Many Beijing-area factories and cement plants will close for two months beginning in late July as a key part of the effort to clean the city’s famously polluted air for the Olympic Games, Chinese officials said. Other clean-air measures include banning the use of half the city’s 3.5 million vehicles, disallowing spray paint and other harsh chemicals to be used outdoors, closing about one-tenth of the city’s gas stations, and halting construction in the Beijing area, which now has about 40 square miles of construction sites. Many heavily polluting industries, including coal-burning power plants, have been asked to cut their pollution by about 30 percent by July or risk getting shut down. If Beijing’s air is still too polluted by the time the games are scheduled to begin, the government said it would take “stringent steps” of one kind or another to shut down major sources of pollution. “We will do everything possible to honor the promise,” said Du Shaozhong of the city’s Environmental Protection Bureau. “Just tell everybody they don’t have to worry.”