[Kayou]

>Xian can experience heavy pollution in winter, with much of the city’s heating relying on coal.

Have they tried to filter directly the air at the source of pollution? For instance, placing filters on the coal heating facilities. It should be more efficient, as you wouldn't release as many particles in the air in the first place.

[seanmcdirmid]

Filtering at the source reduces efficiency and has overhead in capital costs, so most people ignore it, even at big power plants and factories (otherwise they get creamed by their competitors who are ignoring it). China’s pollution problem is basically a rule of law problem, and it is difficult to fix it barring external solutions (this) or inevitable blanket restrictions (the banning of coal completely this year in and around Beijing).

[chapium]

Article mentions coal based heating being a big culprit, which is most likely used residentially in homes and small businesses.

[Kayou]

I thought it was a central source of coal heating, but if it's in every home, it's probably too difficult to filter then.

[seanmcdirmid]

In city heating is typically central, in then countryside it is by home. When I visited Tianjin in 1999, however, the apartment I was staying at had a coal furnace.