PPM - (PARTS-PER-MILLION)
Parts-per-million is the most widely used expression for concentrations of pollutants in air. It is a mixing ratio expression--volume of pollutant over volume of air. At one atmosphere total pressure, one PPM is one micro-atmosphere. Pressure, volume, mass and temperature are related in the gas law: PV = nRT. Refer to physical chemistry texts. From PPM one may calculate the mass of pollutant gas per unit volume. At ordinary temperature and pressure,if m.w. is the molecular weight of the gaseous molecule, the conversion relationship is:
Micrograms per cubic meter = m.w. x PPM x 41.31
Also see the entry for micrograms per cubic meter.