CB6 mechanism regarding SULF and ASO4

CMAQ Chemistry and Aerosols
1

Hello everyone,

I have a question about the CB6r5 mechanism, specifically regarding SULF and ASO4.

'EVERYWHERE', 'ALL', 'SULF', 'SULF', 'GAS',  0., 'UNIT', 'a',
'EVERYWHERE', 'ALL', 'SULF', 'ASO4', 'FINE', 1., 'MASS', 'a',
'EVERYWHERE', 'ALL', 'PSO4', 'ASO4', 'FINE', 1., 'UNIT', 'a',

I have a few questions:

  1. What is the difference between the two SULF species, since they appear in different namelist tables (GC and AE)?

    My understanding is that SULF (gas) represents sulfuric acid (H₂SO₄, molecular weight = 98 g/mol), whereas SULF in the aerosol mechanism ultimately represents sulfate (SO₄²⁻, molecular weight = 96 g/mol).

    Also, am I correct in thinking that ASO4 can be involved for condensation and nucleation, whereas SULF (gas) is involved only in condensation? Or does SULF (gas) also participate in nucleation within the CB6r5 mechanism?

  2. Do SULF (gas) and SULF (FINE) partition between the gas and particle phases, or are they treated as separate species within the mechanism?

  3. I have emissions of volatile sulfate particulate matter (PMSO), referring to particles containing sulfuric acid (H₂SO₄) and sulfate compounds. Is it appropriate to map these emissions only to ASO4, or should some fraction also be emitted as SULF (GAS)? or Any suggestions?

Best regards,

Darby

2

Hi, Darby-

It looks like your example at the top is from the CMAQ_Control_DESID_cb6r5_ae7_aq.nml file. These lines indicate that you are mapping all SULF (gas-phase H2SO4, sulfuric acid) EMISSIONS to the CMAQ ASO4 species (aerosol sulfate) rather than SULF (gas). Check out Appendix B in the CMAQ users’ guide to learn about the structure and usage of these control files.

SULF can be involved in nucleation and condensation. Once SULF condenses to aerosol sulfate (ASO4) it remains in the aerosol phase (i.e., no evaporation back to SULF). ASO4 can also react to form organosulfate species.

I do not know the particulars of your PMSO emissions, but you can change how they are mapped to CMAQ species, if you choose, in your CMAQ_Control_DESID_${MECH}.nml file. You can decide to put it all in SULF, all in ASO4, or a fraction in each.

Hope this helps,

Kathleen