For a uniform mixture of ideal gases, I think that the concentration of a gas reported in parts per million by volume (ppmV) and micro-moles per mole (μmol/mol) would be the same.
When I see atmospheric CO2 discussed, I have seen both units used, depending on the source. See for example, the comments under this answer.
Since multi-atomic gases like N2, O2, H2O and CO2 are not ideal gases (even monatomic gases are not absolutely ideal) I would guess that for realistic atmospheric samples at different altitudes and times, the CO2 concentrations expressed in ppmV and μmol/mol would not be numerically identical. Further, due to the variations in concentrations of the other constituents they wouldn't even be strictly proportional.
Is there a handy, approximate conversion constant, or function, that is more accurate than unity? Is there a nearly-exact one?