How do I convert tons to ppm? I'm trying to do this for CO and NOx.
Ppm means parts per million. It is used for dilute concentrations of one thing in another thing. For gases it usually refers to a dilute solution of a pollutant gas in air. So it is a ratio of how much CO (for example) in how much air. As a minimum you would also have to know how many tons of air.
However, gas parts per million is generally a ratio of mole fractions (it can also be ppm by weight or volume. So if you have masses of each gas, you would have to divide by molecular weight to get moles.