Modeling Aqueous Secondary Organic Aerosol Formation: GAMMA, simpleGAMMA, and Properties Modules


GAMMA schematic 2

  • GAMMA (Gas-Aerosol Model for Mechanism Analysis) is a photochemical box model that features detailed, coupled gas- and aqueous aerosol-phase chemistry. GAMMA was originally designed to allow speciated prediction of secondary organic aerosol (SOA) and organosulfate formation in the aqueous aerosol phase under ambient or laboratory conditions (McNeill et al., 2012).  It has also been applied to study aqueous chemistry, including SOA formation, in cloudwater.
  • simpleGAMMA is a reduced version of GAMMA suitable for coupling with larger scale atmospheric chemistry models (Woo and McNeill (2015)).
  • For questions about GAMMA or simpleGAMMA, or to request the code, contact us.

GAMMA: Mechanism

The chemical mechanism and governing equations of GAMMA are described in McNeill et al (2012). A complete listing of the gas and aqueous phase chemical mechanisms and parameters used can be found in the supporting information here.  GAMMA currently includes the gas-phase photochemistry of isoprene, acetylene, toluene, and xylenes, as well as the inter-phase transport and subsequent aqueous aerosol-phase processing of their water-soluble oxidation products. GAMMA follows over 160 gas- and aerosol-phase species and more than 260 reactions. Both GAMMA and simpleGAMMA are written for MATLAB and use the stiff solver ode15s.


Results from McNeill et al. (2012) indicated that aqueous aerosol SOA mass and organosulfate generation were dominated by isoprene-derived epoxydiol (IEPOX) uptake and processing under low-NOx conditions, or dark uptake of glyoxal under high-NOx conditions. This suggested a natural approach for reducing the GAMMA mechanism to a computationally light version, useful for coupling with large-scale atmospheric chemistry models. simpleGAMMA tracks only 4 aqueous species (IEPOX, IEPOXOS, 2-methyltetrol, and glyoxal) and only 2 species are passed between the gas and aqueous phases (glyoxal and IEPOX).  In test runs, agreement to within 25% of the aqueous aerosol SOA mass predicted by the full model is acheived (20% for low-NOx). Details regarding simpleGAMMA can be found in Woo and McNeill (2015).

Physical Property Modules

Aqueous-phase SOA formation can potentially impact the optical properties and surface tension of the seed aerosol.  We have developed modules based on parameterizations of our laboratory studies to predict the UV-visible light absorption and surface tension based on GAMMA output.  Details can be found in Woo et al. (2013).


“Aqueous-phase Secondary Organic Aerosol and Organosulfate Formation in Atmospheric Aerosols: A Modeling Study” VF McNeill, JL Woo, DD Kim, AN Schwier, NJ Wannell, AJ Sumner, JM Barakat. Environ. Sci. Technol.,  46 (15), 8075–8081 (2012). link to paper

“Aqueous aerosol SOA formation: Impact on aerosol physical properties” JL Woo, DD Kim, AN Schwier, R Li and VF McNeill. Faraday Discuss., 165, 357-367 (2013). link to paper

“Modeling the surface tension of complex, reactive organic-inorganic mixtures” AN Schwier, GA Viglione, Z Li, and VF McNeill. Atmos. Chem. Phys., 13, 10721-10732 (2013). link to paper

“Climate-Relevant Physical Properties of Molecular Constituents Relevant for Isoprene-Derived Secondary Organic Aerosol Material” MA Upshur, BF Strick, VF McNeill, RJ Thomson, FM Geiger. Atmos. Chem. Phys., 14, 10731-10740 (2014). link to paper

“Model Analysis of Secondary Organic Aerosol Formation by Glyoxal in Laboratory Studies: The Case for Photoenhanced Chemistry” AJ Sumner, JL Woo, VF McNeill. Environ. Sci. Technol., 48 (20), 11919-11925 (2014). link to paper

“simpleGAMMA 1.0 – A reduced model of secondary organic aerosol formation in the aqueous aerosol phase (aaSOA)” JL Woo and VF McNeill, Geosci. Model. Dev. 8, 1821-1829 (2015)  link to paper

“Examining the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol formation during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Look Rock, Tennessee, ground site” S.H. Budisulistiorini, X. Li, S.T. Bairai, J. Renfro, Y. Liu, Y.J. Liu, K.A. McKinney, S.T. Martin, V.F. McNeill, H.O.T. Pye, A. Nenes, M.E. Neff, E.A. Stone, S. Mueller, C. Knote, S.L. Shaw, Z. Zhang, A. Gold, and J.D. Surratt, Atmos. Chem. Phys., 15, 8871-8888 (2015) link to paper

“Aqueous-phase mechanism for secondary organic aerosol formation from isoprene: application to the southeast United States and co-benefit of SO2 emission controls” E.A. Marais, D. J. Jacob, J. L. Jimenez, P. Campuzano-Jost, D. A. Day, W. Hu, J. Krechmer, L. Zhu, P.S. Kim, C.C. Miller, J.A. Fisher, K. Travis, K. Yu, T.F. Hanisco, G.M.Wolfe, H. L. Arkinson, H. O. T. Pye, K. D. Froyd, J. Liao, and V.F. McNeill (2016) . Atmos. Chem. Phys., 16, 1603-1618 (2016) link to paper

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