Question about the treatment of dry deposition of dust + seasalt aerosols in GEOS-Chem

[olivian13]

Your name

Olivia Norman

Your affiliation

MIT

Please provide a clear and concise description of your question or discussion topic.

Recently I noticed that dust and sea salt concentrations/deposition are different when chemistry is turned on versus off in the default simulation of GEOS-Chem (v14.2.3) and this has brought up a couple of questions:

1. I've determined that the reason for this is that the dry settling of dust/seasalt ('DRY_SETTLING' in dust_mod.F90 and 'WET_SETTLING' in seasalt_mod.F90) is only called if chemistry is turned on (tracing back to the call for CHEMDUST and CHEMSEASALT in chemistry_mod.F90). This function is only dependent on T and pressure (and RH for sea salt), so is there a reason why this process should be turned on/off with chemistry? The same is true for species in dust and sea salt (e.g. SALACL, SALAAL). Below are some quick plots detailing the differences in annual average surface concentration of DST1, all DST species (DST1+DST2+DST3+DST4), SALA, and SALC when chemistry is on vs off:
   
   
   
   

2. What is the difference between the dry deposition calculated + applied in dust_mod/seasalt_mod versus in mixing_mod? Because there is a comment in mixing_mod.F90 that says 'While all dry deposition rates are calculated either in DO_PBL_MIX2 or DO_TEND, settling of aerosols is still computed in the dust/seasalt modules'. Does that mean that all dry deposition of dust/seasalt is handled in dust_mod/seasalt_mod or are there multiple sub-processes of dry deposition that are accounted for in dust_mod/seasalt_mod and in mixing_mod separately? I ask this because if I turn chemistry off, there is still dry deposition of dust + seasalt aerosols reported, despite the calls for dust + seasalt settling in dust_mod/seasalt_mod not happening.

Thanks!
Olivia

[yantosca]

Hi @olivian13, nice to have met you at IGC11. Thanks for your patience, I am just getting back to work this week.

1. The dust/seasalt settling routines have been included in the "chemistry" for as long as the model has existed. Maybe what we can do is to improve the logic so that these routines get called even when chemistry is turned off. This will probably involve moving the calls to outside of chemistry_mod.F90, to do this properly. Let's add this as a to-do item for a future version.

2. I think the comment settling of aerosols is still computed in the dust/seasalt modules simply means that the code to do the settling is in dust_mod and seasalt_mod. I probably wrote that comment so apologies if it was unclear! Again, maybe we need to group the dust/seasalt settling with mixing instead of with chemistry to totally resolve this.