Coupling of organic and inorganic aerosol systems and the effect on gas–particle partitioning in the southeastern US

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.

Effectiveness of Ammonia Reduction on Control of Fine Particle Nitrate

“…S2 and S3 for this study; similar results are found in other studies (Gao et al, 2016;Gao et al, 2017a;Nah et al, 2018). Pye et al (2018) found only minor difference in predicted ammonia partition when an organic phase was considered. It is also assumed that the particles were internally mixed, and that pH did not vary 30 with size.…”

The acidity of atmospheric particles and clouds

“…One such assumption is that H + accounts for the charge deficit between the watersoluble species present in particles that can be readily measured with an ion chromatograph. This typically includes five cation (NH + 4 , Na + , Ca 2+ , Mg 2+ , and K + ) and three anion (Cl − , NO − 3 , and SO 2− 4 ) species (e.g., Quinn et al, 2006;Sun et al, 1998), though it occasionally includes a limited group of organic anions (Kerminen et al, 2001). A similar definition has also been applied to the nonrefractory inorganic species measured with an Aerodyne Aerosol Mass Spectrometer (NH + 4 , NO − 3 , SO 2− 4 , and partial Cl − ) (Zhang et al, 2005;Quinn et al, 2006).…”

“…This typically includes five cation (NH + 4 , Na + , Ca 2+ , Mg 2+ , and K + ) and three anion (Cl − , NO − 3 , and SO 2− 4 ) species (e.g., Quinn et al, 2006;Sun et al, 1998), though it occasionally includes a limited group of organic anions (Kerminen et al, 2001). A similar definition has also been applied to the nonrefractory inorganic species measured with an Aerodyne Aerosol Mass Spectrometer (NH + 4 , NO − 3 , SO 2− 4 , and partial Cl − ) (Zhang et al, 2005;Quinn et al, 2006). If the aerosol is near neutral (pH ≈ 7) or acidity is dictated by components other than the inorganic ions in the balance (e.g., organic acids), then charge balance will not provide meaningful information (Trebs et al, 2005;Lawrence and Koutrakis, 1996).…”

Modelling organic aerosol over Europe in summer conditions with the VBS-GECKO parameterization: sensitivity to secondary organic compound properties and IVOC emissions

“…Couvidat et al, 2012;Couvidat and Seigneur, 2011;Paulot et al, 2009;Pun et al, 2006b;Surratt et al, 2010), the oligomerization of SVOC in both organic and aqueous phase (e.g. Aksoyoglu et al, 2011;Couvidat et al, 2012;Denkenberger et al, 2007;Dommen et al, 2006;Kalberer et al, 2006;Lemaire et al, 2016;Trump and Donahue, 2014), the non-ideal behaviour of the organic aerosol (Couvidat et 5 al., 2012, Couvidat andSartelet, 2015;Pun et al, 2006;Pye et al, 2018) or the effect of the aerosol viscosity (Couvidat and Sartelet, 2015;Shiraiwa et al, 2013). However, most CTMs still fall short to reproduce SOA concentration with up to date parameterizations (e.g.…”