UNITS FOR USE IN ATMOSPHERIC CHEMISTRY

This document examines the utility and suitability of the Internationa l System of Units (Systeme International, SI) as a framework for units in atmospheric chemistry and presents recommendations regarding use o f 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 pre fixes denoting multiples and submultiples of units, and the applicatio n 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 S I, and are not products of powers of SI base units is discouraged; exa mples are atmosphere, Dobson unit, hectare. However, continued use of such units (especially the Dobson unit) alongside SI units is advisabl e for a time until the SI units become more familiar. Local abundances of substances in air may be expressed as mixing ratios or concentrati ons. Mixing ratio has the advantage of being independent of pressure o r temperature, but concentration has advantages for reaction kinetics and material transport. Amount-based units for mixing ratios or concen trations of substances of known chemical composition are preferable to mass-based units. The dimensionless unit mol per mol of air is recomm ended for mixing ratio of a substance in air, e.g. nmol/mol, rather th an the customary parts per billion (ppb). It is necessary to specify w hether a mixing ratio refers to dry or moist air. Expressing mixing ra tios as ''reduced concentrations'' relative to standard conditions of pressure and temperature leads to ambiguity and should be avoided wher e possible. Gas-phase concentrations may be expressed on the basis of either mole (mol m(-3)) or molecule (molecule m(-3); SI: m(-3)). The u niversal use of a single set of units for gas-phase concentration does not seem forthcoming in the immediate future, although the use of mol (-3) affords advantages of consistency and convenience. An extensive t able of m recommended symbols and SI units for quantities in atmospher ic chemistry is presented.