Atmospheric Pollutants (C9.3)
Pollutants from burning fuels
Burning fossil fuels releases several pollutants in addition to CO₂ and H₂O:
| Pollutant | Source | Effects |
|---|---|---|
| Carbon monoxide (CO) | Incomplete combustion | Toxic — binds to haemoglobin; prevents O₂ transport |
| Sulfur dioxide (SO₂) | Burning fossil fuels with sulfur impurities | Acid rain → harms ecosystems, corrodes stone/metal |
| Nitrogen oxides (NOₓ: NO, NO₂) | High-temperature combustion — N₂ + O₂ react | Acid rain, photochemical smog, respiratory problems |
| Particulates (soot/PM2.5) | Incomplete combustion, diesel engines | Respiratory disease, cardiovascular disease, climate effects (dimming) |
| Unburned hydrocarbons | Inefficient combustion | Contribute to photochemical smog |
Carbon monoxide (CO)
Produced when carbon-containing fuels burn with insufficient oxygen.
Toxicity: CO binds to haemoglobin (in RBCs) ~200× more strongly than O₂ → forms carboxyhaemoglobin → cannot carry O₂ → tissue hypoxia → death.
Colourless, odourless — dangerous because undetectable without alarms.
Sulfur dioxide (SO₂)
Formed when sulfur impurities in coal and oil are burned:
S + O₂ → SO₂
SO₂ dissolves in rainwater → H₂SO₃ (sulfurous acid) → acid rain (pH 4–5).
Effects of acid rain:
- Damages leaves and defoliates trees
- Acidifies lakes and rivers → kills aquatic life
- Reacts with limestone buildings and statues (CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O)
Reducing SO₂: use low-sulfur fuels; remove S from fuel before combustion; flue gas desulfurisation (limestone scrubbers in power station chimneys neutralise SO₂).
Nitrogen oxides (NOₓ)
At high combustion temperatures (e.g. car engines): N₂ + O₂ → 2NO
NO can further oxidise to NO₂.
Effects: contribute to acid rain (HNO₃); photochemical smog with hydrocarbons in sunlight.
Reducing NOₓ: catalytic converters in car exhausts:
2CO + 2NO → 2CO₂ + N₂ (also reduces CO and unburned hydrocarbons)
Particulates (PM2.5, PM10)
Fine particles from incomplete combustion. Can be inhaled deep into lungs → inflammation, cardiovascular disease, premature death. Also reduce visibility (smog) and can cool climate by reflecting sunlight (but soot on ice reduces albedo → accelerates ice melt).
Common exam errors
- Saying CO₂ is a pollutant in the same sense as CO — CO₂ is a greenhouse gas; CO is directly toxic.
- Forgetting that acid rain is caused by SO₂ AND NOₓ (not only CO₂).
- Saying catalytic converters reduce CO₂ emissions — they reduce CO, NOₓ and unburned hydrocarbons, NOT CO₂.
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