Figure 2-9 illustrates the CMAQ modeling system used to simulate the chemistry and transport of pollutants. Figure 2-9 also illustrates how CMAQ relates to other parts of the Models-3: the meteorological models, emissions model, and the analysis packages. For a model simulation, CMAQ needs input information including meteorological and emissions data. Using this information, the CMAQ Chemistry-Transport Model (CCTM) simulates each of the atmospheric processes that affect the transport, transformation, and removal of ozone, particulate matter, and other pollutants. CMAQ uses state-of-the-science techniques to simulate these processes, as described in the following subsections.
Various modules for simulating tropospheric gas-phase chemistry within CMAQ have been developed, ranging from simple linear and nonlinear systems for engineering model prototypes to comprehensive chemistry representations for detailed chemical pathways related to atmospheric acid and oxidant formation. In CMAQ Version 4.6, gas-phase chemistry can be simulated with the Carbon Bond IV (CB-IV), the 2005 update to the Carbon Bond mechanism (CB05) (Yarwood et al., 2005), or the Statewide Air Pollution Research Center, Version 1999 (SAPRC99) photochemical mechanisms. Because of CCTM’s modularity, users can modify the existing photochemical mechanisms or add new mechanisms. To compute time-varying species concentrations and their rate of formation or depletion (called chemical kinetics), equations governing chemical reaction kinetics and species conservation need to be solved for the entire species set. CCTM uses special numerical techniques called chemistry solvers that compute these concentrations and rates at each time step.
Different solution algorithms have been investigated for the chemical kinetics in terms of the optimal balance of accuracy, generalization, and computational efficiency that is required for this component of the atmospheric system. CCTM currently contains three options for solving gas-phase chemical transformations with the Rosenbrock (ROS3) (Sandu et al., 1997), Euler Backward Iterative (EBI) (Hertel et al., 1993), and Sparse Matrix Vectorized GEAR (SMVGEAR) (Jacobson and Turco, 1994) solvers. Version 4.6 includes the provision for mercury and other toxic compounds. This version also includes special routines to track sulfate and organic carbon species.
Different solution algorithms have been investigated for the chemical kinetics in terms of the optimal balance of accuracy, generalization, and computational efficiency that is required for this component of the atmospheric system. CCTM currently contains three options for solving gas-phase chemical transformations with the Rosenbrock (ROS3)24, Euler Backward Iterative (EBI)25 and Sparse Matrix Vectorized GEAR (SMVGEAR)15 solvers.
