*ears tingle at the mention of 'organic chemicals'*
They say that truth is stranger than fiction, but it's also a lot simpler. All the reports of the DOD experiments on the US population have involved things like adding components to public drinking water, directly spraying pathogens on the population with a method akin to crop dusting, and releasing pathogens in subway systems.
In all that I've read, however, the most commonly utilized unwilling or uninformed population in experimentation has been US Military personnel rather than the public at large. The military has quite a bit of control over its enlisted forces, and they have little recourse.
Crap like the chemtrails conspiracy makes it more difficult to talk about unethical experimentation in the US without being dismissed as a crackpot. It makes anyone who is concerned with chasing down an unethical practice sound like they should be wearing a tinfoil hat.
Now someone needs to suggest that the government started and spread the chemtrails theory to take legitimacy away from valid complaints about government abuse.
Clouds and cloud systems can be persistent - hasn't anyone propagating this theory ever seen a hurricane? You can go to the satellite pages and watch the massive cloud systems roll off of Africa, across the lower Atlantic, and form up into tropical storms and hurricanes. I did it all the time when I was in a high risk area.
Volatile organic solvents tend to have a depressed boiling point - it's one of the qualities that make organic chemicals so useful in research and spectroscopy. They would be volatilized by the jet engine and the fumes would rise, not sink.
High heat also denatures proteins. If you've ever scrambled an egg, you can watch protein denaturation in progress. Microbes are made up of proteins, and they can't survive the process. There are some microbes that can survive those kind of temperatures, but if they were being distributed across a wide area, there would be quite a few very sick, sick people since the microbes that can survive heat like that also tend to be extremely nasty.
In short, everything about this theory is insulting to me as a scientist.