A cold front that swept choking smog from northern China couldn't have come sooner for Beijing's mayor.

After days of hazardous pollution forced people to wear masks and huddle indoors, residents of China's capital turned their attention to the mayor, Wang Anshun, and his bold vow last year to clear the air. If pollution wasn't brought under control by 2017, Wang said at the time, he would cut off his own head and present it to the country's leadership.

"Hi Mayor, I'm here, waiting for your head," a person using the Internet handle "I Love Rao Zizhao" wrote Tuesday on a discussion board on the Weibo social network, with dozens of others reposting the comment. Earlier, the official count of PM2.5 — the particles that cause the greatest risk to human health — had exceeded 500 micrograms per cubic meter, more than 20 times the World Health Organization's recommended limit.