Unit Conversion

There are two systems of unit in common use for gaseous and particular pollutants:

Mass per unit volume: usually µg-3. The mass of pollutant is expressed as a ratio to the volume of air. Since the volume of a given parcel of air is dependent upon the temperature and pressure at the time of sampling, the pollutant concentration expressed in these units should, strictly speaking, specify the conditions at the time of sampling.

Volume mixing ratio: usually ppm - parts per million (10-6); or ppb - parts per billion (10-9); or ppt - parts per trillion (10-12). This unit expressed the concentration of a pollutant as the ratio of its volume if segregated pure, to the volume of the air in which it is contained. Ideal gas behaviour is assumed and thus the concentration is not dependent upon temperature and pressure as these affect both the pollutant and the air to the same extent. As a consequence of the gas laws, a gas present at a volume mixing ratio of 1 ppm is not only 1 cm3 per 10-6 cm3 of polluted air, it is also 1 molecule per 10-6 molecules and has a partial pressure of one millionth of the atmospheric pressure.

Since NOX consists partly of NO and NO2, the volume fraction in air ppb equates to a different air concentration in µg m-3 depending on the ratio of NO to NO2. For this reason air concentrations of NOX are normally expressed as µg NOX-NO2 m-3. i.e. all references assume that NOX is in the form of NO2.

Some pollutants (e.g. sulphate, nitrate) are present as particles in the air and the concept of a volume mixing ratio of gases is not obviously applicable. Their concentrations are normally expressed only in µg m-3 units.

Conversion Factors

µg m-3 to ppb:
Equation 1
Pollutant or

Molecular Weight value

Temp (°C): or temp value:
Recalculation required
where: Equation 2

T=absolute temperature (K);
P=atmospheric pressure (hPa)
( Remember that Celcius + 273 = Kelvin)

Kg ha-1 year-1 to kiloequivalents ha-1 year-1:
The unit eq (a keq is 1000 eq) refers to molar equivalent of potential acidity resulting from e.g. sulphur, oxidised and reduced nitrogen, as well as base cations. For example:
1 keq N ha-1 yr-1 is equal to 14 kg N ha-1 yr-1 and
1 keq S ha-1 yr-1 is equal to 16 kg S ha-1 yr-1.

When converting acidifying compounds the valency is important and is connected with the charge on the ion. For example NH4+ or NO3- have a valency of 1. For sulphur, the prevalent ion is sulphate SO42-, which has a double charge (valency). The full process is to convert kg to kmoles, then kmoles to keq. The 'equivalent' refers to the ionic charge on an ion. 1 kmole SO42- is 2 keq, (because of the double - charge) and 1 keq SO4= is 32/2 or 16 kg S.

Recalculation required