CMAQv5.1 Mechanisms

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CMAQ v5.1 Photochemical Mechanisms

Mechanism Definitions

Photochemical mechanisms define the chemical reactions that destroy or produce gas and aerosol species. The modeled chemical species used in each mechanism are listed in three CMAQ files:

  • GC namelist for the gas-phase species
  • AE namelist for the aerosol species
  • NR namelist for the non-reactive species
  • TR namelist for tracer species

The photochemical mechanism does not define the reactions and species used in cloud chemistry.

The directories under $M3HOME/models/CCTM/MECHS contain files that define the chemical species and their reactions for each of the CMAQ chemistry mechanisms. Along with the GC, AE, and NR namelists, a set of other files are used to define the CMAQ mechanisms:

  • The CSQY_DATA_mechanism file contains data used for in-line photolysis rates calculations
  • The mech*.def file lists the photochemical reactions.
  • The RXNS*.F90 files are machine-generated, Fortran source code implementations of the CMAQ mechanisms. The CMAQ utility CHEMMECH uses the mech*.def file to generate the RXNS*.F90 files for use in compiling the different CMAQ programs.
  • The trac* file contains the TR namelists for configuring CMAQ to run with inert tracer species.

Chemistry Solvers

The directories under $M3HOME/models/CCTM/gas contain the Fortran source codes for numerical solvers to calculate species concentrations using the different CMAQ chemical mechanisms.

  • The ros3 and smvgear solvers work for any mechanism and are based on Rosenbrock and Gear methods, respectively.
  • The ebi_mechanism solver only works for the specified mechanism. They are optimized solvers based on a Backward Euler Iteration (EBI) method and analytical solutions.

CMAQ v5.1 Mechanism Listing

The table below lists the chemistry mechanisms available in CMAQ v5.1. The entries in the MECHS Module column correspond to the Mechanism setting in the CMAQ build scripts. Each row in the table defines the aerosol and cloud modules that are compatible with each mechanism. The preceding paragraph states what gas modules work for a specific MECHS module.

MECHS Module Aerosol Module Cloud Module
cb05e51_ae6_aq aero6 acm_ae6 or acm_ae6_kmt
cb05tucl_ae6_aq aero6 acm_ae6 or acm_ae6_kmt
racm2_ae6_aq aero6 acm_ae6 or acm_ae6_kmt
saprc07tb_ae6_aq aero6 acm_ae6 or acm_ae6_kmt
saprc07tc_ae6_aq aero6 acm_ae6 or acm_ae6_kmt
saprc07tic_ae6i_aq aero6i acm_ae6
saprc07tic_ae6i_aqkmti aero6i acm_ae6i_kmt
cb05mp51_ae6_aq aero6_mp acm_ae6_mp
cb05tump_ae6_aq aero6_mp acm_ae6_mp

CMAQv5.1 Mechanism Changes and Updates

Base mechanisms for simulating criteria pollutants

Multi-Pollutant mechanisms for simulating both criteria and hazardous air pollutants.


Golam Sarwar, National Exposure Research Laboratory, U.S. EPA