An excerpt from a section about the capital of Taiwan, in an article I wrote about Air pollution in Taiwan:
Further more, in greater Taipei, the outdoor PM2.5 concentration in the air at ground-level up until the height of three-stories is around ten to twenty times higher than the concentration in the air at the height of four-story buildings and above. Levels of other hazardous suspended particles such as silicon and iron were equally found to be significantly lower when increasing altitude. Professor Chang-Fu Wu (吳章甫) of the same university attributed this to dust and traffic pollution.
So, my question is, can this increasing pollution, when decreasing in altitude, be extrapolated to a negative (as compared to street-level) altitude as well, in a typical city? Intuitively, I would assume that parkings (don't forget about traffic tunnels) can be pretty bad, when thinking of sub-terrain places in a typical city, with regards to pollution? But what I am really interested in here, is finding out more about the increase in pollution in subways (underground transportation), as compared to the street-level.
Is this problem perhaps being mitigated partly by sucking in air from a high altitude? (this shouldn't be seen as the question, it's just a remark)
P.S. (this is also not part of the question, so please don't close this question because I wrote a remark): I think finding higher ground is actually one of the best ways to escape from pollution in cities. But of course many cities will not give this advice to its citizens ... it might be unpopular and sound bad.