Atmospheric mixing ratio
= Atmospheric mixing ratio
{title2=$x_i=n_i/n_{\rm air}$}
= Mixing ratio
{synonym}
A number or mole fraction relative to a specified atmospheric reference population. For ideal gas partial pressures with a consistent wet/dry convention, $x_i=p_i/p$. A quoted ppm or ppb must state whether it denotes this number fraction, rather than a mass fraction.