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

We develop the thermodynamic underpinnings of a two-dimensional volatility basis set (2D-VBS) employing saturation mass concentration (C-o) and the oxygen content (O:C) to describe volatility, mixing thermodynamics, and chemical evolution of organic aerosol. The work addresses a simple question: "Can we reasonably constrain organic-aerosol composition in the atmosphere based on only two measurable organic properties, volatility and the extent of oxygenation?" This is an extension of our earlier one-dimensional approach employing volatility only (C* = gamma C-o, where gamma is an activity coefficient). Using available constraints on bulk organic-aerosol composition, we argue that one can reasonably predict the composition of organics (carbon, oxygen and hydrogen numbers) given a location in the C-o - space. Further, we argue that we can constrain the activity coefficients at various locations in this space based on the O:C of the organic aerosol.