I think I'd buy judicial meanz or HipShot's (I can't remember which posted this) idea that they'd use the choppers to dump a load of something into the a/c air handlers of a major office building. But I think a general dump over the city would be wasted. I say this because of the incredibly strong air currents that flow in and around those buildings. Any agent would be rapidly dispersed and therefore rendered ineffective by dilution. It's like when they hit you with tear gas in the army the first aid is always to face into the wind and the effects will quickly dissipate.
Water balloons dropped from a couple thousand feet? Dropped off of buildings?
Silly, huh.
What about nerve agents?