Causes of Urban Air Pollution (DP IB Environmental Systems & Societies (ESS)): Revision Note

Causes of Urban Air Pollution

What is urban air pollution?

• Human activities that release harmful substances into the atmosphere cause urban air pollution

  • Pollutants in the air can come from many sources and impact both human health and the environment

• Common pollutants include:

  • Nitrogen oxides (NOx)

  • Sulphur dioxide (SO₂)

  • Carbon monoxide (CO)

  • Particulate matter (PM)

    • Particulate matter refers to tiny solid particles or liquid droplets in the air

    • These particles can come from dust, soot, smoke, and vehicle emissions

• Particulate matter can be classified by size:

  • PM2.5: fine particles with a diameter of 2.5 micrometres or smaller

  • PM10: larger particles with a diameter of 10 micrometres or smaller

Primary pollutants

• Primary pollutants are harmful substances that are:

  • Directly emitted from a source

  • Immediately active in the atmosphere

• They enter the air through various activities like burning fossil fuels, industrial processes, or natural events such as volcanic eruptions

Sources of primary pollutants

• Natural sources:

  • Some air pollutants come from natural events that occur without human involvement

    • Forest fires: release smoke, ash, and particulate matter into the air

    • Dust storms: strong winds lift dust from dry areas, which spreads to cities

    • Volcanic eruptions: these produce large amounts of SO₂ and ash

• Anthropogenic (human-made) sources:

  • Many pollutants in urban areas come from human activities, especially those involving the burning of fuels

    • Burning fossil fuels: emissions from vehicles, power plants, and factories produce NOx, SO₂, CO, and PM

    • Agricultural burning and deforestation: these release large quantities of smoke, dust, and other pollutants into the atmosphere

    • Construction sites and roads: create dust and PM from the movement of machinery and vehicles

    • Industrial processes: factories release pollutants like NOx and PM from smokestacks and chemical processing

Common pollutants from urban activities

• The most common pollutants in urban areas are usually linked to the combustion of fossil fuels

  • Particulate matter (PM2.5 and PM10): tiny particles from exhaust fumes, industrial activities, and construction dust

  • CO: released by cars and industrial processes that burn fuels

  • NOx: produced by vehicle emissions and power plants

  • SO₂: released mainly by burning coal and oil

Secondary pollutants

• Secondary pollutants are not emitted directly but form in the atmosphere when primary pollutants react with other chemicals

  • Tropospheric ozone (O₃): forms when nitrogen oxides (NOx) react with sunlight

    • It is a major component of urban smog

Examples of urban air pollution

• Beijing, China: often experiences high levels of PM2.5, mainly due to coal burning for energy and industrial activity

• Los Angeles, USA: struggles with ozone pollution due to a high number of vehicles and sunny weather, which speeds up the reaction that forms ozone

• The burning of crops, industrial activity, and vehicle emissions frequently cause severe air pollution in New Delhi, India