This is correct. CO2 is well-mixed, both laterally and vertically. Mauna Loa observatory is at about 4200 meters, or 0.6 atmospheres. At that atmospheric heigh

This is correct. CO2 is well-mixed, both laterally and vertically. Mauna Loa observatory is at about 4200 meters, or 0.6 atmospheres. At that atmospheric height, CO2 should generally be within ~10ppm of the concentration anywhere above the atmospheric boundary layer (BL). Below the BL, CO2 can be trapped by topography or temperature inversions, or in general, not be as well-mixed vertically. The height of the BL varies with time and location but generally that's a few hundred meters. Well above 0.6 atmospheres -- say, 0.2 atmospheres, or > 12km up -- the CO2 concentration in ppm can decrease noticeably more.