Is there any evidence that jet contrails have anything in them other than byproducts of jet fuel? This may just be another conspiratorial non-sense but I was just curious.
Cecil addressed this about 8 months ago:
Well (with luck, this is the first time I’ve tried it)
this should be Cecil on Contrails.
Also a quick search of the General Questions board for “contrails” found some interesting and occasionally relevant opinions and information.
Grumble, mutter, people who type faster than the Scruff… 
No, there is not. The byproduct in question is water. The pressure changes of the air flowing over the airplane cause condensation, but only in certain weather conditions.
You can observe something similar when riding in an airplane, or watching them land, especially in humid weather. The low pressure zones above the wings or in the vortices formed off the wing tips will have clouds in them. Sometimes the vortex ones will extend quite a ways behind the plane. Contrails are just large scale versions of this.
One more thing to keep in mind - contrails are not new. I’ve seen WWII footage of B-17s with contrails. Imagine riding a B-17 with a giant white contrail pointing to you like the finger of God saying “Here, Nazi fighter planes!” Either the nefarious technology has been around for a very long time, and people were stupid enough to use it to the detriment of bomber crews, or it’s just freaking condensation.