If you're saying that moles/m^3 of CO2 might be constant while pmm of CO2 is increasing, that implies that atmospheric pressure at sea level is dropping. And it isn't.
Also, who says the pressure isn't dropping? Can that even be measured to detect such a small difference?
And the volume and/or temperature of the troposphere could change instead.
Finally, what about water vapor compensating?
Edit: Sorry, should be "total weight of air". My physics is rusty.