Am I doubling the isoprene and other related chemicals when I turn ON the MEGAN?

[Hemraj]

Hi,
I am running CESM2.1.3, FMOZ compset using CAM4, CLM5. I ran the model (default) turning ON the MEGAN (CLM_BLDNML_OPTS="-megan") and found isoprene emission ~ 70 mg/m2/day over Amazon. Checking literature, I can see ~ 30 mg/m2/day for the same reason. Is 70 mg/m2/day a reasonable number or am I doubling it? But how can the default CESM model be set to be doubled? The default emission file read for isoprene emission available under CaseDocs/atm_in is 'emissions.ISOP.surface.1.9x2.5_c110426.nc', having "Biogenic emissions".

I also tried a control simulation changing the emission files by making use of 'emissions-cmip6-ScenarioMIP_IAMC-IMAGE-ssp126-1-1_ISOP_bb_surface_mol_175001-210101_0.9x1.25_c20190225.nc' which have only biomass burning isoprene emissions. The difference of isoprene emission input file from biogenic (default) and biomass burning (changed) have a huge difference (Fig. 1) which is obvious. However, the model output result for isoprene emission is very similar (fig. 2) although using those completely different input files.
Fig. 1 Isoprene emission (mg/m2/day) in input file





Fig. 2 Isoprene emission (mg/m2/day) model output

I am not getting an idea, why the model output result for isoprene emission is doubled, and why the model output results are very similar although the input files are completely different? How to resolve it?

Any suggestions would be highly appreciated.

 

[emmons]

If you are using MEGAN online emissions from CLM, the "biogenic" emissions must be taken out of the 'srf_emis_specifier' list - you will need to create a new list and include it in user_nl_cam.
We provide biomass burning emissions files of ISOP (and MTERP, and other biogenic compounds) and those should be included in the emissions list in the namelist.
The MEGAN online emissions are added to 'SFISOP' in the output. The variable 'MEG_ISOP' contains only the MEGAN biogenic emissions.
Unless there is a specific reason to use CAM4, we highly recommend using a compset such as FCHIST in CESM2, which includes CAM6 and updated chemistry mechanism, with CLM/MEGAN emissions as the default. You could at least build a FCHIST case and check the emissions list and the MEGAN settings, though note that there will be some differences between CAM6 and CAM4, particularly related to the chemical species.
More information is available on the CAM-chem wiki page: https://wiki.ucar.edu/display/camchem/Home