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

3

@kfahey Thank you for the explanation.

I have a few more questions about SULF and PSO4.

The following DESID mapping regarded different emission species to the same CMAQ aerosol species, ASO4:

SULF → ASO4 (FINE), MASS
PSO4 → ASO4 (FINE), UNIT

  1. Do the SULF → ASO4 and PSO4 → ASO4 DESID mean that both input emission species are treated as directly emitted particulate sulfate once they enter CMAQ, even though the original emission species and mapping basis are different? or any differences in molecular weight?
  2. Can I interpret that SULF → ASO4 might represent nucleation and PSO4 → ASO4 might represent condensation, or are both representing just primary emitted particles?
  3. My understanding from aero_subs.F is that gaseous H₂SO₄ (SULF) can form Aitken mode sulfate through new particle formation (NEWPART3), while the remaining H₂SO₄ (SULF) can condense onto existing Aitken and accumulation mode particles (HCOND3), and the portion of nucleation and condensation is determined by atmospheric conditions; is this correct?
  4. Also, are SULF fromSO2 + OH → SULF + HO2 + SULRXN and SULF gaseous emissions regarded as the same gas?
  5. ASO4J is consumed in the IEPOX uptake pathway to form AISO3OSJ. Is this the only pathway that consumes ASO4J to form an organosulfate?

Best regards,

Darby

4

Hi, Darby-

1.) In your example, SULF (gas) and PSO4 (particulate) EMISSIONS are mapped to CMAQ’s ASO4 (particulate) species. Conversion factors are applied to conserve mass (BASIS = “MASS”) when mapping SULF gaseous emissions to the particulate model species, ASO4.

2.) For the emissions mapping rules listed, SULF and PSO4 represent emissions “species”, and ASO4 is the CMAQ species to which they are mapped. These are primary particulate sulfate emissions. The DESID rules specify how emissions are mapped to CMAQ species. Remember that while some emissions variable names are different from CMAQ names, some are the same. “SULF” can be found in emissions files; “SULF” also represents sulfuric acid in the cb6r5 mechanism.

3.) Sulfuric acid may condense onto coarse mode particles as well as Aitken and accumulation modes. The overall condensation rate is the difference between the SULF chemical production rate and the mass production rate from new particle formation (nucleation). Condensation to individual modes is size dependent. The nucleation rate does depend on RH and T.

4.) “SULF” represents H2SO4 in the CMAQ chemical mechanism as well as in the emissions files.

5.) Sulfate conversion to organosulfate in CMAQv5.5 is represented by

IEPOXP + ASO4J = AISO3J in cb6r5

or

IEPOXP + ASO4J = AISO3OSJ in the cracmm mechanisms.

Hope this helps,

Kathleen

5

Thank you for your help!