Units for use in atmospheric chemistry (IUPAC Recommendations 1995)

This document examines the utility and suitability of the International System of Units (Systkme International, SI) as a framework for units in atmospheric chemistry and presents recommendations regarding use of these units in reporting atmospheric chemistry data consistent with the special requirements of this subdiscipline.In general SI is well suited for application to atmospheric chemistry; specifically the wide range of magnitudes that are encountered can be dealt with by the prefixes denoting multiples and submultiples of units, and the application of SI units leads automatically to consistency in unit calculus.The use of special names and symbols for units that are not part of the SI, and are not products of powers of SI base units is discouraged; examples are atmosphere, Dobson unit, hectare.However, continued use of such units (especially the Dobson unit) alongside SI units is advisable for a time until the SI units become more familiar.Local abundances of substances in air may be expressed as mixing ratios or concentrations.Mixing ratio has the advantage of being independent of pressure or temperature, but concentration has advantages for reaction kinetics and material transport.Amount-based units for mixing ratios or concentrations of substances of known chemical composition are preferable to mass-based units.The dimensionless unit mol per mol of air is recommended for mixing ratio of a substance in air, e.g.nmol/mol, rather than the customary parts per billion (ppb).It is necessary to specify whether a mixing ratio refers to dry or moist air.Expressing