Air pollution: sulfur dioxide and carbon monoxide
Where these gases come from
| Gas | Source | Key idea |
|---|---|---|
| Sulfur dioxide (SO₂) | Burning fossil fuels (coal and oil) that contain sulfur impurities — power stations, factories, some vehicles | Causes acid rain |
| Carbon monoxide (CO) | Incomplete combustion of fuels — car exhausts, faulty gas heaters, cigarette smoke | A toxic gas that affects the blood |
Sulfur dioxide → acid rain
- SO₂ is released into the air.
- It dissolves in water droplets in clouds, forming an acidic solution (sulfuric acid).
- It falls as acid rain, often a long way from where the gas was produced.
Biological consequences of acid rain
In lakes and rivers
- The pH of the water falls. Fish eggs and young fish die; many invertebrates cannot survive below about pH 5.
- Acid rain leaches aluminium ions out of soil into the water; these damage fish gills, causing mucus build-up so the fish cannot absorb enough oxygen.
- Fewer species survive → biodiversity falls, and food chains break down.
On land
- Damages the leaves (and conifer needles) of trees directly — yellowing and leaf loss, so less photosynthesis and reduced growth.
- Leaches mineral ions such as magnesium and calcium out of the soil. Without magnesium a plant cannot make chlorophyll, so leaves yellow and growth is stunted.
- Lichens are very sensitive to SO₂, so they are useful indicator species — few or no lichen species grow near heavily polluted areas.
Carbon monoxide → less oxygen in the blood
- CO is colourless and odourless, so people do not notice it.
- It binds to haemoglobin in red blood cells, forming carboxyhaemoglobin. The binding is very difficult to reverse.
- That haemoglobin can no longer carry oxygen, so the oxygen-carrying capacity of the blood falls.
- Less oxygen reaches the tissues → less aerobic respiration, less energy released → headaches, dizziness, drowsiness, and at high concentrations death.
Common mistake: carbon monoxide does not cause acid rain and is not a greenhouse gas. Keep its one job clear: it stops haemoglobin carrying oxygen.
Examiner tip. Examiners repeatedly report candidates writing carbon monoxide when they mean carbon dioxide (and vice versa). Before you write either name, check which effect you are describing: carbon dioxide = greenhouse gas; carbon monoxide = binds to haemoglobin. Also watch the command word — 'describe' acid rain means say what happens; 'explain' means give the reason (e.g. because aluminium ions damage the gills).