title: Formation and transformation of metastable double salts from the crystallization of mixed ammonium nitrate and ammonium sulfate particles.
abstract: Ammonium nitrate (AN) and ammonium sulfate (AS) are ubiquitous components of atmospheric aerosols. Thermodynamic models predict formation of pure (AN and AS) and double salts (3AN. AS and 2AN. AS) for the AN/AS system. Because of the high supersaturation at which a droplet crystallizes, metastable crystal formation is possible. In this study, the identity of the crystals formed from the crystallization of equimolar AN/AS mixed droplets was investigated in an electrodynamic balance coupled with a Raman spectroscopic system. Raman spectra of bulk AN/AS double salts possibly formed in this system are first reported for comparison with the single particle Raman results. The double-salt 3AN. AS, not predicted from thermodynamics, was observed in the freshly crystallized single particles. The degree of metastability can be different among several crystallization processes of the same particles. The metastable salt 3AN. AS gradually transformed into stable 2AN. AS, and the rate of such transformation increased with increasing relative humidity. This study illustrates the possibility of occurrence of metastable salts in atmospheric aerosols.
authors.affiliation: Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: Tsz Yan
authors.initials: TY
authors.lastname: Ling
authors.forename: Chak K
authors.initials: CK
authors.lastname: Chan
mesh_terms: D000645:Ammonium Sulfate / Q000737:chemistry*; D003460:Crystallization; D009566:Nitrates / Q000737:chemistry*; D010316:Particle Size; D012492:Salts / Q000737:chemistry*; D013059:Spectrum Analysis, Raman
chemical_list: D009566:Nitrates; D012492:Salts; D000645:Ammonium Sulfate; C006568:ammonium nitrate