Air Quality and Climate
1. Composition of Clean, Dry Air
- Approximate composition:
- Nitrogen (N₂): 78%
- Oxygen (O₂): 21%
- Remaining ~1%: Mixture of noble gases (argon, neon, helium, krypton, xenon) and carbon dioxide (CO₂)
- Importance of each component:
- Nitrogen: Inert gas, prevents rapid combustion in the atmosphere.
- Oxygen: Essential for respiration and combustion processes.
- Noble gases: Chemically unreactive, used in specialised lighting, welding, and as inert shielding gases.
- Carbon dioxide: Needed for photosynthesis in plants.
2. Sources of Air Pollutants
(a) Carbon dioxide (CO₂)
- Source: Complete combustion of carbon-containing fuels (coal, petrol, diesel, natural gas, wood).
- Equation example: C + O₂ → CO₂
(b) Carbon monoxide (CO) and particulates
- Source: Incomplete combustion of carbon-containing fuels due to insufficient oxygen supply.
- Equation example for CO: 2C + O₂ → 2CO
- Particulates: Tiny solid particles (soot, dust) formed during incomplete combustion.
(c) Methane (CH₄)
- Source:
- Decomposition of vegetation in waterlogged soils and swamps.
- Waste gases from digestion in ruminant animals (e.g., cows, sheep).
- Landfill sites producing methane from rotting organic waste.
(d) Oxides of nitrogen (NO, NO₂)
- Source: High-temperature reactions between nitrogen and oxygen inside car engines and in lightning strikes.
- Equations:
- N₂ + O₂ → 2NO (in hot engines)
- 2NO + O₂ → 2NO₂
(e) Sulfur dioxide (SO₂)
- Source: Burning of fossil fuels containing sulfur compounds (e.g., coal, heavy oil).
- Equation: S + O₂ → SO₂
3. Adverse Effects of Air Pollutants
(a) Carbon dioxide:
- Excess CO₂ traps heat in Earth’s atmosphere → enhanced greenhouse effect → global warming → climate change (melting ice caps, sea-level rise, extreme weather).
(b) Carbon monoxide:
- Toxic gas – binds with haemoglobin in red blood cells to form carboxyhaemoglobin, reducing oxygen transport in the body → can cause headaches, unconsciousness, or death.
(c) Particulates:
- Cause respiratory problems (e.g., asthma, bronchitis) and increase cancer risk.
- Can reduce visibility (smog).
(d) Methane:
- Potent greenhouse gas – more effective at trapping heat than CO₂.
- Contributes to global warming and climate change.
(e) Oxides of nitrogen:
- Cause acid rain → damages vegetation, aquatic life, and corrodes buildings.
- Contribute to photochemical smog → causes respiratory issues and eye irritation.
(f) Sulfur dioxide:
- Causes acid rain when dissolved in rainwater, forming sulfurous (H₂SO₃) and sulfuric acid (H₂SO₄).
4. Greenhouse Effect – CO₂ and CH₄ Role
- Step-by-step process:
- Sun emits shortwave radiation (light energy) which passes through Earth’s atmosphere.
- Earth’s surface absorbs energy and re-emits it as longwave infrared radiation (heat).
- Greenhouse gases (CO₂, CH₄) absorb this thermal energy and re-radiate it back to Earth.
- Less heat escapes into space → global temperature rises.
5. Strategies to Reduce Environmental Issues
(a) Climate change
- Plant more trees → absorb CO₂ by photosynthesis.
- Reduce livestock farming → less methane from digestion.
- Decrease fossil fuel use → switch to renewable energy sources (wind, solar, tidal).
- Use hydrogen fuel instead of petrol/diesel.
(b) Acid rain
- Fit vehicles with catalytic converters to reduce NOₓ emissions.
- Use low-sulfur fuels in industry.
- Use flue gas desulfurisation in power stations → spray flue gases with calcium oxide (CaO) to neutralise SO₂.
- Equation: CaO + SO₂ → CaSO₃
6. Formation and Removal of Oxides of Nitrogen in Vehicles
- Formation: High temperatures in car engines cause N₂ and O₂ from air to combine.
- N₂ + O₂ → 2NO
- 2NO + O₂ → 2NO₂
- Removal: Catalytic converters change harmful gases into harmless ones.
- 2CO + 2NO → 2CO₂ + N₂
7. Photosynthesis
- Definition: Process in plants where carbon dioxide reacts with water in the presence of chlorophyll and sunlight to produce glucose and oxygen.
- Word equation: carbon dioxide + water → glucose + oxygen
- Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂