Atmospheric aerosols play an important role in the Earth-atmosphere system. They can affect the Earth's radiation budget and climate directly through absorption and scattering of solar and terrestrial radiation and indirectly through acting as cloud condensation nuclei (CCN) and modifying cloud properties such as droplet number concentration and size distribution, cloud reflectivity and lifetime, and precipitation frequency. They are also associated with visibility degradation, acid rain, and a number of human health problems. Simulating atmospheric aerosols, on the other hand, poses major challenges due to their complicated physical and chemical properties and processes as well as associated uncertainties. Different aerosol treatments in 3-D models may lead to different gaseous and aerosol predictions and overall computational efficiency. In this study, the 2005 version of Carbon Bond mechanism (CB05) has been implemented into the Weather Research and Forecasting Model with Chemistry (WRF/Chem) version 3.0 and is being coupled with three aerosol modules: the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID), the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC), and the Modal Aerosol Dynamics Model for Europe (MADE) with the secondary organic aerosol model (SORGAM) (referred to as MADE/SORGAM). The three modules differ in several aspects. For example, while both MADRID and MOSAIC use a sectional approach with 4 or 8 size sections to represent aerosol size distribution, MADE/SORGAM uses a modal approach with 3 lognormally-distributed modes. For inorganic aerosol equilibrium, both MADRID and MADE/SORGAM use ISORROPIA (“equilibrium” in Greek) version 1.7, whereas MOSAIC uses the Multicomponent Equilibrium Solver for Aerosols (MESA). For organic aerosol, the current version of MOSAIC in WRF/Chem does not treat secondary organic aerosol (SOA) formation. In MADE/SORGAM and MADRID, 8 and 25 SOA species are produced from the reversible absorption of 5 and 11 categories of volatile organic compounds (VOCs), respectively. For gas/particle mass transfer simulation, MADE/SORGAM uses a full equilibrium approach for HNO₃ and NH₃ but a dynamic approach for H₂SO₄, MOSAIC uses the dynamic approach for all species, and MADRID offers three approaches: full equilibrium, dynamic, and hybrid.

The WRF/Chem simulations with the same gas-phase mechanism (i.e., CB05) but different aerosol modules are being conducted over the continental U.S. for July 1-31, 2001. Simulation results will be compared and evaluated against surface and satellite observations. The likely discrepancies between observations and simulations as well as the sensitivity of gaseous and aerosol predictions to aerosol modules will be examined, with a focus on secondary aerosols.