I've only visited 4 cities long enough to get a definite smell of them. I think it's only after 2 months here that I've become used to Estelí's smell that when I go away I can notice it.
Toronto: has no smell. Or maybe I just grew up with it so I don't notice it.
Hong Kong smells like sewers.
Reykjavík smells like sulfur.
Estelí smells like tobacco.
Mmmm...yummy permeating nose odours...