7.2 Atmospheric Pollution and Its Causes

2026 Syllabus Objectives

By the end of this subtopic, you should be able to:

  1. Describe and explain the causes of atmospheric pollution, with reference to: smog, acid rain, ozone layer depletion, and the enhanced greenhouse effect.
  2. Apply specific knowledge about each type of pollution, including:
    • Smog: volatile organic compounds (VOCs) from industrial processes, vehicle emissions, and the impact of temperature inversion
    • Acid rain: the roles of sulfur dioxide and oxides of nitrogen
    • Ozone layer depletion: the action of chlorofluorocarbons (CFCs)
    • Enhanced greenhouse effect: the roles of greenhouse gases — carbon dioxide, water vapour, and methane

Main Content

https://cambridgeassistant.com

1. Smog

What is Smog?

Smog is a type of air pollution that appears as a thick, hazy, brownish or yellowish fog hanging over cities and industrial areas. The word itself comes from combining "smoke" and "fog." It makes the air look dirty and murky, and it is harmful to breathe. Smog is particularly common in large cities and areas with heavy industry or traffic.

What Causes Smog?

There are two main sources of the chemicals that create smog:

a) Volatile Organic Compounds (VOCs) from Industrial Processes

  • Volatile organic compounds (VOCs) are chemicals that easily turn into gases and escape into the air. Think of them as invisible gases that evaporate quickly — similar to the smell you notice when you open a can of paint or petrol. The word "volatile" just means they change to gas very easily.
  • Industries such as factories, oil refineries, and chemical plants release large amounts of VOCs into the atmosphere during their operations.
  • When VOCs react with other pollutants already in the air — especially in the presence of sunlight — they help form smog.

b) Vehicle Emissions

  • Cars, lorries, buses, and motorcycles burn petrol or diesel fuel. When fuel burns in an engine, it does not burn perfectly cleanly. The exhaust fumes that come out of the vehicle's tailpipe contain a mixture of harmful gases.
  • These gases include nitrogen oxides (NOₓ) — a group of gases produced when nitrogen from the air reacts with oxygen inside the very hot engine — as well as unburned hydrocarbons (which are a type of VOC) and other pollutants.
  • In sunlight, nitrogen oxides and VOCs react together chemically to produce ozone at ground level. This ground-level ozone is a major part of what we call smog, and unlike the ozone high up in the stratosphere (which protects us), ground-level ozone is harmful to human lungs.
  • The resulting smog is sometimes called photochemical smog because sunlight (photo = light) drives the chemical reactions that create it.

Sign in to view full notes