A novel model to predict the physical state of atmospheric H2SO4/NH3/H2O aerosol particles C. A. Colberg, B. P. Luo, H. Wernli, T. Koop, and Th. Peter Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zürich, Switzerland
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
H2SO4/NH3/H2O 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, (NH4)3H(SO4)2, while there is only little
ammonium sulfate, (NH4)2SO4. Since ammonium
bisulfate, NH4HSO4, does not nucleate homogeneously, it
can only form via heterogeneous crystallization. As the ammonia-to-sulfate ratios of the atmospheric
H2SO4/NH3/H2O 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.
Citation: Colberg, C. A., Luo, B. P., Wernli, H., Koop, T., and Peter, Th.: A novel model to predict the physical state of atmospheric H2SO4/NH3/H2O aerosol particles, Atmos. Chem. Phys., 3, 909-924, doi:10.5194/acp-3-909-2003, 2003.