. " Mass concentration means mass per unit volume and is used in the construction mass_concentration_of_X_in_Y, where X is a material constituent of Y. 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 the hydroxyl radical is OH. In chemistry, a 'radical' is a highly reactive, and therefore shortlived, species." . "Climate and Forecast (CF)" . . . "mass concentration of hydroxyl radical in air" .