At some point, you may wish to convert concentrations in micrograms per cubic meter (µg/m3) to concentrations in parts per million (ppm) or parts per billion (ppb). For example, the ISC and AERMOD models generate output in µg/m3 while you may need results in ppm.
This conversion can be achieved using the following equations:
Cppb = Cµg/m3 x 24.45/MW
Cppm = Cµg/m3 x 0.02445/MW
where C = Concentration of the contaminant MW = Molecular weight of contaminant 24.45 = Constant representing the volume, in liters, of one mole of a gas at standard temperature (25°C) and pressure (1 atmosphere).
For example: 100 µg/m3 of SO2, which has a molecular weight of 64, converted to ppm:
Cppm = 100 x 0.02445/64
= 0.0382 ppm = 38.2 ppb
ISC-AERMOD View is capable of performing this conversion for you: simply go to Tools | Concentration Converter, and select µg/m3 to ppm. All you need to do is specify the molecular weight (MW) in grams per mole of the specific pollutant you are modeling and click convert. This will generate a new plotfile with concentrations in ppm.