10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Discrepancies Between the Volatility Distributions of OA in the Ambient Atmosphere and the Laboratory
Eleni Karnezi, Evangelos Louvaris, Evangelia Kostenidou, Kalliopi Florou, Kerrigan Cain, SPYROS PANDIS, Carnegie Mellon University, University of Patras
Abstract Number: 870 Working Group: Carbonaceous Aerosol
Abstract Organic compounds represent a significant fraction of submicrometer atmospheric aerosol mass. Even if most of these compounds are semi-volatile in atmospheric concentrations, the ambient organic aerosol volatility is quite uncertain. This work attempts to synthesize the organic aerosol volatility distribution estimates in both ambient and laboratory studies and quantifies the role of other properties that affect volatility estimates such as the vaporization enthalpy and accommodation coefficient that are usually assumed.
Measurements combining a thermodenuder (TD) and a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) that took place in Paris (summer 2009 and winter 2010), Athens (winter 2013) and Finokalia (summer of 2016) are used for the analysis. These are combined with similar measurements of both primary and secondary organic aerosol (cooking OA and SOA, SOA from VOCs) that took place in the smog chambers of the Institute of Chemical Engineering Sciences in Patras and Carnegie Mellon University in Pittsburgh. The Karnezi et al. (2014) method is applied to these datasets in order to estimate the volatility distribution together with the vaporization enthalpy and the accommodation coefficient for the organic aerosol (OA) and its components (based on positive matrix factorization).
Our results suggest that the laboratory OA is significantly more volatile than the ambient from the same source or precursor. However, a significant part of this discrepancy is due to the different OA concentration levels present in the two types of studies. At the high concentration levels present in the laboratory the semivolatile components with higher volatility partition into the particle phase and the OA becomes more volatile (Donahue et al., 2006). The volatility basis set framework can be used to estimate the volatility distribution of the laboratory aerosol at ambient concentration levels allowing a direct comparison of the two distributions. This technique is applied to all the data sets. The results suggest that the gap is a lot smaller than originally thought. For example, for cooking OA the ambient and laboratory volatilities are practically the same when compared at the ambient concentration levels of a few micrograms per cubic meter. The differences of the volatility distributions of the various OA components determined during the PMF analysis of the AMS measurements at the same factor concentration levels are also lower. These volatility differences can be related to differences in the AMS spectra of the various factors and are probably due to the source mix or the degree of atmospheric processing in each study. The vaporization enthalpies of the OA factors were relatively consistent across the different campaigns examined and the model suggests small mass transfer resistances in most cases.
[1] Donahue, N. M., Robinson, A. L., Stanier, C. O. and Pandis, S. N.: Coupled partitioning, dilution, and chemical aging of semivolatile organics, Environ. Sci. Technol., 40, 2635–2643, 2006. [2] Karnezi, E., Riipinen, I., and Pandis, S. N.: Measuring the atmospheric organic aerosol volatility distribution: A theoretical analysis, Atmos. Meas. Tech., 7, 2953-2965, 2014.