BEIJING - A day after Chinese leader Xi Jinping arrived in Paris for the U.N. climate conference, his country's capital was smothered Monday with some of its worst smog of the year.

Levels of tiny particulates - known as PM 2.5 - topped 600 micrograms per cubic meter in Beijing by late afternoon, according to monitoring by the U.S. Embassy and other institutions. That's about 24 times higher than the level considered safe by the World Health Organization.

The stench of soot hung over the city of 22 million, with the pollution obscuring visibility and prompting some citizens to question the government's claims of working to clean up the air.

"It is almost impossible to breathe in Beijing now," wrote Tianxuan Ke'aiduo, a resident of Beijing's Haidian district, on Weibo, China's main social media platform. "Even our right of breathing freely is deprived. I call for the government to really treat the air."

Beijing's pollution comes partly from vehicle exhaust, but the recent smog is more likely the result of seasonal burning of coal - China's main form of energy and a major source of greenhouse gases, the focus of the Paris summit. Northern China was unusually cold in November, and once the government cranked up coal-fired boilers to provide residential heating, smog levels spiked.

Beijing issued an orange alert for the smog over the weekend, the highest alert of the year. Under the designation, factories must reduce production and heavy vehicles are banned from the city, although enforcement of such measures is thought to be uneven.

According to China's Ministry of Environmental Protection, the heavy smog in Beijing extends across a vast swath of northern China, covering more than 204,000 square miles, an area larger than California. Winds predicted for Wednesday are expected to clear away some of the pollution, but until then, residents are being warned to stay inside.

As part of the Paris talks, China has promised to cut emissions at least 60 percent per unit of GDP compared with levels during 2005.