Atmospheric Pollutants and Their Effects
Sources of Atmospheric Pollutants
Most atmospheric pollutants come from the combustion of fossil fuels — in vehicles, power stations and industry. When fuels burn, they produce a range of harmful substances depending on the conditions.
The Main Pollutants
Carbon Monoxide (CO)
Source: Produced by incomplete combustion of carbon-containing fuels (when there is not enough oxygen for complete combustion).
2C + O₂ → 2CO (incomplete combustion)
Properties and dangers:
- Colourless and odourless — impossible to detect without a detector
- Toxic — it binds to haemoglobin in red blood cells, preventing them from carrying oxygen
- Even low concentrations cause headaches and dizziness; high concentrations can be fatal
- Particularly dangerous in enclosed spaces (e.g. homes with faulty gas boilers)
Sulfur Dioxide (SO₂)
Source: Burning fossil fuels that contain sulfur impurities. Coal and oil contain sulfur compounds. When these burn:
S + O₂ → SO₂
Effects:
- Causes acid rain — SO₂ dissolves in rainwater to form sulfurous acid (H₂SO₃), which can be further oxidised to sulfuric acid (H₂SO₄)
- Acid rain damages limestone buildings and metal structures (corrosion)
- Acid rain makes lakes and rivers acidic, harming aquatic life (fish, invertebrates)
- Acid rain damages trees and soil
- Causes respiratory problems in humans — aggravates asthma and bronchitis
Nitrogen Oxides (NOx)
Source: Formed when nitrogen and oxygen from the air react together at the high temperatures inside car engines and power stations:
N₂ + O₂ → 2NO
2NO + O₂ → 2NO₂
At normal temperatures, nitrogen and oxygen do not react because the N≡N triple bond is very strong. Only the extreme temperatures in engines provide enough energy.
Effects:
- Cause acid rain (form nitric acid, HNO₃, when dissolved in rainwater)
- Cause photochemical smog in cities — a brown haze that irritates lungs and eyes
- Trigger respiratory problems — worsen asthma and lung disease
- NO₂ is a brown/orange gas visible as smog
Particulates (Soot and Fine Particles)
Source: Incomplete combustion of fuels, especially diesel engines and burning wood/coal. Also from construction, quarrying and natural sources.
Effects:
- Respiratory problems — small particles (PM₂.₅ and PM₁₀) penetrate deep into the lungs, worsening asthma and causing lung disease and cancer
- Reduced visibility — cause smog and haze
- Global dimming — particulates in the upper atmosphere reflect sunlight, slightly reducing the amount reaching the Earth's surface
- Blacken buildings — soot deposits darken stone surfaces
- Cardiovascular disease — fine particles enter the bloodstream
Unburned Hydrocarbons
Source: Fuels that pass through engines without burning completely.
Effects:
- Contribute to photochemical smog (react with NOx in sunlight to form ground-level ozone)
- Ground-level ozone irritates the lungs and eyes
- Some hydrocarbons are carcinogenic (cancer-causing)
Reducing Pollutant Emissions
Catalytic Converters
Catalytic converters are fitted to vehicle exhaust systems. They contain a honeycomb structure coated with precious metals (platinum, palladium and rhodium) that catalyse reactions to convert harmful gases to less harmful ones:
2CO + 2NO → 2CO₂ + N₂
This simultaneously removes carbon monoxide and nitrogen monoxide, converting them to carbon dioxide and nitrogen (both naturally present in the atmosphere).
Other Methods
| Method | Pollutant reduced |
|---|---|
| Desulfurisation of flue gases (power stations) | SO₂ |
| Using low-sulfur fuels | SO₂ |
| Catalytic converters | CO, NOx, unburned hydrocarbons |
| Diesel particulate filters | Particulates |
| Using electric/hydrogen vehicles | All combustion pollutants |
| Improving engine efficiency | All pollutants |
| Using renewable energy sources | All fossil fuel pollutants |
Summary Table of Pollutants
| Pollutant | Source | Main effects |
|---|---|---|
| CO | Incomplete combustion | Toxic (binds to haemoglobin) |
| SO₂ | Burning sulfur-containing fuels | Acid rain, respiratory problems |
| NOx | High-temperature combustion (engines) | Acid rain, smog, respiratory problems |
| Particulates | Incomplete combustion | Lung disease, global dimming |
| Unburned hydrocarbons | Incomplete combustion | Smog, carcinogenic |
Exam Tips
- Know the source and effects of each pollutant — exam questions frequently ask for both
- CO is from incomplete combustion; CO₂ is from complete combustion — do not confuse them
- NOx is formed because of the high temperature in engines, not from the fuel itself — nitrogen comes from the air
- Catalytic converters use platinum/palladium/rhodium — be able to write the equation: 2CO + 2NO → 2CO₂ + N₂
- Six-mark questions may ask you to discuss the environmental problems caused by burning fossil fuels — cover multiple pollutants and link each to its specific effects