Chemtrails and spectroscopy

Occam's Razor

Chemtrails would be a very inefficient delivery method of anything and hideously expensive.



since you are talking about 10Km altitude sampling isn't really an option

you would be looking at signatures in the wavelengths of sunlight blocked , and 10km away you would need pretty big telescopes

In theory you could try and use lasers and see what fluoresces but pointing lasers powerful enough to be of use at planes will get you locked up pdq. (lasers pointers, point both ways)

Somewhere like Donegal would average about an hour of sunshine per day in winter. Don't forget that if they wanted to hide from detection they would simply do it at night. If they were clever enough that no one has proof after this length of time they would no doubt have procedures in place to prevent basic errors.



You can be sure that if some researcher had found evidence about unusual species in the atmosphere it would have been published.


Barium
What biological role does Barium have , assuming you could find a way to keep it in the atmosphere as I'd imagine by the time it arrived at ground level acid rain would have converted it to barium sulphate which is fairly inert


detection limit 0.7ng That's one 25 billionth of a teaspoon full

It's also the 14th most common element in the earths crust, being more common than stuff like carbon or sulphur , so would be very hard to detect an increase in it at ground level