Actually, it is most likely that an independent nuke would be a gound burst, just becuase most people will leave it in the car with a long timer, and take a plane. The bulk of the buildings might save your eyes.

That being said, if they really wanted, with a mini nuke, put it on a hand truck and wheel it into an elevator. Not much of an airburst, but it would improve the effects of the weapon. Of course, they could be nice and leave it in a subway tunnel, but if they really wanted to be nice, why would they have a nuke?