"secondary inorganic aerosols" just means that these particles were created from gaseous substances, either by chemical reaction or by attachment of additional matter. In contrast, primary inorganic aerosols come directly from mechanical or thermical processes - for example dust from construction sites, pm from car engines or industrial furnaces ect.
http://en.wikipedia.org/wiki/Aerosol
So this does not state anything about the origin of these 26%. My wild guess would be to blame industry and agriculture. Can't read the primary source though, maybe they explain more about this in chinese.

Anyway, that's only the distribution of pm 2.5. while it's sure a huge portion of the pollution, it's hardly all of it.