How are background methane and emissions methane handled in CMAQ?

I have observed this in CMAQv5.3.2:

  1. The DESID emission control file EmissCtrl_cb6r3_ae7_aq.nml maps the ‘CH4’ species in the emissions files to ‘ECH4’ species.

  2. In the RXNS_DATA_MODULE.F90 file, the ambient methane level is set as
    PARAMETER :: ATM_CH4 = 1.85000D+00

  3. The chemical mechanism definition file mech_cb6r3_ae7_aq.def has separate reactions for ‘CH4’ and ‘ECH4’:

    <R124> CH4 + OH = MEO2 + RO2 # 1.85E-12@1690;
    <R216> ECH4 + OH = MEO2 + RO2 # 1.85E-12@1690;

    ! Chlorine reactions added from base CB05tucl and updated
    <CL10> CL + CH4 = HCL + MEO2 + RO2 # 6.6E-12 @ 1240;

    CONSTANTS
    <C5> ATM_CH4 = 1.85

My questions are:

  1. Looks like background methane (CH4?) and emitted (as in emissions files output from SMOKE) methane (ECH4) are being handled separately in the chemical mechanism. I am assuming it is correct to say that in some of the gridcells, methane will not be at the background level but rather at an elevated level due to the emissions?
  2. If I am to change the background methane level to say 2000 for a future scenario, should I replace the default value of 1.85 (ppb) of both ATM_CH4 parameter in RXNS_DATA_MODULE.F90 file and the constant ATM_CH4 in mech_cb6r3_ae7_aq.def?
  3. I do not think it will be necessary to feed methane in the Boundary Conditions, but I wanted to be sure about it.

The mechanism definition file (mech_*.def) is a human-readable description of the mechanism. It is processed by the CHEMMECH utility that generates mechanism files for CMAQ. It is not read by the CCTM itself. All you need to do is modify ATM_CH4.

CMAQ uses ppmv units for gas-phase species. The default value for methane (ATM_CH4) is 1.85 ppm. If you want to increase it to 2000 ppb, that should be 2.0 ppm.