Quantitative estimates of the volatility of ambient organic aerosol
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 thermodenuder-aerosol mass spectrometer (TD-AMS) 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 "basis-sets") are determined using several assumptions as to the enthalpy of vaporization (Delta H(vap)). We present two definitions of "non-volatile 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 non-volatile components that will not evaporate under any atmospheric conditions; on the order of 50-80% when the most realistic Delta H(vap) assumptions are considered. The sensitivity of the total OA mass to dilution and ambient changes in temperature has been assessed for the various Delta H(vap) assumptions. The temperature sensitivity is relatively independent of the particular Delta H(vap) assumptions whereas dilution sensitivity is found to be greatest for the low (Delta H(vap) = 50 kJ/mol) and lowest for the high (Delta H(vap) = 150 kJ/mol) assumptions. This difference arises from the high Delta H(vap) assumptions yielding volatility distributions with a greater fraction of non-volatile material than the low Delta H(vap) assumptions. If the observations are fit using a 1 or 2-component 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 Delta H(vap) assumptions. Our results also show that the amount of semivolatile gas-phase organics in equilibrium with the OA could range from similar to 20% to 400% of the OA mass, with smaller values generally corresponding to the higher Delta H(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 non-volatile material follows the pattern: biomass burning OA < hydrocarbonlike OA < semivolatile oxygenated OA < low-volatility oxygenated OA. Correspondingly, the sensitivity to dilution and the estimated amount of semivolatile gas-phase material for the OA factors follows the reverse order. Primary OA has a substantial semivolatile fraction, in agreement with previous results, while the non-volatile 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 (gamma(e)) substantially greater than 10(-2); at this point it is not possible to place firmer constraints on gamma(e) based on the observations.