Atmospheric chemists often define the Henry solubility as
Here  is the concentration of a species in the aqueous phase, and  is the partial pressure of that species in the gas phase under equilibrium conditions.
The SI unit for  is mol/(m·Pa); however, often the unit M/atm is used, since  is usually expressed in M (1M = 1 mol/dm) and  in atm (1atm = 101325Pa).