A curved multi-component aerosol hygroscopicity model framework: Part 1 – Inorganic compounds D. O. Topping, G. B. McFiggans, and H. Coe School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, The Sackville street building, Sackville street, Manchester, M60 1QD, UK
Abstract. A thermodynamic modelling framework to predict the equilibrium behaviour of
mixed inorganic salt aerosols is developed, and then coupled with a
technique for finding a solution to the Kohler equation in order to create a
diameter dependent hygroscopic aerosol model (Aerosol Diameter Dependent
Equilibrium Model – ADDEM). The model described here provides a robust and
accurate inorganic basis using a mole fraction based activity coefficient
model and adjusted energies of formation for treating solid precipitation.
The model framework can accommodate organic components, though this added
complexity is considered in a companion paper, this paper describes the
development of the modelling architecture to be used and predictions of an
inorganic model alone. The modelling framework has been developed to
flexibly use a combination of mixing rules and other potentially more
accurate techniques where available to calculate the water content.
Comparisons with other state-of-the-art general equilibrium models and
experimental data are presented and show excellent agreement. The Kelvin
effect can be considered in this scheme using a variety of surface tension
models. Comparison of predicted diameter dependent phenomena, such as the
increased relative humidity for onset of deliquescence with decreasing
diameter, with another diameter dependent model is very good despite the
different approach used. The model is subject to various
sensitivities. For the inorganic systems studied here, the model is
sensitive to choice of surface tension scheme used, which decreases for
larger aerosol. Large sensitivities are found for the value of dry density
used. It is thus likely that the history of the aerosol studied in a
hygroscopic tandem differential mobility analyser (HTDMA), specifically the
nature of the drying process that will influence the final crystalline form,
will create systematic uncertainties upon comparisons with theoretical
predictions. However, the magnitudes of all of the above sensitivities are
potentially less than those introduced when using a semi ideal growth factor
analogue for certain conditions.
Citation: Topping, D. O., McFiggans, G. B., and Coe, H.: A curved multi-component aerosol hygroscopicity model framework: Part 1 – Inorganic compounds, Atmos. Chem. Phys., 5, 1205-1222, doi:10.5194/acp-5-1205-2005, 2005.