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Atmos. Chem. Phys. Discuss., 10, 24091-24133, 2010
www.atmos-chem-phys-discuss.net/10/24091/2010/ doi:10.5194/acpd-10-24091-2010 © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. |
A two-dimensional volatility basis set: 1. organic-aerosol mixing thermodynamics
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*=γ 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.