I thought I might answer my own question.
The absorption of radiation by matter follows a decreasing exponential relationship.
I = Iº exp(−α n X) where
Iº is radiation flux at origin, I is radiation flux at distance X from origin, n is number of molecules per m³, α is absorption cross section m²/molecule.
This equation is derived here http://astrowww.phys.uvic.ca/~tatum/stellatm/atm5.pdf
It is a version of the Beer-Lambert law.
The international standard atmosphere at sea-level is T = 288K, pressure = 101325 Pa.
For a relative volume of CO2 of 0.04%, this gives a partial pressure of CO2 = (101325 x 0.0004) Pa
Solving the ideal gas law pV = nRT using the partial pressure of CO2, and applying Avogadro’s number as the number of molecules per mole, gives 10.2 x 10^21 molecules of CO2 in 1 m³.
Now we can determine the distance to absorb a given proportion of incident radiation, by using the equation above, and solving for X.
We need to know the absorption cross section of CO2.
This is found here http://vpl.astro.washington.edu/spectra/co2.htm from the PNNL link cm2/molecule vs. wavenumbers.
I am interested in the 15µm wavelength (667 per cm), which looks to be about 4.5 x 10^-18 cm2/molecule (or 4.5 x 10^-22 m²/molecule)
So now, for 99% absorption, or I/Iº = 0.01, we can solve for X.
The result is almost complete absorption in 1 metre!
The result is sensitive to the order of magnitude of the absorption cross section. It compares to the sometimes quoted experimental finding of Heinz Hug of complete absorption at 10 metres.