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Units of measurement

Air quality measurement are commonly reported in terms of:

  • micrograms per cubic meter (µg/m3)

  • parts per million (ppm) or parts per billion (ppb)

For particulate matter, sizes are expressed in micron or micrometer.

ppm is a volume-to-volume ratio, which makes it independent of local temperature and pressure.

Unit conversions

Under standard conditions (0° Centigrade, 101.325 kPa), one mole of an ideal gas occupies 22.414 liters. The mass of a pollutant p, Mp in grams can therefor be converted to its equivalent volume Vp in liters:

Vp = Mp/gMW * 22.414 l/g

with MW the molecular weight of the pollutant. For measurements at pressure and temperature other than the standard conditions, corrections to the standard volume must be applied, based on the ideal gas law:

22.414 l/g * (T/273.15K)  * (101.325 kPa/P)

where T and P are the ambient temperature and pressure at the time of measurement, respectively.


ppm = Vp/Va
where Va and Vp are the air and pollutant volume, respectiviely. Combining the equations gives the conversion formula:
ppm = [Mp/gMW * 22.414 * T/273.15K * 101.325kPa/P] / Va * 1,000 l/m3

with the pollutant mass in µg (microgram).

As an example, consider the following conversions:

Substance Mass formula Mol.weight µg/m3 ppm ppb
Nitrogen DioxideNO2 461 0.000500.50
Nitrogen OxideNO 301 0.000770.77
Sulphur DioxideSO2 641 0.000370.37
OzoneO3 481 0.000480.48

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