Stack Exhaust modelling

Bob

hi everyone, I'm trying to model a stack exhaust in CFX5.5 starting with just a simple stack as my geometry. I've setup Boundary conditions to represent an atmospheric boundary layer profile and have set a mass flow rate for the inlet at the top of the stack. My domain is set to Ideal gas, with buoyancy and the thermal Energy equation option and the RNG Turbulence model. My question is regarding the stack inlet boundary condition. At the moment I have a temperature and a mass flow rate prescribed. If I wanted to try and model the dispersion of a particular constituent (eg NO2) of the plume how would I best accomplish this. Can I just prescribe an additional variable ? What else would I need to define for the additional variable, or is this totally the wrong method of approaching this problem ? Looking forward to your response, Bob