In an electrodynamic particle trap, experiments with single levitated H2SO4/NH3/H2O aerosol particles have been performed under atmospheric conditions. Four analytical methods provide independent information on the aerosol composition and structure (measurements of Mie scattering, Raman scattering, scattering fluctuations, and of mass). The morphology of the aerosol particles and the water uptake and drying behavior are investigated including the determination of deliquescence and efflorescence relative humidities. In general, the thermodynamic data derived from our measurements are in good agreement with previous work. The observed solid phase is mostly letovicite [(NH4)3H(SO4)2] and sometimes ammonium sulfate [(NH4)2SO4], whereas ammonium bisulfate [(NH4)HSO4] does not nucleate at temperatures between 260 and 270 K despite supersaturation over periods of up to 1 day. This underlines the atmospheric importance of letovicite, which has been ignored in most previous studies concentrating on ammonium sulfate. When the stoichiometry of the aqueous solution in the droplets is chosen as neither that of ammonium sulfate nor letovicite, the particles forming after efflorescence are mixed-phase particles (solid/liquid), representing the usual case in the natural atmosphere. Upon crystallization these mixed-phase particles reveal a range of different morphologies with a tendency to form complex crystalline structures with embedded liquid cavities, but there is no evidence for the occurrence of crystalline material surrounded by the remaining liquid. This liquid possibly resides in grain boundaries or triple junctions between single crystals or in small pores and shows little mobility upon extensive drying, unless the shell-like surrounding solid cracks.
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