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ResourceSheet-3_ppm_for_gases[1] - CHEM/ENVS 380 Resource...

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CHEM/ENVS 380 Resource Sheet-3: “ppm” for Gases 1. Most commonly used dimensionless "parts per" units for GAS concentrations: parts-per-million volume = ppmv (or ppm) parts-per-billion volume = ppbv (or ppb) parts-per-trillion volume = pptv (or ppt) Note-1: analogous to “%” (parts-per-cent, or parts-per-hundred) Note-2: “volume” re-enforces the fact that these gas concentrations are expressed as ratios of volumes, moles, molecules, or partial pressure (all inter-related) and NOT in mass. 2. Because many gases in the atmosphere behave similarly to an ideal gas, one can use the ideal gas law: PV = nRT where R is Gas Constant, 0.0821 L atm mol -1 K -1 P is pressure (atm) V is volume(L) T is temperature (K) n is moles to re-express the above units to various other forms shown below. In other words, when you are faced with a gas concentration unit given in any of the above, you have the liberty to re-express it in various other ways according to your needs .
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