" 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 ethyne is HC2H. Ethyne is the IUPAC name for this species, which is also commonly known as acetylene." . . . "mass fraction of ethyne in air" . . "Climate and Forecast (CF)" .