The chemical and physical characteristics of aerosols are widely varying and the associated hygroscopic properties have far reaching impacts on our environment and the way we live. Whilst numerous inorganic models have been developed, it is clearly necessary to treat the influence of organics given the upsurge in data which has identified a ubiquitous contribution from such species to aerosol mass loadings. Similarly, the majority of equilibrium models available do not even treat the influence of curvature. In this report a thermodynamic model is presented which was designed to embrace both the chemical and physical characteristics of atmospheric aerosols on a fundamental level (ADDEM - Aerosol Diameter Dependent Model) and calculate the equilibrium composition of an aerosol of known composition and dry size. A direct minimisation of the Gibbs free energy, using a sequential quadratic programming algorithm, allows the calculation of equilibration between the gaseous, solid and aqueous phase components subject to thermodynamic data availability. Subsequently, a general inorganic equilibrium model was developed and found to compare excellently with other state-of-the-art inorganic equilibrium models available. The main crux of the problem of treating mixed inorganic/organic systems stems from not being able to treat the complex interactions taking place in solution between the two fractions. Given this, ADDEM combines two separate thermodynamic models and adds the water content associated with the inorganic and organic fractions. It was found that the additive approach for modelling mixed inorganic/organic systems worked well for a variety of mixtures, as has been found in previous studies for simple binary inorganic/organic systems. Whilst this is likely to remain the only option for some time, further work is still required to assess whether this approach reproduces errors which lie within the experimental uncertainty of observed hygroscopic behaviour for a variety of systems. With regards to the generalised contribution from water soluble organic species in the atmosphere, analysis of fractionated WSOC data from two ambient studies, using the modelling tools developed here, suggested that this portion of the atmospheric aerosol contributes little to the observed water uptake relative to the inorganic fraction. Indeed, whilst further studies are required from a wider range of locations, a combination with observed results that an additive approach for modelling mixed inorganic/organic aerosols works very well suggests that one representative growth factor for the WSOC fraction may be adequate for aged air masses if indeed the inorganic fraction dominates the water uptake behaviour in the sub-saturated humid environment. Whilst simple mixing rules for including organics may be relatively successful at reproducing water uptake characteristics, they rely on data from binary systems that may not be available and thus have to be calculated. This further validates the need for predictive models such as ADDEM. For calculation of non ideality in the organic model, the most widely used group contribution method UNIFAC is employed here. Results suggest that UNIFAC is quite capable of reproducing the water uptake characteristics of various complex species of atmospheric importance in the sub-saturated humid regime. ADDEM currently houses a general equilibrium model within a bisection iterative loop to find a solution to the definition of equilibrium given by the K??hler equation. In doing this, the complexities of including the surface term within the Gibbs energy summation are circumvented allowing any number of surface tension and density mixing rules to be used with ease. Comparisons with another diameter dependent model showed that ADDEM can reproduce diameter dependent phenomena including the onset of deliquescence. Similarly, it is possible to extend ADDEM into the super-saturated humid regime now that the curvature effect is included. ADDEM is subject to various sensitivities derived from the choice of parameters used to describe pure component and mixture properties. Surface tension sensitivity seems to increase as the aerosol dry size decreases and the RH increases. However, larger sensitivities are found for the value of dry density used. It is thus likely that the history of the aerosol studied in a HTDMA, specifically the nature of the drying process that will influence the final crystalline form, will create systematic uncertainties upon comparisons with theoretical predictions.
|Publication status||Published - 2005|