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 levoglu-cosan but decrease particle-phase partitioning for low O: C, monoterpene-derived species.
Highlights
IntroductionThe organic compounds present in the gas and particle phase are diverse and numerous (Goldstein and Galbally, 2007), ranging from relatively unoxidized, long-chain alkanes in fresh emissions to small, highly soluble compounds formed through multiple generations of atmospheric chemistry
Ambient particles consist of organic and inorganic compounds
Community Multiscale Air Quality (CMAQ)-predicted sulfate was relatively unbiased in the southeastern US, but ammonium was high by a factor of 1.5 (Table S10)
Summary
The organic compounds present in the gas and particle phase are diverse and numerous (Goldstein and Galbally, 2007), ranging from relatively unoxidized, long-chain alkanes in fresh emissions to small, highly soluble compounds formed through multiple generations of atmospheric chemistry. The extent to which organic and inorganic components of particulate matter interact within a particle depends on the mixing state (e.g., internal vs external) of the aerosol population as well as the degree of phase separation (or number of phases) within the particle. Mixed populations, as typically assumed in chemical transport models such as the Community Multiscale Air Quality (CMAQ) model, may exhibit one fairly homogeneous liquid phase state or be heterogeneous in composition. Heterogeneous configurations occur as a result of phase separation and may include a liquid and solid phase or multiple liquid phases. A common heterogeneous configuration under conditions of liquid–liquid or solid–liquid phase separation is that of a core-shell morphology; alternatively, partially engulfed morphologies have been predicted by theory and observed in laboratory experiments (Kwamena et al, 2010; Song et al, 2013; Reid et al, 2011)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
