It's fairly simple, it's winter and people are burning cylinders of compressed coal powder by the truckload. I haven't seen any sign of heating oil being used here.
I live in an apartment with nicely designed, modern radiators and the complex is served by a central boiler plant. It may be difficult for me to regulate the heat in my apartment, but the heating works so well that I can barely stand to wear a summer weight shirt at home. Unfortunately, the blasted compressed coal is traditional here and a lot of people use it. All that coal smoke is going straight into the air, with no scrubbers, because it is burned at the household level. The further from the city center one gets, the more you'll find people just burning those cylinders of compressed coal powder in cast iron stoves. They're viewed with a kind of nostalgia or fondness, something like fireplaces and wood burning stoves in the US. I know several restaurants with perfectly good heat that still use the things.
Not only is heat being generated by coal, but the heat isn't conserved and so even more coal gets burned. It doesn't help that "fresh" air is supposed to be a virtue, so the average window in China (if one can get the Chinese to keep it closed) is slightly more effective than cheesecloth for stopping wind, and winds in Northeast China are often fairly strong. Most buildings in the region are brick and the local idea of insulation is ... thicker brick walls.
The air this winter has been particularly bad. Normally, we have strong winter winds that clear the air, but this year we haven't.