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.
Carbon monoxide is largely related to combustion engine usage; CO levels were lowest on the electric trams, and higher on the other modes of transportation. Carbon Dioxide, on the other hand, is largely related to other passengers breathing; CO$_2$ was highest on the crowded vehicles and much lower for pedestrians. CO$_2$, unlike the others, I don't think is a chronic health risk at the observed concentrations.
There is a lot of information in that paper, and I am not really qualified to judge the health risks from one factor against another. I can note that in no health measures does the subway have the worst air quality between the four transportation modes; and there is no other mode that has better air quality than the subway in all measures.
I suppose it is safest to say that, in a statistical sense across multiple measures, the subway air is not significantly more unhealthy than street level air.