"To meet these new standards, especially for ozone and PM2.5, a strategic shift in pollution-reduction measures will be required," says Shao. Unlike sulphuric dioxide and carbon monoxide, which are directly emitted by their sources, ozone and PM2.5 are secondary pollutants that are formed by chemical reactions between a range of different precursors in the atmosphere. “This means we should cast our net much wider rather than focusing on just a few precursors,” says Shao.
Beijing has the most stringent emission controls in China, but a significant fraction of its pollutants come from surrounding regions, says Zhu. During the 2008 Beijing Olympics, for instance, air masses from the south could contribute to 34-88% of peak ozone concentrations in the city. “Beijing’s air quality will not be significantly improved unless the current measures are able to break administrative boundaries,” says Zhu.
Beijing’s approach should be applied in other large cities in the developing world, researchers said at the conference. Otherwise, “megacities will become a main source of global pollution”, says Molina. “How governments meet the challenges of rapid urbanization will determine the quality of life in the future.”