The strongest evidence that particles in the upper atmosphere will have a global cooling effect comes from Volcanic Eruptions in the last three hundred years when we have had temperature records in many parts of the world.
There is lots on this on the internet, but just take this as a typical example “the eruption of Mount Pinatubo in the Philippines in 1991 cooled global temperatures in 1992 by around half a degree.” From http://www.dandantheweatherman.com/Bereklauw/yearnosummer.html
And the year 1816 without a summer, caused by Mount Tamboro.
If you want a real “sitting on the fence” report on how this would relate to aircraft condensation trails try http://www.rcep.org.uk/aviation/DefraAnnex.pdf
But there is lots more. I believe that the volcano evidence is more relevant to the present proposal because those particles remain in the atmosphere for a long time while the condensation trails disperse and evaporate fairly quickly –or they have done up to now!