P_(CO_2) = 0.288"mmHg" if %CO_2 is mole percent. If %CO_2 is weight percent then one will need the mass values of the other components of the air mixture.
Explanation:
Given 0.038% (mole/mole) => X_(CO_2) = (0.0038"mole" CO_2)/(100"moles"AIR") = 0.00038