A time-lapse video shows a wave of smog rolling into China's Beijing capital city in what has been called the "airpocalypse", even as authorities downgraded its smog alert from red to orange.
The 12sec video, posted by YouTube user Chas Pope on the video platform on Monday (Jan 2), shows blue sky turning white and eventually smoky grey as smog invades a part of the city.
The Briton working in China said on his Twitter and Instagram accounts that the smog rolled in within 20 minutes.
Mr Pope said the air quality index was above 400 when the video was taken, The New York Times reported on Tuesday (Jan 3).
Readings of 301 to 500 are regarded by the United States government as "hazardous".
Mr Pope's time-lapse video has been retweeted more than 1,400 times and liked more than 1,100 times.
Twitter user E. J. Coughlin said "that's frighteningly awful", while Didi Kirsten Tatlow said it "looks almost like a sandstorm".
Heavy smog in northern China on New Year's Day caused hundreds of flights to be cancelled and highways closed.
Average concentrations of small particles known as PM2.5 were higher than 500 micrograms per cubic metre in Beijing - 50 times higher than recommendations by the World Health Organisation.
Beijing city officials on Tuesday said air quality was improving overall, citing data for the whole of last year, Reuters said in a report.
The city environment bureau said the orange alert was expected to last until Wednesday.
Government officials have expressed frustration that persistently heavy winter pollution, brought about by unfavourable weather and the use of coal-fired urban heating systems, has overshadowed the genuine progress China has made to reduce smog.