Concentration Units for Air Folks

A couple of customers have recently asked about standard concentrations and converting from ppm_v to mg/m³. Honestly, I have no clue how I have never blogged about this subject, especially considering how fundamental and critical this topic is. So here we go:

Air concentrations at ppm are parts per million by volume and should therefore be expressed as ppm_v. Although sometimes people forget the “v” at the end, it is implied. Unless of course they meant parts per million by mole, which is sometimes used for air as well; and just so happens to be conveniently identical for an ideal gas and practically identical for most compounds in air at standard temperature and pressure (STP).

But for the most part it should go as follows:

Which you may then conceptualize as:

Now… I have encountered some confusion as to the fact that ppm_v may be directly interpreted as mg/m³. Please do not make this mistake! The water and soil folks have the luxury of jumping from ppm to mg/L. But that is because they are not dealing with the density of gases, which are temperature and pressure dependent.

So how do we go from ppm_v to mg/m³ (this is exactly the same for ppb_v to µg/m³)… here is how:

Where T = temperature in K = 273.15 + °C and 0.08205 is the Universal gas constant in L atm K^-1 mol^-1, which you may find on the good ole interweb.

Or you could stick to the short version of the aforementioned, which only requires you to remember an ideal gas will occupy 24.45 L/mol at 1 atm and 25 °C. Yes, not the 22.4 L/mol you see littered all over the internet. Unless of course your STP includes 0 °C. Therefore, the above equation simply reduces to the following:

Remember, use the exact same equation for ppb_v to µg/m³.