Sec 4 Chemistry Maintaining Air Quality notes

Clean, dry air is remarkably uniform: 78% nitrogen, 21% oxygen, about 1% noble gases (mostly argon), and 0.04% carbon dioxide — about 400 parts per million. Water vapour adds a variable extra slice depending on the weather.

To measure the oxygen slice, pass a measured volume of air repeatedly over heated excess copper: the copper ties up the oxygen as black copper(II) oxide, 2Cu + O₂ → 2CuO, and the gas syringe shrinks by exactly the oxygen volume — about 21% of the starting reading. (The old candle-in-an-upturned-jar version shows the same shrinkage, though less cleanly, because the CO₂ product partly dissolves.)

Carbon monoxide, CO: made whenever carbon fuels burn with too little oxygen (blocked vents, idling engines). Colourless, odourless, and deadly — it binds to haemoglobin about two hundred times more strongly than oxygen, so the blood quietly stops delivering oxygen.

Sulfur dioxide, SO₂: released when fuels containing sulfur impurities burn (S + O₂ → SO₂) — coal power stations and smelters are the big emitters. In clouds it becomes acid rain; directly it irritates lungs and worsens asthma.

Oxides of nitrogen, NO and NO₂: air itself is the fuel — inside a hot car engine (or a lightning bolt), N₂ + O₂ → 2NO, which oxidises further to 2NO₂. They acidify rain and cook up photochemical smog with unburnt hydrocarbons in sunlight. The unburnt hydrocarbons themselves are the fourth suspect — wasted fuel that irritates lungs and feeds smog.

Rain is naturally slightly acidic (dissolved CO₂), but SO₂ and NO₂ push it far below that. Sulfur dioxide dissolves to give sulfurous acid (SO₂ + H₂O → H₂SO₃) and is oxidised in air to sulfur trioxide, 2SO₂ + O₂ → 2SO₃, which makes sulfuric acid: SO₃ + H₂O → H₂SO₄. Nitrogen dioxide gives nitric acid: 3NO₂ + H₂O → 2HNO₃ + NO.

The damage bill: lakes acidify until fish eggs fail; forests weaken; and limestone buildings and statues dissolve — CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ — erasing carvings detail by detail. The fix at source: flue-gas desulfurisation, spraying power-station exhaust with limestone slurry so CaCO₃ + SO₂ → CaSO₃ + CO₂ — the SO₂ never reaches the sky.

Sunlight warms the Earth; the warm Earth radiates infrared back towards space. Greenhouse gases — carbon dioxide, methane and water vapour — absorb that infrared and re-radiate it, keeping the surface warm. That blanket is natural and essential: without it, Earth would freeze. The problem is extra CO₂ from burning fossil fuels and extra CH₄ from livestock, rice paddies and leaks — a thicker blanket, a warmer world: global warming.

Consequences on the table: rising sea levels as oceans warm and ice melts, more extreme weather, and shifts in farming zones. Responses run from burning less fossil fuel (renewables, nuclear), through efficiency (better engines, insulation, public transport), to carbon capture schemes — and the individual versions: fewer flights, less waste.

Car exhaust leaves the cylinder carrying CO, unburnt hydrocarbons and NOx. The catalytic converter — a honeycomb coated with platinum and rhodium — sits in the exhaust pipe and lets these meet and redeposit as friendlier chemistry: 2NO + 2CO → N₂ + 2CO₂. Hydrocarbons are oxidised to CO₂ and water too. The metals are catalysts: unchanged, working for years.

Honest accounting: the converter kills the toxic gases (CO, NOx, hydrocarbons) but cannot remove CO₂ — the greenhouse gas rides on. Cleaner cars also need leaner burning engines, better fuels and, ultimately, electrification. Air quality improves pollutant by pollutant.