At 35,000 ft. A jet engine draws in air that is the neighborhood of -30°F. There is quite a bit of moisture up there (clouds). It's mixed with kerosene(jet fuel) that heats that air and that humidity to around 2500°F as it is violently expelled back into the atmosphere that is still at -30. The water vapor condenses to form ice crystals , or what we see as contrails, short for condensate trails. They are most commonly seen when the upper atmosphere is really cold and relatively calm. Any chemicals added to the fuel or engines would endanger the aircraft and the passengers, and it would be instantly vaporized and chemically altered by the heat anyway. Sorry, I don't believe in chemtrails.
Not to mention that spewing any liquid at 35,000 ft would make it so dilute by time it reached the ground that it would be impossible to detect and you would have no idea where to look for it. It would be like homeopathic medicine where you dilute something so much that it's undetectable but people believe it still has magical powers.
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