Following the "airpocalypse" in the city of Harbin this week, a question now hovers in the minds of many residents across northern China. It was summed up in a headline that ran on Thursday with a commentary in People’s Daily, the Communist Party mouthpiece: "In this season of central heating, will PM 2.5 drop?"

The fetid smog that settled on Beijing in January 2013 could join the ranks of these game-changing environmental disruptions. For several weeks the air was worse than in an airport smoking lounge. A swathe of warm air in the atmosphere settled over the Chinese capital like a duvet and trapped beneath it pollution from the region’s 200 coal-fired power plants and 5m cars….

The "airpocalypse" injected a new urgency into local debate about the environment—and produced a green-policy frenzy a few months later.

Dust, smog and heavy wind made Li Yuan's weekend trip to Beijing's southern suburb in search of an affordable apartment a nasty experience….

January's "airpocalypse," as it has come to be known, has pushed sales of PM2.5-blocking masks skyrocketing in the past few months.