Air pollutants and where they come from

Where does the winter smog come from?

Every winter, many cities of Pakistan disappear under a grey blanket of smog. Nobody makes smog on purpose, so what is it made of, and where do its ingredients come from?

An air pollutant is any solid, liquid or gas in the air that harms human health and quality of life. Its amount is given in parts per million (ppm): one part of pollutant in a million parts of the mixture.

Seven pollutants cause over 90% of air pollution. They are carbon dioxide, carbon monoxide, oxides of nitrogen (NOx), oxides of sulphur (SOx), hydrocarbons such as methane, particulates such as dust and pollen, and ozone.

Burning fossil fuels is the biggest source. Complete burning gives carbon dioxide; incomplete burning gives carbon monoxide and particulates; sulphur in the fuel gives sulphur dioxide; car engines give oxides of nitrogen. Nature adds some too: volcanoes release CO₂, SO₂ and particulates, and rotting vegetation and animal digestion release methane.

Ozone near the ground forms when heat and sunlight make oxides of nitrogen react with volatile organic compounds. In winter, smoke mixes with fog to form smog; in Pakistan this comes from industry, vehicle emissions and crop burning.

Most air pollution comes from burning fossil fuels; smog is smoke mixed with fog.

Key terms

Air pollutant

Naming what we measure.

ppm

Measuring small amounts.

Particulates

Dust, pollen, metal compounds.

Smog

Winter pollution.

Short questions with model answers

1. Q1. A fuel burns with too little oxygen. Which pollutants form, and why?

   • Complete burning
   • Incomplete burning

   Carbon monoxide and particulates such as soot.

2. Q2. Which three things are needed for photochemical smog (ground-level ozone)? (Exercise ix, p. 143)

   • Reactants
   • Trigger

   NO₂, volatile organic compounds and sunlight.

Common mistakes

• ✗ All air pollutants come from human activity.

  ✓ Volcanoes and rotting vegetation release some too (p. 132).

• ✗ Carbon monoxide comes from complete burning of fuel.

  ✓ Complete burning gives CO₂; CO comes from incomplete burning.

• ✗ Smog is a summer problem caused by heat alone.

  ✓ Smog forms in winter when smoke mixes with fog (p. 132).

MCQs

1. 1. What is the source of the sulphur behind sulphur oxides in the air? (Exercise ii, p. 142)

   1. (a) Decomposition of vegetation
   2. (b) Waste gases from animal digestion
   3. (c) Photochemical smog
   4. (d) Combustion of fossil fuels
   Show answer

   (d) Sulphur in the fuel burns to SO₂ (p. 134).

2. 2. Which are needed to form photochemical smog? (Exercise ix, p. 143)

   1. (a) CO, NO₂, CO₂
   2. (b) NO₂, volatile organic compounds, sunlight
   3. (c) CO₂, NO₂, sunlight
   4. (d) Volatile organic compounds, NO₂, CO
   Show answer

   (b) p. 132.

3. 3. 1 ppm of a pollutant means:

   1. (a) 1 part in a hundred
   2. (b) 1 part in a thousand
   3. (c) 1 part in a million
   4. (d) 1 gram per litre
   Show answer

   (c) Parts per million (p. 131).

4. 4. Incomplete combustion of fuel mainly produces:

   1. (a) Carbon monoxide and particulates
   2. (b) Oxygen
   3. (c) Nitrogen
   4. (d) Only water
   Show answer

   (a) Learning outcomes, p. 129.

5. 5. Which natural event releases CO₂, SO₂ and particulates?

   1. (a) Rainfall
   2. (b) Volcanic eruption
   3. (c) Snowfall
   4. (d) Photosynthesis
   Show answer

   (b) p. 132.

Quick revision

• Seven pollutants (CO₂, CO, NOx, SOx, hydrocarbons, particulates, ozone) cause over 90% of air pollution.
• Burning fossil fuels is the main source; nature adds methane, and volcanic gases and dust.
• Ground-level ozone needs NOx, volatile organic compounds and sunlight; winter smog is smoke plus fog.