BEIJING — Emergency measures came swiftly in Harbin, the northeastern city blanketed with hazardous smog this week: Schools were shut down, buses ordered off the roads, the airport closed, police roadblocks set up to check tailpipe emissions from cars. City officials even fanned out in the surrounding countryside, ordering farmers to stop burning the cornstalks left in their fields after the harvest. They were reacting to the first notable surge of air pollution in China this autumn. Residents across the nation’s north fear that the smog is a sign of things to come. With winter approaching, cities north of the Huai River are turning on their coal-fired municipal heating systems, whose emissions were found in one study to shorten residents’ life spans by an average of five years. In Harbin, moist air trapped the pollution at ground level, leaving people to walk through a gray miasma wearing face masks. […]