Reformulating atmospheric aerosol thermodynamics and hygroscopic growth into fog, haze and clouds S. Metzger and J. Lelieveld Max Planck Institute for Chemistry, Mainz, Germany
Abstract. Modeling atmospheric aerosol and cloud microphysics is rather complex, even
if chemical and thermodynamical equilibrium is assumed. We show, however,
that the thermodynamics can be considerably simplified by reformulating
equilibrium to consistently include water, and transform laboratory-based
concepts to atmospheric conditions. We generalize the thermodynamic principles
that explain hydration and osmosis – merely based on solute solubilities –
to explicitly account for the water mass consumed by hydration. As a result,
in chemical and thermodynamical equilibrium the relative humidity (RH) suffices
to determine the saturation molality, including solute and solvent activities
(and activity coefficients), since the water content is fixed by RH for a
given aerosol concentration and type. As a consequence, gas/liquid/solid
aerosol equilibrium partitioning can be solved analytically and
non-iteratively. Our new concept enables an efficient and accurate
calculation of the aerosol water mass and directly links the aerosol
hygroscopic growth to fog, haze and cloud formation.
We apply our new concept in the 3rd Equilibrium Simplified Aerosol
Model (EQSAM3) for use in regional and global chemistry-transport and
climate models. Its input is limited to the species' solubilities from which
a newly introduced stoichiometric coefficient for water is derived.
Analogously, we introduce effective stoichiometric coefficients for the
solutes to account for complete or incomplete dissociation. We show that
these coefficients can be assumed constant over the entire activity range
and calculated for various inorganic, organic and non-electrolyte compounds,
including alcohols, sugars and dissolved gases. EQSAM3 calculates the
aerosol composition and gas/liquid/solid partitioning of mixed
inorganic/organic multicomponent solutions and the associated water uptake
for almost 100 major compounds. It explicitly accounts for particle
hygroscopic growth by computing aerosol properties such as single solute
molalities, molal based activities, including activity coefficients for
volatile compounds, efflorescence and deliquescence relative humidities of
single solute and mixed solutions.
Various applications and a model inter-comparison indicate that a) the
application is not limited to dilute binary solutions, b) sensitive aerosol
properties such as hygroscopic growth and the pH of binary and mixed inorganic/organic salt
solutions up to saturation can be computed accurately, and c) aerosol water
is central in modeling atmospheric chemistry, visibility, weather and climate.
Citation: Metzger, S. and Lelieveld, J.: Reformulating atmospheric aerosol thermodynamics and hygroscopic growth into fog, haze and clouds, Atmos. Chem. Phys., 7, 3163-3193, doi:10.5194/acp-7-3163-2007, 2007.