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Loading home…Clean dry air is mainly nitrogen (about 78%) and oxygen (about 21%). Argon is the most abundant noble gas at about 0.93%. Carbon dioxide is present at roughly 0.04% and is rising due to human activity. Trace gases include neon, helium, methane and other minor components. Water vapour varies widely and is excluded from “dry air” percentages. These proportions underpin respiration, combustion and atmospheric chemistry.
Air pollution is the presence of substances in air at levels that harm humans, organisms or materials. Primary pollutants are emitted directly, such as CO, SO₂, NO, and particulate matter. Secondary pollutants form by reactions in the atmosphere, such as ozone in photochemical smog and acid rain components. Sources include vehicles, industry, power plants and biomass burning. Effects include respiratory disease, crop damage and corrosion. Control requires cleaner fuels, emission standards and monitoring.
Acid rain forms when SO₂ and NOₓ dissolve and oxidise in atmospheric moisture to give H₂SO₄ and HNO₃. The resulting precipitation has pH significantly below that of natural rain. It acidifies lakes and soils, harming aquatic life and forests. It corrodes limestone buildings, metals and cultural monuments. Reducing sulfur in fuels and fitting flue-gas desulfurisation cuts SO₂ emissions. Catalytic converters and combustion control reduce NOₓ.
Stratospheric ozone absorbs harmful UV-B radiation, protecting living organisms. Ozone forms and destroys continuously in the Chapman cycle involving O₂ and O atoms. Chlorofluorocarbons (CFCs) release chlorine atoms in the stratosphere that catalytically destroy ozone. Polar stratospheric clouds and cold conditions accelerate seasonal ozone holes. International agreements such as the Montreal Protocol phased down CFCs. Recovery of the ozone layer is gradual but measurable.
The greenhouse effect is warming of Earth when atmospheric gases absorb and re-radiate infrared radiation from the surface. Natural greenhouse effect makes Earth habitable; enhanced effect from human emissions causes climate change. Important greenhouse gases include CO₂, CH₄, N₂O and water vapour; CFCs are also potent. Fossil fuel combustion and deforestation raise CO₂. Agriculture and wetlands contribute methane. Mitigation involves renewable energy, efficiency and protecting forests.
Photochemical smog is a mixture of oxidising pollutants formed when sunlight acts on NOₓ and unburnt hydrocarbons. Ozone, peroxyacyl nitrates (PANs) and aldehydes are typical secondary products. It is favoured by strong sunlight, stagnant air and heavy traffic emissions. Effects include eye irritation, breathing problems and damage to rubber and crops. Control focuses on reducing vehicle NOₓ and VOC emissions. It differs from classical sulfurous smog dominated by SO₂ and smoke.
Carbon monoxide from incomplete combustion binds strongly to haemoglobin, reducing oxygen transport and causing headaches or fatal poisoning at high levels. Sulfur dioxide from burning sulfur-containing fuels irritates airways and contributes to acid rain and sulfate aerosols. Nitrogen oxides from high-temperature combustion form acids and help produce photochemical oxidants. Particulate matter (PM2.5, PM10) penetrates lungs and is linked to cardiovascular and respiratory disease.
Ground-level ozone harms lungs and vegetation even though stratospheric ozone is beneficial. Lead from historic petrol additives damaged nervous systems; unleaded fuel greatly reduced this hazard. Volatile organic compounds contribute to smog and some are carcinogenic.
Environmental effects include acidification, reduced visibility, crop losses and climate forcing by CO₂ and aerosols. Monitoring AQI, enforcing emission limits, and adopting cleaner technology are essential public-health measures.
SO₂ oxidises in air to SO₃, which forms H₂SO₄ with water; NO and NO₂ form HNO₃. These strong acids lower the pH of rain, snow and dry deposition. Geology matters: limestone catchments buffer acidity better than granite regions.
Control of SO₂ includes using low-sulfur fuels, coal washing, and flue-gas desulfurisation with lime or limestone scrubbers. NOₓ control uses staged combustion, low-NOₓ burners, and catalytic reduction (e.g. with ammonia in SCR systems). Vehicle catalytic converters oxidise CO and hydrocarbons and reduce NOₓ.
International cooperation matters because acid rain crosses borders. Continuous emission monitoring verifies compliance. Acid-rain chemistry links combustion, atmospheric oxidation and environmental damage.
In the stratosphere, ozone filters UV radiation. CFCs are stable in the troposphere but photolyse aloft, releasing Cl atoms that catalyse O₃ destruction: Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂. One chlorine atom can destroy many ozone molecules. Bromine from halons is even more efficient per atom.
Polar ozone depletion is enhanced by heterogeneous reactions on polar stratospheric clouds. Policy action under the Montreal Protocol substituted safer refrigerants and foaming agents.
Tropospheric ozone is formed by photochemical reactions of NOₓ and VOCs and is a lung irritant. Thus “ozone” is beneficial high above but harmful near the ground. Location and chemistry determine whether ozone protects or pollutes.
Solar short-wave radiation warms Earth’s surface, which emits infrared radiation. Greenhouse gases absorb IR and re-emit it, warming the lower atmosphere. Without natural greenhouse gases, Earth would be much colder. Human activities increase CO₂, CH₄ and N₂O, strengthening the effect.
Consequences include rising global mean temperatures, melting ice, sea-level rise, shifting weather patterns and stress on ecosystems and agriculture. Ocean uptake of CO₂ causes acidification that threatens coral reefs.
Responses include decarbonising energy, improving efficiency, protecting carbon sinks, and adapting infrastructure. Chemistry students should connect combustion stoichiometry and carbon cycles to climate policy discussions.
Classical (London-type) smog involves smoke and SO₂ in cool, humid, stagnant conditions, forming acidic reducing pollution. Photochemical (Los Angeles-type) smog needs sunlight, NOₓ and hydrocarbons, producing oxidising pollutants such as O₃ and PANs.
Urban prevention combines cleaner public transport, vehicle emission standards, catalytic converters, dust control, industrial scrubbers, and green spaces. Banning open burning and regulating construction dust reduce particulates. Real-time monitoring informs alerts for sensitive groups.
Education and urban planning that reduce congestion complement technology. Sustained improvement requires both science-based standards and enforcement.