Air

Air is the invisible mixture of gases that surrounds Earth. We breathe it, it carries sound, and it helps control Earth’s temperature.

Composition of clean, dry air

• Nitrogen, $N_2$: about $78\%$
• Oxygen, $O_2$: about $21\%$
• Remainder (about $1\%$): noble gases (mainly argon) and carbon dioxide, $CO_2$ (about $0.04\%$)

Clean, dry means no water vapour. Real air often contains water vapour and tiny particles (dust, smoke).

Air pollutants and their sources

• Carbon dioxide ($CO_2$): complete combustion of carbon fuels (e.g. petrol, gas, coal)
• Carbon monoxide (CO) and particulates: incomplete combustion (poor oxygen supply in engines or stoves)
• Methane ($CH_4$): decomposition of vegetation and waste; animal digestion
• Oxides of nitrogen ($NO_x$): high-temperature car engines
• Sulfur dioxide ($SO_2$): burning sulfur-containing fossil fuels

Adverse effects of pollutants

• $CO_2$: higher levels increase global warming, leading to climate change
• CO: toxic gas (reduces blood’s ability to carry oxygen)
• Particulates: higher risk of respiratory problems and cancer
• $CH_4$: higher levels increase global warming, leading to climate change
• $NO_x$: acid rain, photochemical smog, respiratory problems
• $SO_2$: acid rain

Greenhouse gases and global warming

Sunlight reaches Earth’s surface and warms it. The warm surface emits thermal (infrared) energy. Greenhouse gases like $CO_2$ and $CH_4$ absorb, then emit and partially reflect this thermal energy back towards Earth. This reduces thermal energy lost to space, like a thin blanket. Extra greenhouse gases strengthen this effect, causing global warming and climate change.

Reducing air pollution

• Burn fewer fossil fuels: use energy efficiently and choose renewables
• Ensure complete combustion: good ventilation and engine maintenance
• Car catalytic converters: reduce CO and $NO_x$
• Low-sulfur fuels and flue gas desulfurisation: reduce $SO_2$
• Capture landfill gas and manage waste: reduce $CH_4$