Thank you again for your reply. 

I'm trying to model the effect of air ventilation on a gas that is introduced in the fluid domain with two sources. The first source is a constant in kg/m3s which results in a concentration build-up within the domain of the gas and the second negative source is the one I mentioned above which has the overall effect of removing some of this gas. In the end, I'm interested in the effect of both the ventilation and the negative source term on the concentration (kmol/m3) of the gas that is building up inside the domain. The mass fraction has no units so it wouldn't work with my source which must have the units kg/m3s. So there is no micro to access species concentration in kmol/m3 which I can then use with the source I mentioned above?