I have tried a few searches to find this study to no avail as yet. The figures we are given in the media for atmospheric CO2 concentration derive from a height of 13000feet in HAwaii.Seemingly sea-level ambient concentrations are too variable to make a reliable record.
There was an interesting Horizon program (I think, had to be sure since I tend to forget these dumbed down programs with their endless repetitions of the same statements). I'm pretty sure it was in a program which was discussing Global Dimming in which claims were made that 10-20% less of the sun's energy gets through to us now than it did 30 years ago. Never fazed, this was turned to support the Global Warming perspective by pointing out this was due to pollution and that if global cooling pollution was having this much effect and was masking global warming, how much worse must global warming be than we originally thought?
Anyway, it was noted that in the 3 day period following 9/11 when all domestic flights were cancelled that temperature differences The difference between the highs and lows each day) rose by around 1degC. and reverted to norm once flights were resumed.
Paadoxically this suggests to me that aircraft emissions are contributing to global cooling but obviously I must have missed the point because, as reported above, this is actually a sign that global warming is much worse than we thought.