To perform air quality simulations, both initial and boundary conditions are required. Initial conditions (calculated in ICON) are needed to provide concentrations at the first time step of individual chemical species for the modeling domain. Boundary conditions (calculated in BCON) are needed to provide concentrations of individual chemical species at the lateral boundaries of the modeling domain. ICON and BCON can generate these concentrations for multiple chemical species.
ICON and BCON require two inputs (Figure 2-4): a concentration file for the chemical species to be simulated and the chemical mechanism. The concentration file used in ICON and BCON can come from one of two sources as follows:
- Time independent set of vertical Regional Acid Deposition Model, Version 2 (RADM2) (Stockwell et al., 1990) concentration profiles. This approach is usually used when no other information about the initial and boundary concentrations is available.
- Existing CCTM 3-D concentration fields. Usually, this is used when performing a nested model simulation and modeling results from a previous CCTM simulation are available from a coarser grid resolution simulation. Existing CCTM concentration fields are also used when a CCTM simulation is extended in a separate run step. Unlike the profiles mentioned previously, these CCTM concentration files are spatially and temporally resolved.
Both the vertical concentration profile fields and the CCTM concentration fields have specific chemical mechanisms associated with them, which are a function of how they were originally generated. Figure 2-4 assumes the RADM2 chemical mechanism was used to generate the default vertical profiles and is consistent with the profiles distributed with the CMAQ version 4.4 tutorial dataset. Existing CCTM 3-D concentration fields could have been generated using several different chemical mechanisms. The chemical mechanism used in the user's CCTM and CMAQ input processors must be consistent with the mechanism used to generate the ICON and BCON concentration fields, unless you use the option to convert to either Carbon Bond-IV (CB-IV) (Gery et al., 1989) or Statewide Air Pollution Research Center 1999 (SAPRC-99) (Carter, 2000) from RADM2. ICON and BCON are capable of converting from the RADM2 to either the CB-IV or SAPRC-99 chemical mechanisms, as discussed in Chapter 5.
ICON and BCON can linearly interpolate input concentration profiles from one horizontal or vertical coordinate system to the one needed for the model simulation if the input data are in the standard I/O API format. If the interpolation is between two different vertical coordinate systems, the mid-layer height of each vertical layer must also be available.