Why is BCWT001 equal to zero globally?

[pritijadhav]

While processing the MERRA-2 monthly files, I noticed that:

BCWT001 = 0

for every valid grid cell that I checked.

Initially, I observed this over the HKH region, but I subsequently checked the variable over the global MERRA-2 domain, and BCWT001 appears to remain zero everywhere.

BCWT002, in contrast, contains non-zero wet-deposition values.

From the aerosol documentation, I understand that the two BC bins may represent different BC states, with Bin 001 corresponding to hydrophobic black carbon and Bin 002 corresponding to hydrophilic black carbon.

If this interpretation is correct, is BCWT001 = 0 an expected result of the MERRA-2/GEOS aerosol model formulation, because hydrophobic BC is not directly removed through wet deposition until it undergoes hydrophobic-to-hydrophilic conversion?

Or is BCWT001 retained in the MERRA-2 output structure even though this diagnostic is effectively zero in this version of the model?

I would particularly appreciate confirmation of:

What exactly do BC Bin 001 and Bin 002 represent in MERRA-2?
Is BCWT001 expected to be zero throughout the MERRA-2 record?
If yes, what physical/model parameterization causes it to be zero?
Should BCWT001 still be included when writing the mathematical definition of total BC wet deposition, even though its numerical contribution is zero?
Is there an official MERRA-2/GEOS document or publication that explicitly documents this behavior?

[GES DISC - cbattisto]

Dear User,

Thank you for contacting us. Here are some answers from the GMAO team:

• Bin 1 of carbon is hydrophobic, bin 2 is hydrophilic

• BCWT001 is expected to be zero because wet deposition does not act on hydrophobic particles

• For completeness, I always like to include BCWT001 when I write out the equation, but the contribution of the term will be zero

• There may not be a document or publication for MERRA-2, but it should be included in the budget section of the documentation for MERRA-21C

• Total loss = BCDP001 + BCDP002 + BCSD001 + BCSD002 + BCWT001 +BCWT002 + BCSV001 + BCSV002

Regards,
GES DISC User Services