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.

Changchun, in northeastern China's Jilin province, had severe air pollution when its heating season opened on Sunday, covering the city with thick, smelly smog, the Changchun environmental protection bureau said.

From November 2013 to March 2014, the average concentration of PM2.5 in the Beijing-Tianjin-Hebei region was 111 micrograms per cubic meter, 1.27 times the annual level, the ministry said.

On Thursday, the ministry released a statement requiring governments to expand their efforts to reduce polluting emissions during the heating season.

They must phase out outdated coal-fired boilers as planned before the season begins and guarantee that emissions from the boilers meet national standards. Any government leaders responsible for ineffective efforts to curb emissions will face tough penalties, the statement said.

Beijing, which frequently experiences severe air pollution in the heating season, will focus on heating facility inspections in November, Zhao Zhiwei, deputy head of the municipal environmental monitoring team, said on Thursday.

"We will keep closely monitoring emissions from boilers and major companies that burn coal, gas and other fossil fuels," he said.

zhengjinran@chinadaily.com.cn