But on Monday, China’s official English-language newspaper, China Daily, ran a front-page story under a boldfaced headline: “Emergency green plan for Games.” The article warned that officials might force far more vehicles off city streets  possibly 90 percent of the city’s total  and temporarily close more factories.

Image Volunteers in Beijings Olympic Village on Monday, under murky and polluted skies. The authorities were considering emergency measures to clear the air. Credit... Jason Lee/Reuters

No timetable was announced, but a senior city engineer told China Daily that officials would inform the public as early as possible about the details of the plan. The Olympics’ opening ceremony is on Friday of next week, Aug. 8.

Pollution has been a pressing concern for the Games. Local organizers have promised to hold a Green Olympics, despite air that often ranks among the most polluted in the world. Some Olympic teams, including that of the United States, are providing optional breathing masks for their athletes to protect them from respiratory problems.

Before Monday, Beijing officials had taken a determinedly upbeat approach to the pollution situation. At various news conferences, Beijing officials said pollution levels in July had fallen 20 percent compared with the same period a year ago. They blamed the problems on recent weeks of unusually heavy rains that left behind a humid summer haze. Even though emissions have fallen, pollution was still being trapped in the haze because of a lack of dispersing winds, they said.

“We are very confident about the effectiveness,” Du Shaozhong, a deputy director of Beijing’s environmental bureau, said of the traffic and industrial restrictions during a news conference on Sunday at the new Olympic media center. “We are going to ensure a good air quality during the Games.”