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

Urban air pollution results from inputs from human activities to atmospheric systems. The main pollutants are:

• Nitrogen oxides (NOx): formed when nitrogen and oxygen combine at high temperatures in engines and power stations.
• Sulfur dioxide (SO2): from burning fuels that contain sulfur, especially coal and some oil.
• Carbon monoxide (CO): from incomplete combustion, mostly vehicles.
• Particulate matter (PM10, PM2.5): soot, dust and aerosols.

Primary pollutants are released directly. Their sources are natural (volcanoes, wildfires, dust) and anthropogenic (vehicles, industry, power stations, waste burning). In cities, most pollutants come directly or indirectly from combustion of fossil fuels.

A range of management and intervention strategies reduce urban air pollution.

• Reduce production: use public transport, cycling and walking; cleaner fuels; renewable energy; energy efficiency.
• Regulation and charges: emission standards, low-emission zones, congestion charges, bans on older vehicles.
• Reduce at the point of release: catalytic converters on vehicles, flue-gas desulfurisation and particle filters on power stations and diesel engines.
• Reduce the effects: air quality alerts, and planting urban vegetation.

Effectiveness depends on cost, enforcement and public acceptance, so evaluate strengths and limitations.

Sulfur dioxide and nitrogen oxides react with water and oxygen in the air to form sulfuric acid and nitric acid. These fall as acid rain (wet deposition) or as dry deposition of gases and particles. Pollutants can be carried hundreds of kilometres by wind, so the damage often occurs far from the source and can cross national borders.

Ecology: damage to leaves and leaching of nutrients from soils, causing forest dieback (e.g. Ore Mountains, Black Triangle); acidified lakes where aluminium harms fish and diversity falls.

Humans: fine particles formed from SO2 and NOx cause respiratory illness, and acid can corrode water pipes; the loss of forestry, fishing and tourism affects livelihoods.

Buildings: acids dissolve calcium carbonate in limestone and marble and corrode metals, harming historic buildings and statues.

Strategies focus on cutting emissions and reducing damage.

• Emissions reduction: flue-gas desulfurisation, switching to low-sulfur fuel, gas or renewables, catalytic converters, fewer vehicles.
• International agreements and laws because pollution crosses borders (e.g. the Convention on Long-Range Transboundary Air Pollution).
• Treating the effects: adding lime to acidified lakes and soils (temporary, expensive, treats symptoms).

Sulfur dioxide emissions in Europe and North America have fallen sharply since the 1980s, while NOx remains harder to control because of traffic.