How can I calculate the percent dilution of a solute in gas phase after adding air to a sample given the initial and final O2 percent assuming normal atmospheric conditions?

I'm trying to estimate how much would the concentration of a gas (e.g. CO) change after adding air assuming that I can measure the %O2 before and after the addition. Air of course about 21% O2 and 79% N2 and other gases.

Thank you very much in advance.