Mixing ratio

In chemistry and physics, the dimensionless mixing ratio is 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 $\zeta_i$, 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/