According to Urban Dictionary, "city snot" comes from when "you live in a densely urban area with lots of pollution. You come home and blow your nose and see black boogers from all the smog and pollution. City snot - gross!"
In his book Understanding, J. Maarten Troost writes that "Every single person in China excretes physically impossible quantities of spit, snot and phlegm in public every single minute of every single day."
In terms of the capitol city I call home, The New York Times claims that for children, living in D.F. is equivalent to smoking two packs of cigarettes a day. By 1999, residents of the capitol were already breathing in "tiny airborne particles of bus exhaust, industrial smoke, garbage and what the authorities refer to, with scientific detachment, as fecal matter."
While my snot isn't exactly black (and I hope my exposure to fecal matter is limited), there is certainly more of it. Running is particularly a problem because even though I do it in the city's nursery, if I do not spit out my accumulating mucous my ears will be plugged up for the rest of the day.
It is also not uncommon to see people wearing surgeon's masks to block out the contamination, but I have heard it is to no use. This makes me at least feel less guilty for not wearing one.
And yes, the picture above is my own, proving that esmog is not merely a scientifically measurable phenomenon but rather visible to the naked eye on any day descending by plane into Mexico City.