A two-dimensional volatility basis set: 1. organic-aerosol mixing thermodynamics

Abstract

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.