Does a partial rebreather mask breath in CO2?

Answer 1

yes but only the ambient CO2 it is a NITROX mask for elevated O2 between 40 and 60 % the rest is AIR which contains Co2 at atmospheric pressure.. 1 BAR

Addition

Assuming you are talking about some kind of full face diving mask with a regulator attached, you will be rebreathing some gas you have exhaled. So yes, you will be rebreathing CO2 - which will be at a greater concentration as well as a greater partial pressure than the CO2 at atmospheric pressure. But it will be minor amounts. However if you are talking about a hospital type of application then it is a different answer.