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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 1 | Copyright
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

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 Havala O. T. Pye et al.
  • 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 µgm−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.

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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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