MEGAN output unit conversion: from mol/s to mol/s/km2

MEGAN
1

Hi,

The offline MEGAN output isoprene unit is mol/s. I want to convert the unit (mol/s) to mol/s/km2. The spatial resolution is 4km * 4km. Can I do the unit conversion by simply dividing the output isoprene by 16?

Thanks,
Huazhen

2

In mgn2mech.F the conversion from mol/s/m2 to mol/s is calculated using garea = xcell3d * ycell3d, so you should be able to divide by 16 km2 to recover the per area flux. That will be for the computational grid. For the real earth space you would need to adjust by the map scale factor squared.

If you’re using these for CMAQ then you have to be careful about setting LAREAADJ for the emission stream. This tells CMAQ whether the projection distortion has already been accounted for in the emission stream.

What does the MSFX2 field look like for your domain (GRIDCRO2D file)?

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3

Hi Willison,

Thanks for your reply. My MSFX2 field is shown in the Figure. I will use the MEGANv3.21 generated biogenic emissions for WRF-Chem simulation.

GRIDCRO2D
GRIDCRO2D2153×1144 337 KB

Thanks,
Huazhen

4

I’m not familiar with how WRF-Chem handles emission gridding. Presumably it wants real Earth area so you would calculate area as

  4*4*MSFX2

but those are very low map scale factors so it’ll only be off by 1-2% in the very south either way.

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