Air quality in Beijing is expected to remain at the optimum standard in the coming days, as heavily polluting factories have curbed or stopped production before the World War II victory anniversary parade on Sept 3, environmental authorities said on Monday.

"In all, 1,927 factories in Beijing were required to reduce air polluting emissions by at least 30 percent starting on Aug 20," said Mu Liang, head of a monitoring team at the municipal environmental monitoring center.

"Our teams are conducting random inspections round-the-clock these days, more frequently than usual."