Mixing ratio

In chemistry and physics, the dimensionless mixing ratio is defined as the abundance of one component of a mixture relative to that of all other components. The term can refer either to mole ratio or mass ratio.^[1]

Mole ratio

In atmospheric chemistry, mixing ratio usually refers to the mole ratio r_i, which is defined as the amount of a constituent n_i divided by the total amount of all other constituents in a mixture:

$r_i = \frac{n_i}{n_{tot}-n_i}$

The mole ratio is also called amount ratio.^[2]

If n_i is much smaller than n_tot (which is the case for atmospheric trace constituents), the mole ratio is almost identical to the mole fraction.

Mass ratio

In meteorology, mixing ratio usually refers to the mass ratio ζ_i, which is defined as the mass of a constituent m_i divided by the total mass of all other constituents in a mixture:

$\zeta_i = \frac{m_i}{m_{tot}-m_i}$

The mass ratio of water vapor in air can be used to describe humidity.

References

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "mixing ratio".
2. ^ http://www.iupac.org/publications/pac/80/2/0233/