Maintaining Air Quality

Dry air contains about 78% nitrogen and 21% oxygen by volume. Most of the remaining 1% is noble gases, mainly argon, with carbon dioxide and other trace gases. Dry air excludes water vapour; the water-vapour content of actual air varies.

Distinguish CO from CO₂. Both contain carbon and oxygen, but CO is a toxic incomplete-combustion product. Complete combustion produces CO₂, which is important in the carbon cycle and greenhouse effect. A fuel containing no sulfur cannot directly supply sulfur dioxide simply by burning; identify which elements the source provides.

Carbon monoxide binds strongly to haemoglobin and reduces the blood's ability to carry oxygen. It is not the same hazard as carbon dioxide's role in climate change. Because incomplete combustion can produce CO, a fuel burning does not guarantee that its exhaust contains only carbon dioxide and water.

Nitrogen dioxide and sulfur dioxide react in the atmosphere with water and oxygen to produce acidic substances that enter rain. Acid rain can damage plants and aquatic ecosystems and attack buildings containing carbonate stone. These gases also irritate the respiratory system; health effects are not confined to rainwater.

For a limestone surface, carbonate reacts with acid: CaCO₃(s) + 2H⁺(aq) -> Ca²⁺(aq) + H₂O(l) + CO₂(g). Solid material is gradually lost. This connects the visible damage to an acid-carbonate reaction rather than merely saying that pollution is bad.

A catalytic converter speeds reactions that turn some exhaust pollutants into less harmful products. Carbon monoxide is oxidised to carbon dioxide while nitrogen monoxide can be reduced to nitrogen: 2CO(g) + 2NO(g) -> 2CO₂(g) + N₂(g). Unburned hydrocarbons can also be oxidised to carbon dioxide and water. The catalyst increases rate; it is not used up overall.

This reduces CO, nitrogen oxides and unburned hydrocarbons, but does not make the exhaust free of environmental impact: carbon dioxide still enters the atmosphere. Nitrogen atoms end up mainly in N₂, not in an unspecified harmless gas.

A simplified sequence is CaCO₃(s) + SO₂(g) -> CaSO₃(s) + CO₂(g), followed by 2CaSO₃(s) + O₂(g) -> 2CaSO₄(s). Actual flue-gas systems often use a wet slurry; the equations show the net chemical changes.

The stratospheric ozone layer absorbs much harmful ultraviolet radiation, helping protect living organisms. Ground-level ozone is an air pollutant that can irritate the respiratory system. Saying ozone is always beneficial or always harmful ignores its location.

Chlorofluorocarbons, CFCs, are chlorine-containing compounds that can persist long enough to reach the upper atmosphere. Ultraviolet radiation can release reactive chlorine species, which react with ozone and promote its destruction. Chlorine can be regenerated, allowing repeated ozone breakdown rather than only a one-for-one removal.

Ozone depletion allows more harmful ultraviolet radiation to reach the surface, increasing risks to living organisms. It is a different issue from greenhouse warming: ozone loss concerns UV protection, while greenhouse gases affect the escape of outgoing infrared radiation.

More ultraviolet radiation can damage cells and genetic material, increasing risks such as skin cancer and cataracts. It can also damage plants and plankton, affecting growth and food chains. CFCs persist for a long time, so reducing new releases does not instantly remove the compounds already in the atmosphere; ozone protection addresses a long-lived cause as well as its present effects.

Photosynthesis takes carbon dioxide from air and incorporates its carbon into plant material. Respiration returns carbon dioxide as organisms release energy from organic compounds. Combustion of biomass and fossil fuels also returns carbon dioxide. These flows help regulate atmospheric carbon dioxide, but they do not guarantee a constant concentration if inputs exceed removals.

Carbon dioxide and methane are greenhouse gases. They absorb outgoing infrared radiation and contribute to warming of the atmosphere and surface. More greenhouse gas strengthens this effect. Fossil-fuel combustion and deforestation add CO₂ or reduce its removal. Methane sources include livestock, landfill decomposition, wetlands and fossil-fuel leakage.

Potential consequences of increased greenhouse-gas levels include more extreme weather events and melting polar ice. Melting land ice contributes to rising sea level; floating sea ice and land ice should not be treated as identical sea-level mechanisms. Warming also changes rainfall patterns and stresses habitats.

For bioethanol, plant growth can take up carbon dioxide later released by burning the fuel. The comparison with fossil fuels depends on how the crop is grown, processed and transported and whether land-use changes release additional carbon. A carbon-flow explanation is stronger than the claim that renewable fuels produce no carbon dioxide.

Track carbon flows in amount as well as direction: atmospheric CO2 is steady only when additions and removals balance. Biofuel combustion emits CO2 even when plant growth offsets part of the release.