Can anybody come up with a chart showing CO2 atmospheric
concentrations as a function of altitude?

CO2 is 150% of the density of air so it should sink to the ground.

http://en.wikipedia.org/wiki/Carbon_dioxide "At standard temperature
and pressure, the density of carbon dioxide is around 1.98 kg/m³,
about 1.5 times that of air."

If you have a open top container of CO2 it will remain in the
container until it is poured out like you would pour water.

Paul Lamar

There is actually a limit on upper atmosphere, called "Turbopause" :) -
where this mixing ceases.
http://en.wikipedia.org/wiki/Turbopause

Related thing:
The mesosphere lies above the maximum altitude for aircraft and below
the minimum altitude for orbital spacecraft. It has only been
accessed through the use of sounding rockets. As a result, it is the
most poorly understood part of the atmosphere. This has caused
scientists to refer disparagingly to the mesosphere and lower
thermosphere as the ignorosphere
http://en.wikipedia.org/wiki/Mesosphere

Mesosphere and upper stratosphere play a significant role on long-term
global temperature trends, and still, they are not understood too
thoroughly.

Upper atmosphere... the final frontier :)
(At least on "garage aerospace" budget)
I'm on this :)

Cheers, Petri