Unusual weather patterns this winter may lead to more smoggy days in Northern China as emissions from coal-fired boilers become trapped close to the ground, the national environmental watchdog warned on Thursday.

Thermal inversions due to the El Nino weather phenomenon - which keep warm and dry air near the ground - and an increase in windless days will reduce the dispersal of air pollution during the heating season, the Ministry of Environmental Protection said.

China, especially its northern region, has a 20 percent higher concentration of PM2.5 - particulate matter with a diameter less than 2.5 microns - in winter than the annual average mainly because of coal-fired heating, a major cause of smog, the ministry said.