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THERMODYNAMIC MODELING OF LIQUID AEROSOLS CONTAINING DISSOLVED ORGANICS AND ELECTROLYTES
Edney, E O., S. L. Clegg, AND J. H. Seinfeld. THERMODYNAMIC MODELING OF LIQUID AEROSOLS CONTAINING DISSOLVED ORGANICS AND ELECTROLYTES. Presented at American Association for Aerosol Research, Charlotte, NC, October 7-11, 2002.
1. Using laboratory and field study data generated during FY99-FY04, develop a science version of a PM chemistry model for predicting ambient concentrations of water, inorganics, and organics in PM2.5 samples. The model will include the Aerosol Inorganic Model for predicting concentrations of inorganic compounds and a computational chemistry-based method for predicting concentrations of organic compounds.
2. Identify and evaluate methods for analyzing the polar fraction of PM2.5 samples.
3. Carry out short term field studies in Research Triangle Park, North Carolina in the summer and the winter to determine the composition of the organic fraction of ambient PM2.5 samples, with special emphasis placed on identifying and determining ambient concentrations of polar compounds.
4. Conduct laboratory studies to establish the chemical composition of secondary organic aerosol (SOA) and to determine source signatures for aromatic and biogenic SOA.
5. Conduct laboratory and theoretical investigations of thermodynamic properties of polar organic compounds.
6. Evaluate the science version of the PM chemistry model using laboratory and field data generated under this task as well as other available data in the literature.
7. Conduct PM chemistry-related special studies for OAQPS
Many tropospheric aerosols contain large fractions of soluble organic material, believed to derive from the oxidation of precursors such alpha-pinene. The chemical composition of aerosol organic matter is complex and not yet fully understood.
The key properties of soluble inorganic aerosols - water uptake, deliquescence of the salts present, and equilibrium with gases such as NH3 and HNO3 - can readily be calculated using existing thermodynamic models. It is desirable to include soluble organic compounds and so treat internally mixed inorganic/organic aerosols. There are two main difficulties. First, the lack of fundamental thermodynamic data for organic compounds and their solutions in water. Second, a suitable theoretical approach must be found for aqueous inorganic/organic mixtures, applicable to systems of complex composition and high concentration (low relative humidity).
A comprehensive model of soluble aerosols containing water, and both inorganic and organic components should also have the following properties. First, for the two limiting cases of an aqueous inorganic solution, and a mixture of organic compounds in water, it should yield results as accurate as existing models for the two different types of systems. Second, the effect on aerosol solution behaviour of the interaction of dissolved ions and organic molecules should be included in a flexible way. This allows varying levels of complexity, and therefore accuracy, in the way in which the interactions are represented. It is particularly necessary as few thermodynamic data are currently available for many aqueous inorganic/organic mixtures of relevance to aerosol chemistry, thus requiring initially very simple approaches.
We have previously proposed a practical, thermodynamically self-consistent, method for predicting the properties of soluble mixed inorganic/organic aerosols (J. Aerosol Sci. 32 (6): 713-738, 2001). Here we describe an alternative approach which is likely to be particularly useful for systems for which thermodynamic data for ternary mixtures (i.e., salt-organic-water) are unavailable, and for high liquid phase concentrations. The new method is also likely to be computationally efficient, which is necessary for inclusion in atmospheric codes. The method is based upon an extension of the Zdanovskii-Stokes-Robinson relationship to include parameters for ternary (salt-salt-solvent, organic-organic-solvent, and salt-organic-solvent) interactions, combined with a well proven mixing rule for specifying the composition of a multicomponent electrolyte mixture in terms of salts rather than ions. The extended ZSR relationship is used to calculate both solvent and solute activities. The equations are formulated in such a way that existing models for electrolyte-water and/or organic-water mixtures can be incorporated into the method without loss of thermodynamic consistency.
Examples of the predicted deliquescence behaviour of salt-organic solutions, effects of salts on the vapour/liquid partitioning of organic compounds ("salting out"), and liquid/liquid phase separation will be discussed.
This work has been funded fully, or in part, by the United States Environmental Protection Agency, under Contract 1D-5700-NATX to Dr. Simon L. Clegg. It has been subjected to Agency review and approved for publication.