SPECIATION OF AMMONIUM-CONTAINING SPECIES IN ATMOSPHERIC AEROSOLS

A method for the speciation and separate determination of inorganic ammonium salts in atmospheric aerosols has been developed. It is based on sampling by an annular denuder-filter-pack system, in order to avoid chemical artifacts followed by thermal dissociation of NH4NO3 and NH4Cl from the Teflon pre-filter, and collection of evolved gases on back-up filters. The predominant form of thermostable sulfate partially or totally neutralized by NH3 is then identified and quantified from pH and [H+]/[SO42-] ratio measurements. Results of field experiments performed near Rome show that NH4NO3 and (NH4)2SO4 constitute the major species of the fine particle fraction. NH4Cl represents approximately one-fifth of NH4NO3 and one-tenth of the total ammonium species in terms of mass concentration.