| In this work, existing and modified activity coefficient models are examined
in order to assess their capabilities to describe the properties of aqueous
solution droplets relevant in the atmosphere. Five different
water-organic-electrolyte activity coefficient models were first selected
from the literature. Only one of these models included organics and
electrolytes which are common in atmospheric aerosol particles. In the other
models, organic species were solvents such as alcohols, and important
atmospheric ions like NH4+ could be missing. The predictions of
these models were compared to experimental activity and solubility data in
aqueous single electrolyte solutions with 31 different electrolytes.
Based on the deviations from experimental data and on the capabilities of the
models, four predictive models were selected for fitting of new parameters
for binary and ternary solutions of common atmospheric electrolytes and
organics. New electrolytes (H+, NH4+, Na+,
Cl-, NO3- and SO42-) and organics (dicarboxylic
and some hydroxy acids) were added and some modifications were made to the
models if it was found useful. All new and most of the existing parameters
were fitted to experimental single electrolyte data as well as data for
aqueous organics and aqueous organic-electrolyte solutions. Unfortunately,
there are very few data available for organic activities in binary solutions
and for organic and electrolyte activities in aqueous organic-electrolyte
solutions. This reduces model capabilities in predicting solubilities.
After the parameters were fitted, deviations from measurement data were
calculated for all fitted models, and for different data types. These
deviations and the calculated property values were compared with those from
other non-electrolyte and organic-electrolyte models found in the
literature. Finally, hygroscopic growth factors were calculated for four 100 nm
organic-electrolyte particles and these predictions were compared
to experimental data and to predictions from other models.
All of the newly fitted models show good agreement with experimental water
activity data in binary and ternary solutions. One of the models is for
activities of non-electrolytes only, but the other three models show quite
small deviations from measured electrolyte activities. Because there were not
enough experimental data for organic and electrolyte activities, some models
show bigger deviation for mutual deliquescence relative humidities of
organic-electrolyte particles, but calculated growth factors for liquid
droplets are quite close to the experimental data. Even in cases with
somewhat bigger deviations, the results can be considered satisfactory,
because they were calculated based mainly on the predictive properties of the
models. |