It's been happening since 1992, if not sooner. Generally, they add stuff to the jet fuel. It is possible that not all of the pilots are aware of all of the ingredients in their jet fuel or they believe in whatever that they think they are spraying, if they are spraying. Some of it may be spray and some of it may be simply jet exhaust that had other things added to it. Things like Lithium. People who don't know about geoengineering should investigate whether they want to digest lithium, etc. NASA confessed that they are spraying poisonous chem-trails according to this 2016 article.