8.3 Urban air pollutionIB Environmental Systems and Societies HL: Revision notes

Urban air pollution results from inputs from human activities to atmospheric systems. Key pollutants are nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO) and particulate matter. Primary pollutants are released directly from natural (dust, wildfires, volcanoes) and anthropogenic (vehicles, power stations, industry) sources. Most urban pollutants come directly or indirectly from combustion of fossil fuels.

Strategies reduce pollution at different points: reduce production (public transport, cycling, cleaner fuels, renewable energy, congestion charges or low-emission zones), regulate (emission standards, cap-and-trade, vehicle restrictions), reduce at point of release (catalytic converters, flue-gas desulfurisation, particle filters) and reduce effects (air quality alerts, liming). Evaluate cost, enforcement, side effects and effectiveness.

SO2 and NOx react with water and oxygen to form sulfuric and nitric acids (acid rain). Impacts: ecology (acid lakes, aluminium harming fish, damaged leaves, nutrient leaching and forest decline), humans (respiratory harm from fine particles, economic losses) and buildings (limestone and marble dissolve, metals corrode). Strategies cut SO2 and NOx at source (scrubbers, low-sulfur fuel, catalytic converters), use regulation (e.g. the US cap-and-trade scheme for SO2) and treat effects (liming). Example: Adirondack lakes.

Photochemical smog forms when sunlight acts on primary pollutants (nitrogen oxides and volatile organic compounds / hydrocarbons) and transforms them chemically into secondary pollutants, notably tropospheric ozone and other irritants, producing a brown haze. Smog is worst in sunny, warm weather and on days when traffic emissions are high. Example: Los Angeles.

Smog formation is intensified by:

• Intense sunlight and warm temperatures, which drive and speed up reactions.
• Light winds that fail to disperse pollutants.
• Temperature inversions, where warm air lies over cooler air and traps pollution near the ground.
• Topography: basins or valleys ringed by mountains limit air movement and trap pollution.

Examples: Los Angeles and Mexico City (a high basin ringed by mountains).

Tropospheric ozone is a pollutant at ground level, unlike stratospheric ozone, which protects from ultraviolet.

Direct impacts

• Biological: damages lung tissue (asthma, breathing difficulty), and enters plant stomata, damaging leaf tissue and reducing photosynthesis and crop yields.
• Physical: degrades rubber, plastics and some materials.

Indirect impacts: societal costs (healthcare costs, lost working days) and lost economic output (reduced crop and timber yields, repair costs).