Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
1680-7367
1680-7375
2
6
2002
10.5194/acpd-2-2449-2002
http://www.atmos-chem-phys-discuss.net/2/2449/2002/
http://www.atmos-chem-phys-discuss.net/2/2449/2002/acpd-2-2449-2002.html
http://www.atmos-chem-phys-discuss.net/2/2449/2002/acpd-2-2449-2002.pdf
2449
2487
2002-12-16
A novel model to predict the physical state of atmospheric H<sub>2</sub>SO<sub>4</sub>/NH<sub>3</sub>/H<sub>2</sub>O aerosol particles
C. A. Colberg
B. P. Luo
H. Wernli
T. Koop
Th. Peter
Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zürich, Switzerland
The physical state of tropospheric aerosol particles is largely
unknown despite its importance for cloud formation and for the aerosols' radiative properties. Here we show the first systematic
global modelling study of the physical state of the H<sub>2</sub>SO<sub>4</sub>/NH<sub>3</sub>/H<sub>2</sub>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<sub>4</sub>)<sub>3</sub>H(SO<sub>4</sub>)<sub>2</sub>, while there is only little
ammonium sulfate, (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>. Since ammonium
bisulfate, NH<sub>4</sub>HSO<sub>4</sub>, does not nucleate homogeneously, it
can only form via heterogeneous crystallization. As the ammonia-to-sulfate ratios of the atmospheric 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 global importance of
letovicite, whose role as cloud condensation nucleus (CCN) and as scatterer of solar radiation remains to be scrutinized.