China warned residents across a large part of northern China on Friday to prepare for a wave of choking smog arriving over the weekend, the worst of which is expected to over capital Beijing, prompting the city to issue its second ever "red alert".
The National Meteorological Centre said the smog would stretch from Xian, home to the Terracotta Warriors, across part of central China, through Beijing and up into Shenyang and Harbin in China's frigid northeast.
The air pollution will begin rolling in from about Saturday evening and last until Tuesday, with visibility in the worst affected areas such as Beijing likely to fall to less than one kilometre, it said.
The red alert is the highest tier of a four-colour warning system. It will be the second time the highest level alert has been issued in Beijing since the city established a pollution warning system in 2013.
In Beijing and parts of Hebei province, which surrounds the capital, the pollution index will probably exceed 500, it said. At levels higher than 300, residents are encouraged to remain indoors, according to government guidelines.
The Beijing city government issued its first "red alert" last week, declaring emergency pollution measures following scathing public criticism that previous bouts of smog had failed to trigger the highest warning level.
Counts of PM2.5 - harmful microscopic particles that penetrate deep into the lungs - regularly exceeded 300 micrograms per cubic metre during last week's red alert, according to the US embassy, which issues independent readings.
The World Health Organization's recommended maximum exposure is 25 over a 24-hour period.
A red alert is triggered when the government believes air quality will surpass a level of 200 on an air quality index that measures various pollutants for at least three days. The US government deems a level of more than 200 "very unhealthy".
In Beijing, a red alert means around half the vehicles are removed from the roads with an odd-even licence plate system enforced, schools are recommended to close and outdoor construction banned.
Beijing is not the only city to have a coloured alert system, and the restrictions rolled out in the most severe cases are broadly similar.
Beijing's neighbouring city of Tianjin also aims to remove about half of all cars from the road in the event of a red alert.
Shenyang said it was issuing an orange warning for the weekend, meaning it recommended people not spend too much time outdoors, while the Harbin government said it expected generally clear skies over the coming days with some smog spells.
After decades of unbridled economic growth, China's leadership has vowed to crack down on severe levels of air, water and soil pollution, including the heavy smog that often blankets major cities.
Beijing's second red alert comes after a landmark climate agreement was reached in Paris earlier this month, setting a course to move away from a fossil fuel-driven economy within decades in a bid to arrest global warming.
City residents have previously criticised authorities for being too slow to issue red alerts for heavy smog, which often exceeds hazardous levels on pollution indices.
Environmental Protection Minister Chen Jining vowed this month to punish agencies and officials for any failure to implement a pollution emergency response plan quickly, the state-run Global Times tabloid said.
Many cities around China suffer from high levels of pollution, with Shanghai schools banning outdoor activities and authorities limiting work at construction sites and factories earlier this week.