Air pollution

By Matt Burdett, 31 December 2019

On this page, we look at air pollution patterns and its management.

  • Visible air pollution on the Buriganga River at Sadarghat in Dhaka, Bangladesh. Air pollution is created by hundreds of thousands of motor vehicles on land and water. The vast majority are extremely old and burn fuel inefficiently. Source: By the author.

Defining air pollution

Air pollution is particles in the atmosphere which are either harmful, or are not caused naturally. Urban areas often experience worse air pollution than rural areas because the emissions that cause air pollution are higher in urban areas, from vehicles, energy production, factory emissions and so on.

In general, air pollution is worse in cities in lower income countries. This is mainly because of the lack of effective government regulation. It is also because poorer communities cannot afford the newer technology that has reduced emissions from vehicles. In some cases, vehicles that no longer conform to the standards of higher income countries are sold in poorer countries. For example, in El Salvador the majority of public transport is old American school buses, which are too old, unsafe or polluting to be allowed on American roads.

Air pollution isn’t entirely from vehicles and factories. The burning of forests in Asia has led to a massive haze for several years. However, it is still air pollution because it is harmful to humans and the environment. The graphic below shows some other types of atmospheric pollution.

The size of pollution is very important because smaller particles are able to pass through the membranes of cells in the human body. In general, most natural pollutants are too large to pass into the bloodstream, and are expelled from the body in mucus. Artificial pollutants such as those found in vehicle exhausts are often small enough to pass into the body and go on to cause health problems.

  • The relative size of pollutants. Source: EPA, 2019.

Measuring air pollution

Air quality is measured using several metrics including the amount of specific gases in the atmosphere, most commonly sulphur dioxide and nitrous oxide. It’s also not unusual to see PM2.5 and PM10 particulates being used.

A common alternative is to use the World Air Quality Project’s maps which show the real-time air pollution. Below is a map showing the air pollution globally at 10pm GMT on New Year’s Eve 2019. It’s clear to see where the clock has already struck midnight as the festive fireworks have already added to the pollution!

  • Global air pollution at 10pm UTC New Year’s Eve 2019. Source: WAQI, 2019.

Air pollution patterns

In urban areas, air pollution patterns are almost always linked to transport networks because vehicles tend to be the biggest source of pollutants. The map below shows measurements in Oslo in 2011, which clearly follow transport routes.

Oslo, like many cities, has a relatively ‘calm’ period at its centre. The Central Business District is often a part of the city where pedestrian access is more efficient than vehicles, which reduces vehicle emissions.

It’s not always vehicles that cause the majority of the emissions, as shown by the diurnal (daily) rise and fall of pollutants. In most cities, the peak air pollution would be during the morning and afternoon rush hours. However, in Ulaanbaatar in Mongolia, many people heat their homes and workplaces using wood and fossil fuel burning stoves. The graph below shows the concentration of PM2.5 particles in Ulaanbaatar, with the peaks occurring at times when people are more likely to be at home rather than moving around.

Managing air pollution

Cities can do many things to reduce air pollution levels. A 2016 study (The Nature Conservancy, 2016) found air pollution was mainly managed through five broad approaches:

  • Tree planting
  • Transport
  • Point (limiting very specific sources of air pollution)
  • Nonpoint (limiting general sources of air pollution that are widely distributed, such as construction sites and individual households)
  • Specific gas reduction (NO2 and SO2 mainly)


Cities can do these things through specific strategies such as:

  • A ‘Green Zone’ in which polluting vehicles are not allowed to enter city centres, such as the one in London.
  • Limits on cars being allowed to drive. These include permission based on the number or letters of the registration plate on the car.
  • No car days.
  • Plant trees.
  • Minimum emission standards on vehicles.
  • Develop public transport.
  • Plant trees.
  • Reduce congestion (so less time idling).

The role of trees: dry deposition

‘Dry deposition’ occurs when trees slow down the wind speed. As a result, there is less wind energy to keep the pollutants suspended in the air, and they are deposited on the surfaces of leaves and branches (and by extension, other nearby surfaces where the wind speed has been slowed down by the trees). The particles are then absorbed into the leaves themselves, or are eventually washed off by precipitation. Although it is widely agreed that this process does occur, there is currently little consensus regarding its importance (The Nature Conservancy, 2016).


EPA, 2019. Particulate Matter (PM) Basics. Accessed 31 December 2019.

Guttikunda, 2013. Air Pollution in Ulaanbaatar, Mongolia – Emissions, Dispersion, and Health Impacts Modeling (Journal Article). Accessed 31 December 2019.

Mieszko the first, 2016. Airborne particulate size chart. Accessed 31 December 2019.

Schneider et al., 2017. Mapping urban air quality in near real-time using observations from low-cost sensors and model information. In Environment International Volume 106, September 2017, Pages 234-247. Accessed 31 December 2019.

The Nature Conservancy, 2016. Planting Healthy Air – A global analysis of the role of urban trees in addressing particulate matter pollution and extreme heat. Accessed 31 December 2019.

WAQI, 2019. World’s Air Pollution: Real-time Air Quality Index. Accessed 31 December 2019.

Air pollution: Learning activities


  1. Define “air pollution”. [2]
  2. Explain why the size of the air pollutant is important. [2]
  3. Identify the case of the most common pattern of air pollution in cities. [1]
  4. Describe how air pollution typically varies during the day in
    1. Cities in high income countries [2]
    2. Ulaanbaatar [2]
  5. Identify at least four strategies to reduce air pollution in urban areas. [4]
  6. Explain the role of trees in dry deposition. [4]

Other tasks

Visit and compare the current air pollution for at least four cities of your choice. Suggest reasons for the differences. In addition to the level of development of the cities, consider these other factors:

  • Time of day
  • Prevailing weather conditions (remember that rainfall will remove pollutants from the air as they are absorbed into water droplets, while anticyclonic conditions will trap pollutants near the ground level)
  • Geographical factors. For example, Bogota in Colombia is bordered on one side by mountains, trapping pollutants.
  • The location of each individual measuring station (e.g. is it by a main road, or in a park?)
  • The source of each measurement (click on individual measurements to find the source in the pop-up box)
  • Variations within the results from different stations – find the US embassy and you’ll often find a higher level of reported pollution as they have more sensitive equipment!

© Matthew Burdett, 2019. All rights reserved.

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