Atmos. Chem. Phys. Discuss., 10, 24091-24133, 2010
© Author(s) 2010. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
A two-dimensional volatility basis set: 1. organic-aerosol mixing thermodynamics
N. M. Donahue, S. A. Epstein, S. N. Pandis, and A. L. Robinson
Carnegie Mellon University, Center for Atmospheric Particle Studies, Pittsburgh, USA

Abstract. We develop the thermodynamic underpinnings of a two-dimensional volatility basis set (2-D-VBS) employing saturation concentration (Co) and the oxygen content (O:C) to describe volatility, mixing thermodynamics, and chemical evolution of organic aerosol. This is an extension of our earlier one-dimensional approach employing C* only (C*Co, where γ is an activity coefficient). We apply a mean-field approximation for organic aerosol, describing interactions of carbon and oxygen groups in individual molecules (solutes) with carbon and oxygen groups in the organic-aerosol solvent. In so doing, we show that a linear structure activity relation (SAR) describing the single-component Co of a molecule is directly tied to ideal solution (Raoult's Law) behavior. Conversely, non-ideal solution behavior (activity coefficients) and a slightly non-linear SAR emerge from off-diagonal (carbon-oxygen) interaction elements. From this foundation we can build a self-consistent description of OA mixing thermodynamics, including predicted saturation concentrations and activity coefficients (and phase separation) for various solutions from just four free parameters: the carbon number of a hydrocarbon with a 1 μg m−3 Co, and the carbon-carbon, oxygen-oxygen, and non-ideal carbon-oxygen terms. This treatment establishes the mean molecular formula for organics within this 2-D space as well as activity coefficients for molecules within this space interacting with any bulk OA phase described by an average O:C.

Citation: Donahue, N. M., Epstein, S. A., Pandis, S. N., and Robinson, A. L.: A two-dimensional volatility basis set: 1. organic-aerosol mixing thermodynamics, Atmos. Chem. Phys. Discuss., 10, 24091-24133, doi:10.5194/acpd-10-24091-2010, 2010.
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