We are surrounded by military bases where I live. When there was a call up a few years back, there were jets taking off and flying over continuously for hours. A lot of air traffic here are military jets, not commercial flights. Plus private jets can fly any time. There's still no reasonable motive for spraying the whole country with chemicals. They could just as easily put it in the water or in the food. In Flint Michigan and other places you can see the chemicals in the water. There's no secret about it.

An interesting topic. I read the full report cited in the Smithsonian article.
http://iopscience.iop.org/article/10.1088/1748-9326/11/8/084011/meta

The conclusions were based on surveys and scientist reviews of chemtrails websites and claims. Part of the problem seems to me to be that the majority of what is seen in the sky are actually contrails, ice crystals formed by jet exhaust water vapor (which can last for hours depending on humidity and atmospheric conditions), so the chemtrails, also known as SLAP, Secret Large scale Atmospheric Spraying, if they exist, would be secret operations hidden in plain sight.

The other problem is one of logic, it's not possible to prove a negative assertion. So the assertion that chemtrails don't exist can not be proven. In the study, 78 of 79 scientists concluded that they fouund no evidence for the existence of secret atmospheric spraying programs, not that they don't exist. Much the same, no one can prove UFO's don't exist, or that I'm not a millionaire, even though no evidence has been brought forward to conclusively prove any of these assertions.

Finally, I for one don't see any motive for the existence of chemtrails. Industry is already polluting the planet in plain sight. Why engage in secret pollution as well? And from a practical point of view, whether all, some or none of the assertions are true, hiding from cell phones, wifi, electro magnetic radiation, chemicals in food and water, air, mass brainwashing by the media, etc. is just too much trouble. It's not like we all aren't going to die anyway.

While I do believe in many things that the news would label conspiracy theories my feeling about chemtrails is that it ought to be easy enough to prove or disprove whether they are harmful or not. Unlike UFO's anyone with a helicopter or private plane can fly through a chemtrail and collect air samples.
The fact there are so many in the sky all day long ought to make it easy to conduct unbiased testing of the chemtrail's components. But to my knowledge no one has done that and published test results. Environmental scientists are all over ground, air, soil and water polution, but none that I know of have published peer reviewed studies on chemtrail pollution.