ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-3303-2011 A two-dimensional volatility basis set: 1. organic-aerosol mixing thermodynamics Donahue N. M. 1 Epstein S. A. 1 Pandis S. N. 1 Robinson A. L. 1 Carnegie Mellon University Center for Atmospheric Particle Studies, Pittsburgh, USA 07 04 2011 11 7 3303 3318 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/3303/2011/acp-11-3303-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/3303/2011/acp-11-3303-2011.pdf

We develop the thermodynamic underpinnings of a two-dimensional volatility basis set (2D-VBS) employing saturation mass concentration (<i>C</i><sup>o</sup>) 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 (<i>C</i><sup>*</sup> = &gamma; <i>C</i><sup>o</sup>, where γ 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 <i>C</i><sup>o</sup> – O:C 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.

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