Question about fluid dynamics (calculating concentration)

Hi all,

I have a question about calculating the concentration of continuous air and carbon monoxide flowing through a pipe. If the gas cylinder originally has 20% CO and 80% air and it will be diluted by another gas cylinder of air, how do you account for temperature, pressure, and 'viscosity' of the gases when calculating the concentration in ppm?

Any advice would be appreciated.
Thanks for your help.