Reducing urban air pollution (ESS HL)

Reducing urban air pollution at Higher Level: The same strategies as SL, through different cities: Paris, Mexico City, Stockholm, Shenzhen, Medellin and London's 2023 ULEZ. At HL, judge each one through law and economics: who pays, how it is enforced, and whether people accept it.

Three places to act: Air pollution can be managed where it starts (the activity), where it escapes (the release) or where it does harm (the impact). The pollution management model puts these in order.

Real example: Paris acts at all three levels. It closed the riverside expressway along the Seine to cars in 2016 and built hundreds of kilometres of cycle lanes (level 1); its Crit'Air stickers keep the oldest vehicles out of the city (level 2); and it plants trees in squares and schoolyards (level 3).

The most sustainable strategies change how people move, so less fuel is burned in the first place. This is level 1.

Reading a transport graph: Car use rises with incomes and status, and falls with caps, charges and better public transport. Cycling rises with safe lanes and subsidies; it falls as roads get busier and people can afford cars.

Real example: in 1989 Mexico City began Hoy No Circula ('today it does not drive'): each car is banned on one weekday, chosen by the last digit of its number plate. Studies found little effect on air quality, because many families bought a second, often older, car.

A ban needs an alternative: Limiting cars works only if people have another way to travel. Without good public transport, people find a way round the rule.

Laws, charges and money can make polluting more expensive and cleaner choices cheaper. They act at levels 1 and 2.

Real example: Stockholm tested a congestion tax in 2006, and traffic crossing the charging line fell by about a fifth. People were doubtful at first, but after the trial they voted to keep it, and it became permanent in 2007.

Technology can clean up exhausts and chimneys without anyone changing their habits. This is mostly level 2, controlling the release.

Real example: Shenzhen, China, replaced its whole fleet of more than 16,000 buses with electric buses by the end of 2017, so they release no exhaust in the city's streets.

Limits of the technology: Catalytic converters cost money, still release carbon dioxide, and use rare metals (platinum, palladium, rhodium) that are mined and often stolen. Electric vehicles need charging points, and their electricity may come from coal.

Plants can clean some of the air once it is polluted, and they make cities nicer to live in. This is mostly level 3, clean-up.

Remember it as: Trees help, but no city can plant its way out of its traffic.

Real example: from 2016 Medellín, Colombia, planted 30 Green Corridors. The city reports streets about 2 °C cooler, and the trees trap some of the traffic's particles.

Do not overclaim: Trees and green walls clean only a small share of a city's air. In an evaluation, say so: they treat the symptom, not the cause.

Many questions ask you to evaluate a strategy, or to judge one approach against another. Use the same checklist every time, and finish with a judgement.

Real example: when London extended its ULEZ to the whole city in August 2023, the dirtiest vehicles became rarer and City Hall reported falling NO₂. But many outer-London drivers and businesses opposed it, and a scrappage scheme was needed to help poorer owners replace old cars.

How this comes up: HL Paper 1: a fact file on a city, then 'Describe how PM2.5 could be managed' [4]: four different strategies, each matched to the city.