In an electrodynamic particle trap, experiments with single levitated H 2 SO 4 /NH 3 /H 2 O 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 [(NH 4 ) 3 H(SO 4 ) 2 ] and sometimes ammonium sulfate [(NH 4 ) 2 SO 4 ], whereas ammonium bisulfate [(NH 4 )HSO 4 ] 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.

Abstract. The physical state of the tropospheric aerosol is largely unknown despite its importance for cloud formation and for the aerosol's radiative properties. Here we use detailed microphysical laboratory measurements to perform a systematic global modelling study of the physical state of the H 2 SO 4 /NH 3 /H 2 O aerosol, which constitutes an important class of aerosols in the free troposphere. The Aerosol Physical State Model (APSM) developed here is based on Lagrangian trajectories computed from ECMWF (European Centre for Medium Range Weather Forecasts) analyses, taking full account of the deliquescence/efflorescence hysteresis. As input APSM requires three data sets: (i) deliquescence and efflorescence relative humidities from laboratory measurements, (ii) ammonia-to-sulfate ratios (ASR) calculated by a global circulation model, and (iii) relative humidities determined from the ECMWF analyses. APSM results indicate that globally averaged a significant fraction (17-57%) of the ammoniated sulfate aerosol particles contain solids with the ratio of solid-containing to purely liquid particles increasing with altitude (between 2 and 10 km). In our calculations the most abundant solid is letovicite, (NH 4 ) 3 H(SO 4 ) 2 , while there is only little ammonium sulfate, (NH 4 ) 2 SO 4 . Since ammonium bisulfate, NH 4 HSO 4 , does not nucleate homogeneously, it can only form via heterogeneous crystallization. As the ammonia-to-sulfate ratios of the atmospheric H 2 SO 4 /NH 3 /H 2 O aerosol usually do not correspond to the stoichiometries of known crystalline substances, all solids are expected to occur in mixed-phase aerosol particles. This work highlights the potential importance of letovicite, whose role as cloud condensation nucleus (CCN) and as scatterer of solar radiation remains to be scrutinized.

Since the product of experimental volume and suspension time is equal to that of stratospheric droplets over an entire winter, the homogeneous formation of the octahydrate at quasi-thermodynamic equilibrium can be practically excluded.

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