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 ${\displaystyle r_{i}}$, which is defined as the amount of a constituent ${\displaystyle n_{i}}$ divided by the total amount of all other constituents in a mixture:

${\displaystyle r_{i}={\frac {n_{i}}{n_{tot}-n_{i}}}}$

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

If ${\displaystyle n_{i}}$ is much smaller than ${\displaystyle n_{tot}}$ (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 ${\displaystyle \zeta _{i}}$, which is defined as the mass of a constituent ${\displaystyle m_{i}}$ divided by the total mass of all other constituents in a mixture:

${\displaystyle \zeta _{i}={\frac {m_{i}}{m_{tot}-m_{i}}}}$

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