial smog.

Legislation

A variety of legislation can be used to reduce atmospheric pollution. However, for legislation to be effective it requires appropriate enforcement and policing.

International agreements

International agreement set goals that are adopted into national policies (e.g. 1999 Gothenburg Protocol to abate acidification, eutrophication and ground level ozone sets minimum emission standards for sulphur dioxide, nitrogen oxides, volatile organic compounds and ammonia).

Legislation setting emission standards

Legislation can be used to set more stringent emission standards for industry, power generation and vehicles. This can encourage use of low sulphur fuel, such as gas or adoption of renewable energy sources such as solar or wind in order to meet the more stringent standards.

Building regulations

Building regulations can set stringent standards that require improvements in the energy efficiency of new and existing buildings. For example, improved insulation of house to reduce loss of heat and adoption of more efficient appliances such as boilers.

Planning regulations

Planning regulations can be used to locate industry and power stations outside of urban areas and in places where conditions are least likely to allow air pollutants to accumulate (for example, outside a valley). Planning regulations can also promote alternatives to private car use, such as:

• Incorporation of cycle paths.

• Road lanes restricted to public transport or those adopting vehicle-sharing (such as shared transit or car-pooling).

• Adoption of vehicle free zones and park and ride schemes.

Figure 1. Cycle lane next to cycle rental stop.

Clean-up measures and restoration

Devices called "scrubbers" can be used by industry and power stations to filter emissions prior to discharge to remove primary pollutants such as particulate matter and sulphur dioxide. However, once the photochemical smog is formed, ozone and PAN are highly reactive and are removed by reactions with other chemicals, hence are not long lasting. In polluted areas, levels of ozone tend to fall overnight as it reacts with the nitrogen oxide to form oxygen and nitrogen dioxide.

Restoration may involve reforestation and replanting of areas affected by the smog.

Theory of Knowledge

Is state control of the number of cars available for purchase justified by the benefits it contributes to the environment and society?

Industrial smog

Prior to large car ownership and when coal was heavily used, industrial smog was more common. Industrial smog differs from photochemical smog. It is caused by smoke and sulphur dioxide emissions mixing with fog. Historically, many industrial cities suffered from industrial smog.

In December 1952, London experienced industrial smog over a four day period which contributed to around 4,000 deaths. There was also a significant increase in the number of deaths from bronchitis and other respiratory illnesses. Perhaps surprisingly, there was also a rise in deaths from drowning. The lack of visibility caused by the smog resulted in some people accidentally falling into the River Thames.

Figure 2. As a result of poor visibility during the day guides with torches were used to lead buses and ambulances.

This smog incident led to UK legislation focused on cutting air pollution emissions by reducing sulphur dioxide and smoke emissions. It allowed regional governments to designate clean air areas where smokeless fuel replaced traditional coal used in homes. The levels of both smoke and sulphur dioxide emissions have fallen since The Clean Air Act of 1956.

With increase in vehicle use and nitrogen oxide emissions, London is now more likely to experience photochemical smog.

The following video provides an overview of this event: