WRF-Chem V4.5 chem_opt=300: optical-depth overflow only when GOCART background, gas chemistry, and aerosol radiative feedback are enabled

[FirasVV]

I am using WRF-Chem V4.5 to quantify dust-induced changes in radiative fluxes during a June 2024 dust event by comparing dust-on and dust-off simulations.

The simulation uses two nested domains, and the chemistry setup is based on the simple GOCART option: chem_opt = 300, 300

The problem is that the model produces extremely large aerosol optical-depth warnings and does not proceed normally when the following three components are enabled together:

auxinput8 GOCART background = enable gaschem_onoff = 1, 1 aer_ra_feedback = 1, 1
The warnings include messages such as:

WARNING: Large total optical depth of ********
WARNING: Large total lw optical depth of ********
and the optical-depth diagnostics eventually contain very large or infinite values.
This warning first appears at 32 minutes, then the simulation continues for 4 more minutes before it appears again.by 40 minutes into the simulation, the individual rsl files were around 1 gb. I have attached the rsl.error.0000 and rsl.out.0000 and the full namelist. only an initial portion of the rsl files

I performed the following controlled tests, changing only the stated options:

	Anthropogenic emissions through auxinput5	GOCART background through auxinput8	gaschem_onoff	aer_ra_feedback	Result
	On	On	On	On	Fails
	On	Off	On	On	Works
	Off	On	On	On	Fails
	On	On	Off	On	Works
	On	On	On	Off	Works

Could you please advise on the following?

1. Is this a known issue or incompatibility for chem_opt=300when auxinput8, gaschem_onoff=1, and aer_ra_feedback=1 are used together?
2. Is it acceptable to run a dust-on/dust-off direct radiative flux comparison with gaschem_onoff=0?

I would appreciate guidance on whether the issue is more likely related to the background-field values, their interpretation by the chemistry mechanism, or the chemistry–radiation coupling.

Thank you.