Modeling Tip – August 2005

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:

C_ppb = C_µg/m3 x 24.45/MW

C_ppm = C_µg/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:

C_ppm = 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.