Lingering smog has severely reduced the visibility in northeast China's Liaoning Province, where air and highway traffic was forced to be partially closed on Thursday morning. The Shenyang Taoxian International Airport closed runways Thursday morning citing low visibility, and notified stranded passengers that the traffic may be resumed at 10 a.m., given weather permits. More than 10 flights via the airport were delayed Thursday morning. Air quality in the northern China including Beijing has dropped sharply since Sunday, and the smog has extended to eastern and central China regions, such as Shandong and Hubei provinces, which saw parts of the expressway traffic partially closed in the past two days. The national capital of Beijing has maintained a yellow smog alert since Tuesday. By 7 p.m. on Thursday, the density readings of PM2.5 -- airborne particles measuring less than 2.5 microns in diameter -- reached 390 in urban districts, according to the Beijing Municipal Environmental Monitoring Center. A cold air can be expected on weekend, when most of Beijing and the neighboring area will have favorable weather to diffuse air pollutants, according to the municipal meteorological center.