Many scientific studies have linked increased particulate matter (PM) levels to a series of significant health problems and reduced visibility (haze) in many US cities. In addition to regional dust events originating from the continental US, today it is well documented that Asian and African dust plumes (often mixed with high levels of pollution) can also be engaged in a long-range transport, significantly affecting PM levels in the continental United States and Caribbean. Our ability to assess the risks and to predict the behavior and possible adverse impacts of the increased levels of PM mainly depends on modeling studies. Despite their significant environmental and health impacts, the treatment of aerosol composition in airshed and global climate models is a computationally expensive task; this is largely due to thermodynamic equilibrium calculations required in every aerosol model because mass transport of volatile species (e.g. water) between gas and aerosol phases is driven from the difference between ambient and equilibrium concentrations. There is strong need for models that quickly and accurately predict aerosol composition and phase state over a wide range of temperature and relative humidity (RH). This work presents the new aerosol thermodynamic equilibrium model “ISORROPIA II” for K+ – Ca2+ – Mg2+ – NH4+ – Na+ – SO42- – NO3- – Cl- – H2O systems. ISORROPIA II is based on the ISORROPIA model (Nenes et al., 1998; Nenes et al., 1999) with appropriate extensions to account for the presence of crustal species. The goal is the development of a module that maximizes computational efficiency with minimal impact on model accuracy. Similar to ISORROPIA, the solution is obtained using the bisection method; most cases are solved using only one level of iteration which ensures maximum computational efficiency. Compared to ISORROPIA, Ca2+, K+, Mg2+ has been added in the aqueous phase, and CaSO4, Ca(NO3)2, CaCL2, K2SO4, KHSO4, KNO3, KCL, MgSO4, Mg(NO3)2, MgCL2 in the solid phase. The number of species (i.e., the number of equilibrium reactions solved) is determined by the relative abundance of each species and the ambient relative humidity. Binary activity coefficients in the new code are calculated using the Kusik-Meissner relationships, (Kusik and Meissner, 1978) while the multicomponent activity coefficients are calculated using Bromley's formula (Bromley, 1973). Water uptake is calculated using the ZSR relationship (Robinson and Stokes, 1965). ISORROPIA II is evaluated extensively against other thermodynamic models that can treat crustal species, like SCAPE2 (Kim and Seinfeld, 1995), MTEM (Zaveri et al., 2005). The intercomparison will focus on the code performance, both in terms of its accuracy and computational efficiency. The model will also be evaluated against in situ data from the MILAGRO (MIRAGE-Mex) campaign in Mexico City, during March 2006. The results of this intercomparison and evaluation will be presented.

Keywords: Thermodynamic equilibrium, inorganic aerosol, crustal material, activity coefficients

REFERENCES

Bromley, L.A., (1973) Thermodynamic properties of strong electrolytes in aqueous solutions. AIChE J., 19, 313-320. Kim, Y.P., and J.H., Seinfeld, (1995) Atmospheric gas aerosol equilibrium: III. Thermodynamics of crustal elements Ca2+, K+, and Mg2+. Aerosol Science and Technology 22, 93 - 110. Kusik, C.L., and H.P., Meissner, (1978) Electrolyte activity coefficients in inorganic processing. AIChE Symp. Series, 173, 14-20. Nenes, A., Pilinis C., and S.N., Pandis, (1998) ISORROPIA: A new thermodynamic model for inorganic multicomponent atmospheric aerosols. Aquat. Geochem., 4, 123-152. Nenes, A., Pandis S.N., and C., Pilinis, (1999) Continued development and testing of a new thermodynamic aerosol module for urban and regional air quality models. Atmos. Environ., 33, 1553-1560. Robinson, R.A. and R.H., Stokes, (1965) Electrolyte solutions. Second Ed., Butterworths, London. Zaveri, R. A., Easter R.C., and A.S., Wexler, (2005) A new method for multicomponent activity coefficients of electrolytes in aqueous atmospheric aerosols. J. Geophys. Res., 110.

ACKNOWLEDGEMENTS This work was supported by the National Oceanic and Atmospheric Administration under contract NMRAC000-5-04017.