Unseasonable weather swept over Beijing during the summer, covering the capital in smog and haze for 18 days during June and resulting in the highest number of high-pollution days in the month for 50 years, according to the China Meteorological Administration.

It's still more than two months before the start of the winter heating season, during which huge amounts of coal are burned and, with ongoing efforts to relocate almost all the polluting industries outside the city, vehicle exhaust fumes have been pinpointed as a key source of pollution during bad weather.

The government has brought forward the timetable for upgrading the quality of both gasoline and diesel fuels. The move came in response to statements by environmental experts that oil-based fuels are the biggest obstacles to solving pollution caused by motor vehicles.