What's your city smell like?

DARPA is looking for a way to detect chemical attacks by gathering data on what various cities smell like. First they plan to make smell profiles for major urban areas. Then they'll need to take this data and figure out how to put it into some sort of map that a constant monitoring system could use. Theoretically these systems could detect any changes in the chemicals in a city's atmosphere rather quickly. I'm just curious to see the results once they have the data.