Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 18, 357-370, 2018
https://doi.org/10.5194/acp-18-357-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
12 Jan 2018
Coupling of organic and inorganic aerosol systems and the effect on gas–particle partitioning in the southeastern US
Havala O. T. Pye1, Andreas Zuend2, Juliane L. Fry3, Gabriel Isaacman-VanWertz4,5, Shannon L. Capps6, K. Wyat Appel1, Hosein Foroutan1,7, Lu Xu8, Nga L. Ng9,10, and Allen H. Goldstein5,11 1National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, North Carolina, USA
2Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Québec, Canada
3Department of Chemistry, Reed College, Portland, Oregon, USA
4Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
5Department of Environmental Science, Policy, and Management, University of California, Berkeley, California, USA
6Civil, Architectural, and Environmental Engineering, Drexel University, Philadelphia, Pennsylvania, USA
7Department of Biomedical Engineering and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
8Department of Environmental Science and Engineering, California Institute of Technology, Pasadena, California, USA
9School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
10School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA
11Department of Civil and Environmental Engineering, University of California, Berkeley, California, USA
Abstract. Several models were used to describe the partitioning of ammonia, water, and organic compounds between the gas and particle phases for conditions in the southeastern US during summer 2013. Existing equilibrium models and frameworks were found to be sufficient, although additional improvements in terms of estimating pure-species vapor pressures are needed. Thermodynamic model predictions were consistent, to first order, with a molar ratio of ammonium to sulfate of approximately 1.6 to 1.8 (ratio of ammonium to 2  ×  sulfate, RN∕2S  ≈  0.8 to 0.9) with approximately 70 % of total ammonia and ammonium (NHx) in the particle. Southeastern Aerosol Research and Characterization Network (SEARCH) gas and aerosol and Southern Oxidant and Aerosol Study (SOAS) Monitor for AeRosols and Gases in Ambient air (MARGA) aerosol measurements were consistent with these conditions. CMAQv5.2 regional chemical transport model predictions did not reflect these conditions due to a factor of 3 overestimate of the nonvolatile cations. In addition, gas-phase ammonia was overestimated in the CMAQ model leading to an even lower fraction of total ammonia in the particle. Chemical Speciation Network (CSN) and aerosol mass spectrometer (AMS) measurements indicated less ammonium per sulfate than SEARCH and MARGA measurements and were inconsistent with thermodynamic model predictions. Organic compounds were predicted to be present to some extent in the same phase as inorganic constituents, modifying their activity and resulting in a decrease in [H+]air (H+ in µg m−3 air), increase in ammonia partitioning to the gas phase, and increase in pH compared to complete organic vs. inorganic liquid–liquid phase separation. In addition, accounting for nonideal mixing modified the pH such that a fully interactive inorganic–organic system had a pH roughly 0.7 units higher than predicted using traditional methods (pH  =  1.5 vs. 0.7). Particle-phase interactions of organic and inorganic compounds were found to increase partitioning towards the particle phase (vs. gas phase) for highly oxygenated (O : C  ≥  0.6) compounds including several isoprene-derived tracers as well as levoglucosan but decrease particle-phase partitioning for low O : C, monoterpene-derived species.

Citation: Pye, H. O. T., Zuend, A., Fry, J. L., Isaacman-VanWertz, G., Capps, S. L., Appel, K. W., Foroutan, H., Xu, L., Ng, N. L., and Goldstein, A. H.: Coupling of organic and inorganic aerosol systems and the effect on gas–particle partitioning in the southeastern US, Atmos. Chem. Phys., 18, 357-370, https://doi.org/10.5194/acp-18-357-2018, 2018.
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Short summary
Thermodynamic modeling revealed that some but not all measurements of ammonium-to-sulfate ratios are consistent with theory. The measurement diversity likely explains the previously reported range of results regarding the suitability of thermodynamic modeling. Despite particles being predominantly phase separated, organic–inorganic interactions resulted in increased aerosol pH and partitioning towards the particle phase for highly oxygenated organic compounds compared to traditional methods.
Thermodynamic modeling revealed that some but not all measurements of ammonium-to-sulfate ratios...
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