Carbon dioxide concentration from pressure changes
If you know the partial pressures of the gases, then you can calculate the mass fractions.  In the present case, assume you have 1 mole of gas.  Then the number of moles of CO2 is 0.167 moles and the number of moles of air is 0.833.  The mass of CO2 is (44)(0.167)=7.33 grams and the mass of air is (29)(0.833)=24.2 grams.  So the mass fraction of CO2 is $$\frac{7.33}{(7.33+24.2)}=0.233$$
So the ppm CO2 by volume = 167000 and the ppm CO2 by mass = 233000.