06 April 2014

#151 Air pollution by sulfur dioxide (SO2), acid rain

SO2 released into the air when coal and oil are burned. Power stations burn large amounts of these fossil fuels. SO2 dissolves in the water vapour in clouds, forming sulphuric acid (H2SO4). When it rains, the rain is acidic.

The combustion of petrol in car engines also contributes to acid rain, but this is mainly due to the production of oxides of nitrogen in the exhaust fumes, rather than SO2.

Acid rain

The main causes of acid rain are process that release SO2 and oxides of nitrogen into the atmosphere. These include:
  • burning of fossil fuels, such as coal and gas, by power stations
  • combustion of petrol in car engines.

Problems caused by acid rain include:
  • Damage to plant leaves, eventually killing the plants. Whole forests of pine trees have been destroyed by acid rain.

Plant leaves damaged by acid rain. 

  • Acidification of lakes: as the water become more acidic, some animals such as fish cannot survive and fish stocks are destroyed.
  • Increased risk of asthma attacks and bronchitis in humans.
  • Corrosion of stonework on buildings.
  • Release into soil of soluble aluminium ions that are toxic to fish when washed into lakes.
Ways of reducing the incidence of acid rain:
  • changing the types of power stations that generate electricity from coal and oil to gas or nuclear power, or using more renewable energy sources such as wind
  • using ‘scrubbers’ in power station chimneys – these remove most of the SOpresent in the waste gases
  • using catalytic converters in car exhausts – these convert oxides of nitrogen to harmless nitrogen.

Common misconceptions

Remember that car engines do not  make large amount of SO2 – but they are responsible for producing large amount of oxides of nitrogen, CO2 and CO.

Examiner’s tip

When describing the effects of car exhaust fumes on the environment, don’t make a list of the chemicals and then link them all to acid rain or global warming. Be specific:
  • oxides of nitrogen leas to acid rain;
  • CO can reduce the ability of haemoglobin to carry O2;
  • CO2 increases can lead to global warming.

Try this

1. SO2 is a major  pollutant of the air. Which process is mainly responsible for the release of SO2 into the air?                                                       [1 mark]

2. SO2 is one of the gases which contributes to acid rain. Acid rain can affect trees and their surrounding soil in a variety of ways. Figure below shows where these effects can occur.

Suggest how each of the following affects the tree and explain how it can lead to its death.

i) Damage of the leaves                                                       [2 marks]
ii) Damage of the fine roots                                                 [2 marks]
iii) Death of the soil microorganisms                                   [2 marks]


1. Combustion of fossil fuels.

2. i) The leaves are unable to photosynthesise, so it cannot make food.

ii) One suggestion and explanation from:

- the roots are unable to absorb water, so cells will become flaccid, or the tree will wilt, or transport of materials will not happen

- the roots are unable to absorb mineral salts which are needed, e.g. for formation of chlorophyll, or for growth. 

iii) One suggestion and explanation from:

- less decomposition will occur, so there will be less minerals available to the plant, e.g. magnesium ions for formation of chlorophyll.

- there will be no nitrogen-fixing bacteria, so there will be less nitrates for the roots to take up, which are needed for protein formation.