Gas-phase chemical mechanisms are implemented in CMAQ using FORTRAN include files. These files are in a machine-readable ASCII format and include all of the mechanism parameters including gas-phase species, reaction stoichiometry, and kinetics information. To invoke chemical mechanisms in CMAQ, these files are included in the compilation of the different CMAQ programs to generate mechanism-specific executables. CHEMMECH takes a mechanism definition file, often named mech.def, and generates the mechanism and species include files that define the chemistry parameters for the CMAQ programs. The file “mech.def” is a human-readable ASCII file that is easy to understand and modify. Figure 2.6 demonstrates the relationship between CHEMMECH and the rest of the CMAQ modeling system.