7.5 Case Study: Atmospheric Pollution

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Study the causes, impact, and management of a specific example of atmospheric pollution.

🌍 What Is Atmospheric Pollution?

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Atmospheric pollution means harmful gases or particles being released into the air by human activities (or sometimes nature). These pollutants can travel huge distances and damage ecosystems, human health, and even buildings.

The specific example we are studying is acid rain.

☁️ PART 1: Causes of Acid Rain

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What Is Acid Rain?

Acid rain is any form of precipitation — rain, snow, sleet, or fog — that has a pH below 5. (Normal, clean rain has a pH of about 5.6, which is only very slightly acidic. Acid rain is much more acidic than this.)

pH is a scale that measures how acidic or alkaline something is. The lower the pH number, the more acidic it is. Pure water is pH 7 (neutral). Battery acid is around pH 1. Acid rain sits around pH 4 or below.

The Main Gases That Cause Acid Rain

Two gases are responsible for forming acid rain:

  • Sulphur dioxide (SO₂)
  • Nitrogen oxides (NOₓ) — this is a group name for different gases containing nitrogen and oxygen

Where Do These Gases Come From?

Human sources (the main cause):

  • Power stations and factories that burn fossil fuels (coal, oil, gas) release large amounts of SO₂. When the sulphur inside the fuel burns, it combines with oxygen in the air to make sulphur dioxide.
  • Vehicle exhausts (cars, lorries, buses) release NOₓ. This happens because inside a car engine, temperatures get extremely hot. At these high temperatures, nitrogen from the air reacts with oxygen to form nitrogen oxides.
  • Industrial furnaces also produce NOₓ for the same reason — extremely high temperatures cause nitrogen and oxygen to react.

Natural sources (a smaller cause):

  • Volcanic eruptions can release sulphur dioxide naturally, though this contributes far less than human activity on a day-to-day basis.

How Does Acid Rain Actually Form?

Once SO₂ and NOₓ are released into the atmosphere, they do not immediately fall as acid rain. Here is the step-by-step process:

  1. SO₂ and NOₓ are released into the atmosphere from factories, power stations, and vehicles.
  2. These gases mix with water vapour and oxygen high up in the atmosphere.
  3. Chemical reactions occur:
    • SO₂ + water + oxygen → sulphuric acid (H₂SO₄)
    • NOₓ + water + oxygen → nitric acid (HNO₃)
  4. These weak acids dissolve into rain droplets, snowflakes, or fog.
  5. The acidic precipitation then falls back to the ground — this is acid rain.

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