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.

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.

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.