"Climate and Forecast (CF)" . . . . " Mass fraction is used in the construction mass_fraction_of_X_in_Y, whereX is a material constituent of Y. It means the ratio of the mass of X to the mass of Y (including X). A chemical species denoted by X may be described by a single term such as 'nitrogen' or a phrase such as 'nox_expressed_as_nitrogen'. The chemical formula for CFC12 is CF2Cl2. The IUPAC name for CFC12 is dichloro-difluoro-methane." . "mass fraction of cfc12 in air" .