8.3: Urban Air Pollution
Urban air pollution is one of the world's largest health and environmental problems. In this section of the course, we delve into understanding how this sort of pollution is formed, what effects it can have on both ecosystems and societies and what steps can be taken to reduce it.
Primary and Secondary Air Pollutants
Air pollutants can be categorised into two groups based on their formation: primary and secondary.
Primary pollutants are directly emitted into the atmosphere from identifiable sources. These sources can be natural, such as volcanic eruptions, wildfires (both natural and anthropogenic - caused by human activity), and windblown dust from natural sources or construction activities. Anthropogenic sources of primary pollutants are significant and include emissions from vehicles, industrial processes, and fuel combustion for heating and power generation.
Secondary pollutants are not directly emitted but form in the atmosphere through chemical reactions between primary pollutants. Sunlight often plays a crucial role in these reactions. Examples include tropospheric ozone, which forms from the interaction of nitrogen oxides and volatile organic compounds (VOCs) in the presence of sunlight, and acid rain, which results from the reaction of sulphur dioxide and nitrogen oxides with water vapour in the atmosphere.
| Type of air pollution | Primary or Secondary pollutant | Sources | impacts | Examples |
|---|---|---|---|---|
| Nitrogen oxides (NOx) | Primary | combustion of fossil fuel | primary pollutant for acid precipitation (rain) | Beijing, China; Los Angeles, USA |
| Sulphur dioxide | Primary | combustion of fossil fuel | primary pollutant for acid precipitation (rain - secondary); Respiratory problems, acidification of lakes and rivers, damage to vegetation | Industrial regions of China and India |
| Carbon monoxide | Primary | combustion of fossil fuel | Dizziness, headaches, nausea, impaired coordination | Major cities globally |
| Particulate matter (PM 2.5 and PM 10) | Primary | combustion of fossil fuel | Respiratory problems, heart disease, lung cancer; Reduced visibility, impact on ecosystems | Beijing, China; Los Angeles, USA |
| Tropospheric ozone | Secondary | Reaction of NOx & VOCs in sunlight | Respiratory problems, cardiovascular issues; Damage to vegetation, reduced crop yields | Mexico City, Mexico; California, USA |
Application of skills: Plan an experiment to use an indicator species as a correlate for pollution in the local
environment.
Increasing public transport. There is a big push across all major cities to reduce air pollution and so this means having increased accessibility to public transport. Here's a great video to outline some of the ways cities around the world are doing this. I particularly like Bangkok's push towards electric Tuk Tuk's!
Improved Infrastructure for Cycling
Copenhagen is one of the best cities in the world for infrastructure for cycling. Their newest bridge, " Lille Langebro" (pictured on left) helps cyclists cross from one section of the city to another and is used by over 10,000 people per day. In fact 62% of Copenhagen residents use bikes to commute.
Growing Trees
It has been proven that there is up to 24% reduction in particulate matter near a mature tree[1]. In this diagram we can see the many benefits of increasing the number of trees can have in urban environments
The Aarhus municipality in Denmark[2] invests about €600,000 per year on afforestation projects on former agricultural land to provide much needed recreational areas for a growing population in the second largest city in Denmark. One project, the Hasselager Wild Forest creates diverse "forest landscapes" in suburbs. This goes beyond tree lines, incorporating meadows, streams, and open spaces. This fosters a richer ecosystem for wildlife and improves air quality. Public pathways encourage nature interaction.Funding comes from a "pay-per-tree" model, where companies like Holmris B8 contribute to sustainability by offsetting their footprint. This cost-effective approach covers initial planting and maintenance.
Green Screens
Similarly, the use of green screens will protect an area by helping to absorb polluting gases, such as nitrogen dioxide, and particulate matter released by transport on nearby roads. You can build your own green screen by following the instructions on this page by the Natural History Museum. This may also generate a few IA ideas if you can measure the impact.
Compulsory catalytic converters
In a car's exhaust system, the catalytic converter uses hot exhaust gases and precious metals like platinum and palladium as catalysts. These metals accelerate chemical reactions within the converter, transforming harmful pollutants like nitrogen oxides and hydrocarbons into less polluting byproducts, reducing air pollution.
Catalytic converters have been mandatory on all new gasoline cars sold in the European Union and the United Kingdom since January 1, 1993 in order to comply with the Euro 1 emission standards. Likewise but much earlier in the USA, the clean air act (1970) required a 90% reduction in emissions from new cars by 1975, with no exceptions. These air-quality standards strictly limited levels of six pollutants: sulfur dioxide, nitrogen dioxide, particulate matter, carbon monoxide, ozone, and lead. To reach these regulations automakers had to design vehicles that could run on unleaded gasoline and incorporate a new device—the catalytic converter—to reduce carbon monoxide and hydrocarbons from car exhaust by 90%.
Limited car use
London implemented a congestion charge zone in central areas, leading to a significant drop in traffic and a 10% reduction in nitrogen dioxide levels within the zone.
Pedestrianised town centres
Completed in 2017, the project carved out 2.5 acres of pedestrian-only space at Manhattan’s core zone, transforming a notoriously congested intersection into a world-class pedestrian space.
What Does Acid Mean?
The pH scale is a non-linear scale (negative logarithmic scale) of the concentration of hydrogen ions in a solution. For each decreasing point on the pH scale there are ten times more hydrogen ions in the solution. The pH scale runs from 0 – 14 with 0 being the strongest acid, 7 being neutral and 14 being most basic (alkaline).
Rain water is normally a weak acid as the carbon dioxide in the atmosphere dissolves in the rain creating a weak solution of carbonic acid. This is not considered to be acid rain even with increasing amounts of carbon dioxide in the atmosphere although there is increasing evidence of the dangers of ocean acidification. Nitrogen in the atmosphere becomes oxidised by the action of lightning and can then dissolve in rain to form nitric acid. This is an important source of nitrogen for plants. Certain pollutants can dissolve in this rain water causing pH readings below pH 2.4[3]
The main primary pollutants leading to acid rain are sulphur dioxide (SO2) and nitrous oxides (NO, NO2, NOx). The origin of these is the burning of fossil fuels in transportation, industry and electricity generating plants. Any fossil fuel will contain both sulphur and nitrogen due to its origins as living organic matter. Nitrogen in the atmosphere is also oxidised by the high temperatures of combustion.
Ammonia from farming and the nitrates in inorganic fertiliser are also an important and possibly underestimated source of acid rain and contributors to global warming.
The primary air pollutants of SO2 and NOx
both react with water in the atmosphere forming secondary pollutants.
Sulphur dioxide can react with oxygen to form sulphur trioxide. Both sulphur dioxide and sulphur trioxide (SO3) react with water to form sulphurous acid (H2SO3) and sulphuric acid.
SO2 + H2O → H2SO4
Nitrogen oxides react with water to form nitric acid.
NOx + H2O → HNO3
These secondary pollutants can be deposited as dry particles relatively close to their source (dry deposition) but when rain is available they are very soluble and can fall as wet deposition.
Once in the atmosphere, the pollutants can travel relatively long distances, across country boundaries, although they remain a regional rather than global problem. This pollution is moved by wind patterns and so the prevailing wind direction is important. For example, pollution formed in the United Kingdom was blown to Scandinavia and caused great problems there.
What are the Effects of Acid Deposition?
1) Soil Ecology
The effect that acidic rain has on soil depends upon the base rock (geology). If the base rock forming the soil is calcium carbonate, e.g. limestone, chalk and serpentine, then these soils tend to be alkaline and will buffer the acidic rain (neutralise the hydrogen ions). If the soils are derived from acidic rocks, e.g. granite and rhyolite, then there will be few available ions to neutralise the acidic rain and therefore acidic soil is very sensitive to acidic rain.
Acid rain reduces a soils ability to hold onto nutrients such as calcium, magnesium and potassium ions. These are leached out of the soil reducing the productivity of the soil. This is combined with the reduced effectiveness of symbiotic bacteria which further decreases the availability mineral ions for plants. This is classified as an INDIRECT NUTRIENT EFFECT.
Acidic rain increases the solubility of heavy metals and toxic aluminium ions, leaching them out of soil. These enter water courses and have an effect on aquatic organisms. This is classified as an INDIRECT TOXIC EFFECT.
2) Forest Ecology
Acidic rain has caused dieback of trees in forests around the world including the Black Forest in southern Germany, forests across Sweden, central Europe, north-east USA, and north-east China. Both coniferous and deciduous can be affected. Leaves and buds can show yellowing (loss of chlorophyll) and damage in the form of lesions, thinning of wax cuticles and needle dropping in coniferous trees. This results in reduced growth and productivity. Trees become increasingly vulnerable to pathogens and low temperatures. This is classified as a DIRECT EFFECT.
3) Fresh Water Ecology
Some aquatic organisms are very vulnerable to acidification of fresh water systems. Trout and Bass are sensitive to acidic water while shellfish are very sensitive and frogs are much less sensitive. Most fish eggs won’t develop below pH 5. Some acidic lakes contain no fish. This is classified as a DIRECT EFFECT.
At low concentrations, the aluminium that is leached into water systems affects the ability of fish to regulate the amount of water and salts in their bodies. This can affect the intake of oxygen and salt; the fish slowly suffocating. It also causes chronic stress, reducing body weight and the ability to compete for food and habitat. At higher concentrations a solid is formed on the fish’s gills leading to suffocation[4]. This is classified as an INDIRECT TOXIC EFFECT.
4) Lichen Ecology
Lichens are indicator species for air pollution, particularly sulphur dioxide. Very few species of lichen can tolerate heavy air pollution while some species are only found in the cleanest air. Lichen indicator species can be used to estimate the amount of air pollution and maps have been produced of air quality based on the distribution of lichens.
5) Effect on Buildings
Acid rain damages limestone buildings and statues causing their degradation; the acid reacts with the calcium carbonate releasing carbon dioxide. This can cause economic impacts as the cost of repair can be large.
Impacts of Acid Deposition Cross Country Boundaries
Acid deposition is considered to be a regional problem but solutions are sometimes sought globally as the problems cross international boundaries.
It is not only industrialised areas which suffer from acid rain. The primary pollutants contributing to acid rain are produced in these industrialised areas but then winds blow these particles, sometimes thousands of kilometres away from the source. This leads to regional problems but not to global problems due to the distance travelled and the pollutants eventually being washed out of the air by precipitation.
In Europe Sweden has suffered from acidic rain originating in the the north of England and Germany. The soils in Sweden are particularly vulnerable as they are largely of acidic nature. Globally, North East USA, Europe and Eastern China are particularly hit by acidic rain but there are many areas with acidic soils which could become vulnerable if emissions from fossil fuel combustion increase in these regions.
Here is a case study published by the United Nations on Acid Rain in Canada and the solutions implemented in the US and Canada to resolve the problems.
This Sciencedaily article compares sulphur dioxide emissions and coal use leading to acid rain in China and India with solutions.
This paper published in BioScience gives a good review of Acid Deposition in the North Eastern US.
We can employ various strategies to minimise the damage caused by sulfur dioxide (SO2) and nitrogen oxides (NOx). These approaches mirror those used for other pollutants:
Alterin Human Activity: Transitioning to cleaner energy sources like solar or wind power reduces emissions from the outset.
Controlling at the point of release: Technologies like scrubbers in power plants or catalytic converters in vehicles capture these pollutants before they enter the atmosphere.
Ecosystem Restoration: For environments already impacted, efforts may involve adding limestone or fertiliser to lakes (for acid rain effects) or undertaking healthcare initiatives to support damaged ecosystems.
This table summarises each type of solution
| Strategy | Examples | Evaluation | EVS |
|---|---|---|---|
| Altering Human Activity | Replace car transport with low emission transport such as bicycles, electric vehicles, alternative energy driven transportation. Car pooling Improving public transportation networks | Need to encourage bicycle use through sharing stations – requires public investment (low cost) and needs to be within easy reach. Need charging stations – public investment in infrastructure – simply moves the source of pollution elsewhere unless electricity produced by renewable sources Reduces cars on road but requires people to change habits and organize time with others. Expensive but very effective. Some resistance may come from political beliefs against a social way of living. | Ecocentric / Technocentric Ecocentric Ecocentric / Technocentric. |
| Controlling release of Pollutant | Reduce the sulfur content of fossil fuels Use catalytic converters on car exhausts Removing sulphur from emissions using scrubbers which are fitted to chimneys of power generation plants. These spray limestone powder into the chimney which reduces the sulphur content of the emissions. Water then washes this out of the chimney and the product is collected. The calcium carbonate of the limestone produces pH neutral calcium sulphate that is physically removed from the scrubber. Switch to use of renewable energy sources. | Increases cost as low sulfur fuel more expensive and requires technological investment. Can be anthropocentric if government requires the sulphur content to be reduced. This has been the most successful way of reducing acid deposition in the US. Very effective at reducing NOx but costs more and uses heavy metals which need to be mined. Scrubbers / CATS expensive to install and maintain. Catalysts need replacing frequently. Effective but requires investment in the technology though not particularly high-tech. Desulphurisation of coal is costly. Requires diversification of energy supply in order to ensure constant supply. Requires massive investment in infrastructure and political will to do this. Many alternative sources have disadvantages e.g. aesthetic aspects of solar/wind generation. Problems of disposing of nuclear waste. Many governments dependent on tax revenue from extraction/use of fuels; especially in LEDCs with rapidly rising populations/aspirations. Many alternatives are costly and require advanced technology. Production of solar panels may involve some pollution – e.g. heavy metals needed. | Technocentric / Anthropocentric Technocentric Technocentric / Anthropocentric as usually mandated by government echnocentric when thinking about the changes in technology and future carbon zero economy. Anthropocentric when governments require the switch. Ecocentric when individuals make the choice to change their source of energy, e.g. installing solar panels on the roof. |
| Clean-Up and Restoration of Damaged Systems | Adding limestone powder to acid lakes neutralises the acid present and increases the pH of the water. Restocking of lakes after remediation Healthcare | Effective and cheap solution to restoring fresh water ecosystems but does not remove the cause of the problem and therefore needs to be repeated. There is an environmental impact from the quarrying of the limestone. Needs careful management and unlikely to restore the same ecosystem that existed prior to the pollution event due to the nature of complex systems and alternate stable state theory. Respiratory diseases like asthma or chronic bronchitis are linked to acid rain and can be treated with medicine. | Technocentric Technocentric |
Formation of Photochemical Smog
When fossil fuels are burned, and in this context we are largely talking about vehicles, primary pollutants are produced. This include:
carbon dioxide
black carbon - soot
unburned hydrocarbons (chemicals with a C-H structure)
oxides of nitrogen (usually represented as NOx)
oxides of sulfur (mostly sulfur dioxide SO2)
The high energy ultraviolet radiation in sunlight splits nitrogen dioxide to release an atom of highly reactive oxygen. This oxygen atom (O) reacts with molecular oxygen (O2) to form ozone (O3). As this is produced by a series of chemical reactions, the tropospheric ozone is classed as a secondary pollutant.
Although ozone is the most important secondary pollutant for our ESS course, there are a variety of nasty secondary pollutants produced from the reaction of primary pollutants in sunlight in the air. They all contribute to the dangerous effects of urban air pollution. for instance peroxyacyl nitrates (PANs) are powerful respiratory and eye irritants At higher concentrations they cause extensive damage to vegetation. PANs are said to be mutagenic, as they can be a factor causing skin cancer.
This mix can contain about one hundred different chemicals but ozone is the main pollutant and nitrous oxides form the brown haze typical of this type of smog. The primary pollutants are at their highest concentrations in the morning and evening rush-hours, but the photochemical smog is at its maximum in the early afternoon when the sunlight has been strongest.
Thermal Inversions Trap Pollution
Thermal inversions are when a layer of air sits on top of an area preventing the slightly cooler air below from rising and dispersing. In some areas this is typical of fine winter days when the amount of radiation from the earth may exceed that from the winter sun but it can occur at other times of the year too. It is also caused when warm moist heavy air, associated with a warm front moves across an area and is sometimes associated with oceanic upwellings such as the one off the Californian coast.
Location can Increase Air Pollution Problems
The accumulation of the smog is greatest when there is strong sun so cities suffer most on hot clear days. Low winds will prevent the pollution from being dispersed and rain washes the pollution out of the air.
The topography of a region will affect the amount of air movement and so some cities are particularly prone to photochemical smog as the air tends to be trapped over the city and not dispersed. Los Angeles has a Mediterranean climate, lies on the Californian coast where an upwelling occurs and to the east is surrounded by mountains. Los Angeles also has a very poor public transport infrastructure and a high use of cars, creating the precursors for photochemical smog accumulation.
Other cities to suffer from photochemical smog regularly include Santiago, Chile, Mexico City, Rio de Janeiro, São Paulo, and Beijing.
Application of skills: Use graphs showing diurnal changes in urban air pollutants. Use secondary databases
to study change over time in local air quality, using a statistical tool to test the significance of any change.
Ozone in the lower troposphere is toxic and an oxidising agent. "High concentrations of ozone cause plants to close their stomata. This slows down photosynthesis and plant growth", thus reducing primary productivity.
"Ozone may also enter the plants through the stomata and directly damage internal cells."[5]
In animals, ozone irritates eyes, can cause breathing difficulties in humans and may increase susceptibility to infection. Environmental monitoring bodies, such as the EPA in the US, monitor levels of ozone in the lower atmosphere and give alerts if ozone levels are above recommended safe levels. Children, elderly people, and those suffering from asthma are particularly prone to high levels of ozone pollution.
As ozone is highly reactive it can also attack fabrics and rubber materials. making them become brittle and crack. In fact ozone is actually used in the fashion industry to deliberately degrade textiles.
Ozone is also a greenhouse gas in the upper troposphere and is included in the Kyoto protocol and in the IPCC's reports.
Economic Effects of Urban Air Pollution
China is reported to be suffering the economic consequences of terrible urban air pollution. It is also suggested to be driving many of the "green" policies that China is implementing. An unhappy population could produce political instability and so China's government is taking massive steps to combat the air pollution.
Here is an interesting article and video about the economic impacts of urban air pollution in China.
Disproportionate effects on the Poor
A Double Burden: Poverty and Air Pollution
A significant portion of the global population breathes unsafe air. This problem is particularly harsh for those living in extreme poverty, with an estimated 716 million – roughly 10% of those exposed to unhealthy air – struggling to survive on less than $1.90 a day. This injustice is concentrated in Sub-Saharan Africa, where over half (405 million) of these poorest individuals face this double burden.
Furthermore, exposure to air pollution carries a heavier weight for the poor. While everyone suffers health consequences from polluted air, the lack of resources and limited access to healthcare in impoverished communities significantly worsen the impact. Essentially, the same level of air pollution translates to a greater risk of severe health problems for those already struggling financially[6].
- Participate in citizen science air-quality projects by installing a networked weather station in the school.
- Advocate for improved walking and cycling options for the school.
- consider fundraising to build a green screen in an appropriate location.
Footnotes
- ^ https://greenblue.com/gb/urban-tree-planting-modal-shift/
- ^ https://invest4nature.eu/2023/06/02/afforestation-projects-and-financing-mechanisms-within-the-aarhus-living-lab/
- ^ http://www.epa.gov/acidrain/effects/surface_water.html
- ^ undefined
- ^ http://www.ucar.edu/learn/1_7_1.htm
- ^ https://blogs.worldbank.org/en/developmenttalk/air-pollution-kills-evidence-global-analysis-exposure-and-poverty#:~:text=Further%2C%20275%20million%20people%20living,their%20livelihoods%20and%20well%2Dbeing.