[1] The aerosol component of the Community Multiscale Air Quality (CMAQ) model is designed to be an efficient and economical depiction of aerosol dynamics in the atmosphere. The approach taken represents the particle size distribution as the superposition of three lognormal subdistributions, called modes. The processes of coagulation, particle growth by the addition of mass, and new particle formation, are included. Time stepping is done with analytical solutions to the differential equations for the conservation of number, surface area, and species mass. The component considers both PM2.5 and PM10 and includes estimates of the primary emissions of elemental and organic carbon, dust, and other species not further specified. Secondary species considered are sulfate, nitrate, ammonium, water, and secondary organics from precursors of anthropogenic and biogenic origin. Extinction of visible light by aerosols is represented by two methods: a parametric approximation to Mie extinction and an empirical approach based upon field data. The algorithms that simulate cloud interactions with aerosols are also described. Results from box model and three-dimensional simulations are exhibited.

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The Regional Acid Deposition Model has been modified to create the Regional Particulate Model, a three‐dimensional Eulerian model that simulates the chemistry, transport, and dynamics of sulfuric acid aerosol resulting from primary emissions and the gas phase oxidation of sulfur dioxide. The new model uses a bimodal lognormal distribution to represent particles in the submicrometer size range. In addition to including the horizontal and vertical advection and vertical diffusion of the aerosol number concentration and sulfate mass concentration fields, the model now explicitly treats the response of the distribution parameters to particle coagulation within and between the modes, condensation of sulfate vapor onto existing particles, formation of new particles, evaporation and condensation of ambient water vapor in the presence of ammonia, and particle‐size‐dependent dry deposition. The model has been used to study how the degree of sulfuric acid neutralization by ambient ammonia affects the total aerosol concentrations and particle size distributions over eastern North America. Preliminary results for three representative locations, rural, near‐source, and nominal downwind of source, show that the effect is greatest for the rural and smallest for the near‐source regions, which corresponds with the largest and smallest values, respectively, of ammonium‐to‐sulfate molar ratios. The results indicate that the model could provide a tool for investigating the effects of various pollution control strategies, as well as new or alternative formulations of important aerosol processes.

Abstract. Air quality models such as the EPA Community Multiscale Air Quality (CMAQ) require meteorological data as part of the input to drive the chemistry and transport simulation. The Meteorology-Chemistry Interface Processor (MCIP) is used to convert meteorological data into CMAQ-ready input. Key shortcoming of such one-way coupling include: excessive temporal interpolation of coarsely saved meteorological input and lack of feedback of atmospheric pollutant loading on simulated dynamics. We have developed a two-way coupled system to address these issues. A single source code principle was used to construct this two-way coupling system so that CMAQ can be consistently executed as a stand-alone model or part of the coupled system without any code changes; this approach eliminates maintenance of separate code versions for the coupled and uncoupled systems. The design also provides the flexibility to permit users: (1) to adjust the call frequency of WRF and CMAQ to balance the accuracy of the simulation versus computational intensity of the system, and (2) to execute the two-way coupling system with feedbacks to study the effect of gases and aerosols on short wave radiation and subsequent simulated dynamics. Details on the development and implementation of this two-way coupled system are provided. When the coupled system is executed without radiative feedback, computational time is virtually identical when using the Community Atmospheric Model (CAM) radiation option and a slightly increased (∼8.5 %) when using the Rapid Radiative Transfer Model for GCMs (RRTMG) radiation option in the coupled system compared to the offline WRF-CMAQ system. Once the feedback mechanism is turned on, the execution time increases only slightly with CAM but increases about 60 % with RRTMG due to the use of a more detailed Mie calculation in this implementation of feedback mechanism. This two-way model with radiative feedback shows noticeably reduced bias in simulated surface shortwave radiation and 2-m temperatures as well improved correlation of simulated ambient ozone and PM 2.5 relative to observed values for a test case with significant tropospheric aerosol loading from California wildfires.

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ABSTRACT. A comparative review of algorithms currently used in air qu ality models to simulate aerosol dynamics is presented. This review addresses coagulation, condensational growth, nucleation, an d gas r r r r rparticle mass transfer. Two major approaches are used in air qu ality models to represent the particle size ( ) distribution: 1 the sectional approach in wh ich the size distribution is discretized into sections and particle properties are assumed to be constant over particle size ( ) sections and 2 the modal approach in wh ich the size distribution is approximated by several modes and particle properties are assumed to be uniform in each mode. The section al approach is accur ate for coagulation an d can reproduce the major ch aracteristics of the evolution of the particle size distribution for condensational growth with the moving-center an d hybrid algorithms. For coagulation and condensation al growth, the modal approach provides more accurate results when the standard deviations of the modes are allowed to vary than it does when they are ® xed. Predictions of H SO nucleation rates are highly sensitive to environ -2 4 mental variables and simulation of relative rates of condensation on existing particles and nucleation is a preferable approach. Explicit treatment of mass transfer is recommended for cases where volatile species undergo different equilib-( rium reactions in different particle size ranges e.g., in the presence of coarse salt ) particles . The results of this study provide useful information for use in selecting algorithms to simulate aerosol dynamics in air qu ality models and for improving the accuracy of existing algorithms.

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