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Coupling of organic and inorganic aerosol systems and the effect on gas–particle partitioning in the southeastern US
Citation:
Pye, H., A. Zuend, J. Fry, G. Isaacman-VanWertz, S. Capps, W. Appel, H. Foroutan, L. Xu, N. Ng, AND A. Goldstein. Coupling of organic and inorganic aerosol systems and the effect on gas–particle partitioning in the southeastern US. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 18:357-370, (2018).
Impact/Purpose:
In this work, conditions over the eastern US were examined with a focus on gas–particle partitioning during the Southern Oxidant and Aerosol Study. Gas–particle partitioning of ammonia and several isoprene-, monoterpene-, and biomass burning derived organic compounds was examined using common air quality modeling treatments and advanced approaches. Results address the degree to which techniques accounting for organic–inorganic interactions, deviations in ideality, and phase separation reproduce observations. Models were evaluated for their ability to predict ammonia vs. ammonium as well as gas–particle partitioning of organic compounds. In addition, the effects of organic compounds on aerosol pH were examined.
Description:
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.
