Composition of clean air, atmospheric pollutants and their sources and effects, the greenhouse effect and global warming, acid rain formation and consequences, ozone layer depletion by CFCs, and the principles of green chemistry.
Human activity, primarily the combustion of fossil fuels, has changed the composition of the atmosphere in ways that drive three linked environmental problems: climate change, acid rain, and ozone depletion. Understanding the chemistry behind each is required for CSEC and increasingly relevant to the world these topics describe.
Clean, dry air is approximately:
| Gas | Percentage by volume |
|---|---|
| Nitrogen (N₂) | ~78% |
| Oxygen (O₂) | ~21% |
| Argon and other noble gases | ~1% |
| Carbon dioxide (CO₂) | ~0.04% |
Variable components — water vapour, dust, and pollutants — are not included in these figures.
That is clean air. Combustion and industry have added significant amounts of other substances, each with its own sources and effects.
| Pollutant | Sources | Effects |
|---|---|---|
| Carbon monoxide (CO) | Incomplete combustion (cars, gas appliances) | Binds haemoglobin; fatal in enclosed spaces |
| Sulfur dioxide (SO₂) | Burning fossil fuels containing sulfur; volcanic eruptions | Causes acid rain; respiratory irritant |
| Oxides of nitrogen (NOₓ) | High-temperature combustion in engines | Cause acid rain; contribute to smog |
| Carbon dioxide (CO₂) | All combustion, deforestation, respiration | Greenhouse effect; climate change |
| Chlorofluorocarbons (CFCs) | Refrigerants, aerosols (now banned in most countries) | Ozone layer depletion |
| Particulates / soot | Diesel engines, coal combustion | Respiratory disease; reduces visibility |
| Unburnt hydrocarbons | Inefficient combustion | Contribute to photochemical smog |
Carbon dioxide is in the table above, but its effect works differently from SO₂ or soot. Rather than causing direct local damage, it gradually changes how the atmosphere retains heat across the whole planet.
The greenhouse effect is a natural process essential for life on Earth. Greenhouse gases (CO₂, methane, water vapour, and others) in the atmosphere absorb infrared radiation emitted by the Earth's surface after it is warmed by sunlight. These gases then re-radiate the energy in all directions, including back toward Earth's surface. Without this effect, Earth's average temperature would be about −18 °C instead of the current +15 °C.
Enhanced greenhouse effect (global warming) results from increasing concentrations of CO₂ and other greenhouse gases from human activity. Consequences include:
Main human sources of CO₂: burning fossil fuels (coal, oil, natural gas) and deforestation (fewer trees means less CO₂ is absorbed by photosynthesis).
Sulfur dioxide and nitrogen oxides have a more direct and faster effect: they dissolve in rainwater and make it acidic.
Sulfur dioxide and nitrogen oxides dissolve in rainwater to form acids. Normal, unpolluted rain has a pH of about 5.6 (slightly acidic due to dissolved CO₂ forming carbonic acid). Acid rain can have a pH as low as 4 or below.
Effects of acid rain:
Catalytic converters in cars convert harmful exhaust gases to less harmful ones before they leave the exhaust pipe:
Unburnt hydrocarbons are also oxidised to CO₂ and H₂O. The catalyst is typically a mixture of platinum and rhodium on a ceramic support.
A separate problem plays out much higher up. In the stratosphere, CFCs have been breaking down ozone, which is the gas that filters UV radiation before it reaches the surface.
The ozone layer (O₃) in the stratosphere absorbs harmful ultraviolet (UV) radiation from the sun, protecting living organisms from DNA damage, skin cancer, and cataracts.
CFCs (chlorofluorocarbons) — once widely used in refrigerants and aerosol propellants — rise into the stratosphere and react with ozone in the presence of UV light. Free chlorine radicals break down ozone in a chain reaction:
One chlorine radical can destroy thousands of ozone molecules before it is deactivated. The Montreal Protocol (1987) committed nations to phasing out CFC production. Even so, CFCs already in the stratosphere continue to damage the ozone layer, particularly over the polar regions (the "ozone hole" over Antarctica).
All three problems above came from chemical processes that were never designed with their environmental effects in mind. Green chemistry is the field that tries to prevent this from the start rather than manage the consequences after.
Green chemistry is the design and application of chemical processes that reduce or eliminate the use and generation of hazardous substances during the manufacture and use of chemical products.
Core principles include:
Green chemistry represents a shift in how chemists think: rather than disposing of hazardous waste safely after it is made, the goal is to design processes that do not generate hazardous waste in the first place.