Nanjing's purple haze has put a spell on Chinese social media as much of the country suffers its latest bout of choking smog.

Buildings in the city loomed large against a violet sky in pictures that circulated on Weibo, a Chinese version of Twitter, and were even tweeted by the official Xinhua news agency.

The tint seemed to be the result of a particularly colourful sunset refracted by the cloud of pollution that settled over the city earlier this week, experts said in online comments.

Counts of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- peaked at 370 micrograms per cubic metre on Wednesday, according to official data from Jiangsu province.

The World Health Organization's recommended maximum exposure is 25 over a 24-hour period.

The purple pictures left many seeing red. Chinese anger at pollution has grown to new levels following a December that has seen some of the worst smog in years.

Beijing declared its first maximum red pollution alert this month, after a thick haze rolled into the capital, blocking the sun and effectively turning day into night.

Nanjing's purple haze, reminiscent of the Jimi Hendrix classic song, had social media users worried at the implications of the psychedelic sunset.

"This is comparable to the London smog during the industrial revolution", said one commentator on Sina Weibo, referring to toxic clouds the covered the UK capital in the early 1950s. "We have to ask why this is happening! Why?!!!"

"When I think about how future children might live under 'gray' skies, unable to even see the stars, my mood also becomes gray", said another user.

Others, however, considered the city's colourful smog to be a gas.

"Let's go to Nanjing and inhale that haze", said one user in a comment that probably would have been appreciated by the drug-loving Hendrix.

"It's really high quality", he added.