Unit conversions
There are two main systems of unit in common use:
Mass per unit volume: usually µg m-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:
T=absolute temperature (K);
P=atmospheric pressure (hPa)
( Remember that Celcius + 273 = Kelvin)
Kg ha-1 year-1 to kiloequivalents ha-1 year-1:
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 N ha-1 yr-1.