Question Regarding `force_use_old_data` and Emission Compatibility in WRF-Chem v4.6.0

[Naser]

Dear WRF-Chem Support Team,

I am currently running WRF-Chem v4.6.0. Previously, I successfully ran WRF-Chem v3.9.1. To migrate to v4.6.0, I updated several settings in my namelist.input.

One of the options I changed is:

force_use_old_data = .true.

Before enabling this option, I encountered an error when running WRF-Chem v4.6.0. After setting force_use_old_data = .true., the model ran successfully.

My main question is about the emission files. I generated my anthropogenic emissions using the EDGAR-HTAP emission preprocessor provided with older versions of WRF-Chem.

I would appreciate it if you could clarify the following:

1. Is the EDGAR-HTAP emission preprocessor intended only for WRF-Chem versions prior to v4?
2. Is there a different or updated method for generating emission files for WRF-Chem v4.6.0?
3. What exactly does force_use_old_data = .true. do? Does it simply allow WRF-Chem to read older-format emission files, or does it ignore certain compatibility checks or modify the way the emission data are processed?
4. Fundamentally, why do emission files generated for older WRF-Chem versions cause errors in v4.x? Were there changes in the emission file format or data structure?
5. I also noticed that the PM₂.₅ and PM₁₀ outputs are different between WRF-Chem v3.9.1 and v4.6.0, even though I am using the same model configuration and emission files. Could you please explain why these two versions produce different PM₂.₅ and PM₁₀ results? Are there any changes in the aerosol module, diagnostics, or PM calculation methods in WRF-Chem v4.6.0 that could account for these differences?

For reference, I have attached my namelist.input files for both WRF-Chem v3.9.1 and WRF-Chem v4.6.0.

I would greatly appreciate your guidance on these issues.

Thank you very much for your time and assistance.

Best regards,

Naser