"A semi-truck, or even an SUV both would be able to carry myuch more of that bad stuff, and to disperse it equally well -- or better. Get some common sense."
Well, you couldn't be more wrong. A ground vehicle would disperse either an explosion or chemical agents far worse than an aircraft, regardless of scale. The one's lacking common sense on this thread are you and anyone who agrees with you. I am definitely glad you are not in charge of my security.
If the plane was a crop duster that's a different story, perhaps. It wasn't. Even then -- a ground vehicle by not being noticed while releasing it's slow-acting agents -- can be much more effective. It can get closer to crowds.
Air releases are not that effective -- the evil Iraqis who sprayed the Kurds had to make many passes -- the stuff blows away from the targets too easily.
The Germans in WWI has similar problems with ground releases -- the wind changes and the chlorine and mustard gases was back on top of them. Proximity and confined spaces are ideals for chemical and biological weapons. Airborne is not the way.
We were secured by folks like you, and still are, eh? Exmine every grandmother up the waz at airports, but keep the borders open. I'm not happy at all that folks with that rare combination of hubris and chicken-little-ism still run too much of domestic security. Bureacratic inertia and grinding procedureal indignities punctuated by knee-jerk panic.