C12: Maintaining Air Quality

Dry air is about $78\%$ nitrogen and $21\%$ oxygen, with most of the remainder made up of noble gases, especially argon, plus carbon dioxide. Water vapour is excluded from dry-air composition. Common pollutants include carbon monoxide, methane, nitrogen oxides, ozone, sulfur dioxide, and unburned hydrocarbons.

Carbon monoxide forms through incomplete combustion of carbon-containing fuels. Nitrogen oxides can form in lightning and high-temperature engines. Sulfur dioxide comes from volcanoes and sulfur-containing fossil fuels. Source identification matters because the control method must target the correct process.

Carbon monoxide is toxic because it reduces the blood's oxygen-carrying capacity. Nitrogen dioxide and sulfur dioxide contribute to acid rain, respiratory harm, and damage to buildings. Catalytic converters use redox reactions to reduce vehicle pollutants. Calcium carbonate can reduce acidity and is used in flue-gas desulfurisation.

The ozone layer absorbs harmful ultraviolet radiation. Chlorine-containing species released from CFCs can catalyse ozone depletion. Keep stratospheric ozone protection separate from ground-level ozone pollution and from the greenhouse effect; these are different environmental mechanisms.

Photosynthesis removes carbon dioxide from the atmosphere, while respiration and combustion return it. Transfers among atmosphere, organisms, fuels, and other stores form a carbon cycle that helps regulate atmospheric carbon dioxide. Human activity can shift the rates and sizes of these transfers.

Carbon dioxide and methane are greenhouse gases. Sources include combustion, decomposition, agriculture, and fuel systems. Increased concentrations can contribute to warming, more extreme weather, and melting polar ice. A complete discussion identifies source, gas, mechanism or trend, consequence, and limits of the evidence supplied.