In all, 17 Shandong cities were on high pollution alerts and roughly 7,500 inspectors had been sent to factories and building sites to assess emergency efforts to improve air quality, Shandong’s environmental protection administration said.

Chief CMA forecaster Ma Xuekuan said the smog in the north and east was expected to disperse by Saturday with the arrival of a strong cold front.

In Beijing, residents woke to polluted grey skies that did not clear until 2pm.

Concentrations of PM2.5 – very fine particles that pose the greatest threat to health – were about 117 micrograms per cubic metre near Tiananmen Square by 2pm yesterday, down from around 300 at 11am. The World Health Organisation recommends daily average exposures of no more than 25 micrograms.

But the authorities lifted a red alert imposed on Saturday.

Beijing traffic officials said more than 112,800 motorists were issued tickets for violating restrictions over the four days of the red alert, the highest in the four-tier system. Private cars were ordered off the roads on alternate days to reduce emissions.

Meanwhile, residents in Nanjing (南京), in Jiangsu province, found the city shrouded in a purple haze on Tuesday afternoon as it issued its first orange alert.

Visibility was down to less than 2,000 metres until Wednesday afternoon, the city’s meteorological observatory said.

Photographs of the smoggy sunset circulated widely online, with some people worried that the colour had been caused by a new type of pollutant.

“It’s the first time I’ve breathed in such rose-red smog,” one internet user said.

“Is this a new type of smog?” another asked.

Liu Hongnian, a professor of atmospheric sciences at Nanjing University, said the purple colour in the air was caused by the blending of the grey smog and the evening’s reddish sunlight.