Why couldn't airlines pay to get a study done which disproves the "nutters'" theories on chemtrails?
Get a university group to do it (cf. a consultancy which will be expensive) and write the expense as CSR. Of course, the chemtrail group might view this as just buying results, notwithstanding the tenets of academic integrity and independence (no matter where the money is coming from). Can't cost a stupidly large amount (even with the cost of testing, time and apparatus) if a number of airlines get into it. I can't even see it as being too awkward (though it'd be nice to collect "trail" samples - probably a bit too difficult).
Also, we all know jet fuel burnt leaves emissions etc. (because of the content of any fuel and your chemistry telling you what happens in a combustion reaction), but the chemtrail group is arguing that governments or other clandestine bodies are putting other chemicals inside that are being sprayed. As long as they aren't confusing the two, that's important.
With the amount of "heated passion" going through this chemtrail group, you'd think they'd be more active agents to get scientific testing or at least a better scientific explanation done? And no, a simple reference to geoengineering plus a few videos from a distance is not scientific; just like an object in the sky in a fuzzy photo that looks unusual and UFO shaped doesn't denote the existence of aliens.
That said, gauging (conjecture perhaps) from the few references and single video I've listened to, I feel the chemtrail group are more of the "can't do right to the wrong people" kind; i.e. we can have the scientific testing with independence, generate peer-reviewed journal papers out of it etc. and they will still quote geoengineering, population control (very Malthusian, by the way) and their videos as "superior" evidence - can't do anything right.....