I am trying to analyze isoprene emissions (MEG_isoprene) from the CLM5 and MEGAN but have found conflicting information on what units this variable defaults to in the daily from my model run. Can anyone provide insight into the units the emissions from MEGAN (e.g., 'MEG_<species>') are in? Thank you!
-
The www2.cesm.ucar.edu website is currently down for maintenance
Units of MEG_isop
- Thread starter demurray
- Start date
The model's technical documentation should have reliable information:
If you discover a mistake, please let us know.
If you discover a mistake, please let us know.
Hi @slevis -- thanks for this information and apologies for the double post. If I am interpreting the information on the link above correctly, isoprene emissions (MEG_isoprene) are expressed in ug C/m2/ hour -- and already expressed as the mass of carbon.
However, when I go to my emission files associated with my model and run 'ncdump -h *.clm2.h0.2023-12-07-00000.nc' to get the metadata, the units for MEG_isoprene are kg/m2/s. It is unclear whether the mass is expressed as the total mass of C5H8 or just the carbon component. I have been assuming that the correct units are those in the emissions file metadata (kg/m2/sec) but any clarification on this would be great! Thanks in advance.
However, when I go to my emission files associated with my model and run 'ncdump -h *.clm2.h0.2023-12-07-00000.nc' to get the metadata, the units for MEG_isoprene are kg/m2/s. It is unclear whether the mass is expressed as the total mass of C5H8 or just the carbon component. I have been assuming that the correct units are those in the emissions file metadata (kg/m2/sec) but any clarification on this would be great! Thanks in advance.
I don't have a definitive answer for you, I'm going to move this to the CAM-Chem Forum. But I see this in VOCEmissionMod.F90
call hist_addfld1d ( fname='MEG_'//trim(meg_cmp%name), units='kg/m2/sec', &
avgflag='A', long_name='MEGAN flux', &
ptr_patch=meg_out(imeg)%flux_out, set_lake=0._r8, set_urb=0._r8 )
If I look for meg_out(imeg)%flux_out I see:
! assign to arrays for history file output (not weighted by landfrac)
meg_out(imeg)%flux_out(p) = meg_out(imeg)%flux_out(p) &
+ epsilon * gamma * megemis_units_factor*1.e-3_r8 ! Kg/m2/sec
The code comments say that epsilon has units of ug/m2/hr, gamma is unitless, megemis_units_factor converts from ug/m2/hr to g/m2/s, and 1e-3 converts from g to kg.
So the units of the second term on the righthand side are in kg/m2/s.
Which means that meg_out(imeg)%flux_out(p) must be kg/m2/s.
I see this also:
real(r8), pointer, private :: flux_out(:) ! patch MEGAN flux [ug C m-2 h-1]
So the ug C implies it's in carbon units. However, it seems to me that flux_out is already in kg C, not ug C, so maybe the comment is incorrect? Or there's a conversion I'm not seeing.
call hist_addfld1d ( fname='MEG_'//trim(meg_cmp%name), units='kg/m2/sec', &
avgflag='A', long_name='MEGAN flux', &
ptr_patch=meg_out(imeg)%flux_out, set_lake=0._r8, set_urb=0._r8 )
If I look for meg_out(imeg)%flux_out I see:
! assign to arrays for history file output (not weighted by landfrac)
meg_out(imeg)%flux_out(p) = meg_out(imeg)%flux_out(p) &
+ epsilon * gamma * megemis_units_factor*1.e-3_r8 ! Kg/m2/sec
The code comments say that epsilon has units of ug/m2/hr, gamma is unitless, megemis_units_factor converts from ug/m2/hr to g/m2/s, and 1e-3 converts from g to kg.
So the units of the second term on the righthand side are in kg/m2/s.
Which means that meg_out(imeg)%flux_out(p) must be kg/m2/s.
I see this also:
real(r8), pointer, private :: flux_out(:) ! patch MEGAN flux [ug C m-2 h-1]
So the ug C implies it's in carbon units. However, it seems to me that flux_out is already in kg C, not ug C, so maybe the comment is incorrect? Or there's a conversion I'm not seeing.
Thanks for the input @oleson, that is very helpful to confirm the units are in kg/m2/s in the emissions output file. It may be a good idea to update the documentation that @slevis provided as these are conflicting?
As for whether the isoprene emissions are already in units of carbon, I don't think the units are in kg Carbon already, as per this post (How to convert emissions (molecules/cm2/s) to mass emission (kg/m2/s)?) which lists a file with the molecular weight for all output variables from CAM-chem: Using CAM-chem Output - CAM-Chem - wiki.ucar.edu, including ISOP as the weight of 68.1142 g/mol.
But if anyone can confirm this that would be great, thank you in advance.
As for whether the isoprene emissions are already in units of carbon, I don't think the units are in kg Carbon already, as per this post (How to convert emissions (molecules/cm2/s) to mass emission (kg/m2/s)?) which lists a file with the molecular weight for all output variables from CAM-chem: Using CAM-chem Output - CAM-Chem - wiki.ucar.edu, including ISOP as the weight of 68.1142 g/mol.
But if anyone can confirm this that would be great, thank you in advance.