Then, imagine a location on a nearby highway that you know of that is a bottleneck to traffic, such as the beltways that circle a major city.
Just before rush hour, in a coordinated fashion, they dump the school buses on their side, and set them on fire (maybe they have some extra diesel fuel, which burns slowly and is less explosive). Now, the city is clogged, there is no way to drive in or out of the city. If there is a decent rail system, that is easy enough to block using the same method.
Mix in the diesel fuel some of the bad stuff, as it burns a lot of it will be burned off, but their will be enough that is not burned off but just becomes airborne.
Take a few more diesel vehicles with engines that spit out blue smoke (poor tuning, rings shot, whatever). Add the aerosol delivery system to the exhaust, not the fuel, so that as the truck burns off fuel, mixed in the exhaust is the bad stuff. Some of the vehicles are stuck in traffic, and infect the people in cars. Some of the vehicles drive around the city in an ambling fashion, using city streets (since the highways are closed anyways).
You can buy used buses and diesel trucks (cube vans, whatever) for less than $5k each - so $100,000 will get you 20 of them.
Scared yet? (I reposted from thread 1 since I didn't see the pointer to Thread 2).