I have observed this in CMAQv5.3.2:
The DESID emission control file EmissCtrl_cb6r3_ae7_aq.nml maps the ‘CH4’ species in the emissions files to ‘ECH4’ species.
In the RXNS_DATA_MODULE.F90 file, the ambient methane level is set as
PARAMETER :: ATM_CH4 = 1.85000D+00
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;
<C5> ATM_CH4 = 1.85
My questions are:
- 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?
- 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?
- I do not think it will be necessary to feed methane in the Boundary Conditions, but I wanted to be sure about it.