^{1}Department of Civil and Environmental Engineering, University of California, Davis, CA, USA ^{2}Cooperative Institute for Research in the Environmental Sciences (CIRES), and Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA Abstract. Measurements of the sensitivity of organic aerosol (OA, and its components) mass to changes in temperature were recently reported by Huffman et al.~(2009) using a tandem thermodenuderaerosol mass spectrometer (TDAMS) system in Mexico City and the Los Angeles area. Here, we use these measurements to derive quantitative estimates of aerosol volatility within the framework of absorptive partitioning theory using a kinetic model of aerosol evaporation in the TD. OA volatility distributions (or "basissets") are determined using several assumptions as to the enthalpy of vaporization (ΔH_{vap}). We present two definitions of "nonvolatile OA," one being a global and one a local definition. Based on these definitions, our analysis indicates that a substantial fraction of the organic aerosol is comprised of nonvolatile components that will not evaporate under any atmospheric conditions; on the order of 50–80% when the most realistic ΔH_{vap} assumptions are considered. The sensitivity of the total OA mass to dilution and ambient changes in temperature has been assessed for the various ΔH_{vap} assumptions. The temperature sensitivity is relatively independent of the particular ΔH_{vap} assumptions whereas dilution sensitivity is found to be greatest for the low (ΔH_{vap} = 50 kJ/mol) and lowest for the high (ΔH_{vap} = 150 kJ/mol) assumptions. This difference arises from the high ΔH_{vap} assumptions yielding volatility distributions with a greater fraction of nonvolatile material than the low ΔH_{vap} assumptions. If the observations are fit using a 1 or 2component model the sensitivity of the OA to dilution is unrealistically high. An empirical method introduced by Faulhaber et al. (2009) has also been used to independently estimate a volatility distribution for the ambient OA and is found to give results consistent with the high and variable ΔH_{vap} assumptions. Our results also show that the amount of semivolatile gasphase organics in equilibrium with the OA could range from ~20% to 400% of the OA mass, with smaller values generally corresponding to the higher ΔH_{vap} assumptions. The volatility of various OA components determined from factor analysis of AMS spectra has also been assessed. In general, it is found that the fraction of nonvolatile material follows the pattern: biomass burning OA < hydrocarbonlike OA < semivolatile oxygenated OA < lowvolatility oxygenated OA. Correspondingly, the sensitivity to dilution and the estimated amount of semivolatile gasphase material for the OA factors follows the reverse order. Primary OA has a substantial semivolatile fraction, in agreement with previous results, while the nonvolatile fraction appears to be dominated by oxygenated OA produced by atmospheric aging. The overall OA volatility is thus controlled by the relative contribution of each aerosol type to the total OA burden. Finally, the model/measurement comparison appears to require OA having an evaporation coefficient (γ_{e}) substantially greater than 10^{−2}; at this point it is not possible to place firmer constraints on γ_{e} based on the observations. Citation: Cappa, C. D. and Jimenez, J. L.: Quantitative estimates of the volatility of ambient organic aerosol, Atmos. Chem. Phys., 10, 54095424, doi:10.5194/acp1054092010, 2010. 
