I don't mean to be alarmist, but it seems to me that an aerosol would not be that hard to produce. (I assume you are talking a mean hydraulic radius on the order of 10 microns).
According to what I've read, the aerosol is hard to produce. You need a big lab and lots of equipment which costs a lot of money. What I want to know is if this stuff is so easy to produce and set off, how come some random nut (not even Arab, but just some loser chemist who's pissed off at the world) hasn't done this before? Those psycho Japanese who let of the sarin gas in Tokyo had millions of dollars to work with and tried to set off anthrax a few times, but didn't kill anyone with it. Unless the technology has got that much better in the last 10 years (it probably has got better, but I don't believe it is that advanced yet), I just don't see it happening on a large scale basis without the help of some rogue nation.
A terrorist would be better off blasting a shotgun into a schoolbus or just dumping toxins into sinkholes where they could enter the local water supply.
Besides, if the objective is to terrorize, you don't need to actually commit additional attacks once you've bombed the WTC. At least not for a while. You can thrive off of the scares and panics quite well, as they are just as disruptive. Just stealing a crop duster is good enough to set off a panic. Or just shoot it down or saboutage it and get rid of the evidence, then report it missing so the government can't deny it. People will assume the worst all by themselves. Steal a hazmat truck... make sure the press knows. People will assume the worst, even though you can easily use any kind of truck as a delivery device. Scaring people is easy- and cheap- once you have already committed one atrocity.
Once people cry wolf enough times and grow jaded, you can always pull off a real act of terrorism to start the cycle again.