Beijing, Jan 15 (IANS) A thick blanket of smog is set to return to Beijing and north Chinese cities from Sunday night after a string of clean and sunny days.The National Meteorological Centre said smog will hit parts of north and central China starting Sunday. Xinhua news agency reported. The smog will last till Thursday. Heavy smog shrouded the capital and several cities in north, prompting the authorities to issue red alert for air pollution in the last week of December. Beijing will see mild pollution, the main pollutant being PM2.5 on Sunday, according to the local authorities. Pollution will become severe on Monday and hit medium levels on Tuesday. Air quality will improve on Wednesday with the arrival of cold air.Recently, the Chinese government admitted failing to curb the smothering air pollution with its Environment Minister saying that he felt "guilty" for not being able to contain the deadly smog. Beijing's air is the dirtiest in the world. The city government has taken several measures, but these have not yielded desired results.