CB6 mechanism regarding Semi-volatile POA

CMAQ Chemistry and Aerosols
1

Hi Everyone,

I have questions about the treatment of semi-volatile POA in the CMAQ_DESID file.

! Gas Species : VLVPO1 VSVPO1 VSVPO2 VSVPO3 VIVPO1
! Particle Species : ALVPO1 ASVPO1 ASVPO2 ASVPO3 AIVPO1
! C* (µg m⁻³) : 0.1 1 10 100 1000

‘EVERYWHERE’, ‘ALL’, ‘POC’, ‘VSVPO1’, ‘GAS’ ,0.045,‘MASS’,‘a’,
‘EVERYWHERE’, ‘ALL’, ‘POC’, ‘VSVPO2’, ‘GAS’ ,0.14 ,‘MASS’,‘a’,
‘EVERYWHERE’, ‘ALL’, ‘POC’, ‘VSVPO3’, ‘GAS’ ,0.18 ,‘MASS’,‘a’,
‘EVERYWHERE’, ‘ALL’, ‘POC’, ‘VIVPO1’, ‘GAS’ ,0.50 ,‘MASS’,‘a’,
‘EVERYWHERE’, ‘ALL’, ‘POC’, ‘ALVPO1’, ‘FINE’,0.09 ,‘MASS’,‘a’,
‘EVERYWHERE’, ‘ALL’, ‘POC’, ‘ASVPO1’, ‘FINE’,0.045,‘MASS’,‘a’,

  1. Gas and particle representation of SVPO1

    In the CMAQ_DESID file, both VSVPO1 and ASVPO1 are assigned a saturation concentration (C* ) of 1 µg m⁻³. However, the CMAQ User’s Guide (Appendix B.2.2.1) states that “Species with C* = 10 can generally have mass split between the gas and particle phases.”

    Question: Since SVPO1 has C = 1 µg m⁻³, why is it represented as both a gas species (VSVPO1) and a particle species (ASVPO1) in CMAQ DESID file? Does the statement in the User’s Guide simply describe a typical example, or is there another reason that species with C = 1 µg m⁻³* are also represented in both phases?

  2. Reversible gas-particle partitioning of POA emissions

    My understanding is that the vapor-particle pairs (e.g., VSVPO1 ↔ ASVPO1) undergo dynamic and reversible gas-particle partitioning within the CMAQ aerosol module. However, primary organic aerosol (POA) emissions are emitted directly into the aerosol species (ALVPO1, ASVPO1, ASVPO2, ASVPO3, and AIVPO1).

    Question: Does this mean that POA mass emitted directly into these aerosol species does not participate in reversible partitioning, while only the corresponding vapor species (VLVPO1, VSVPO1, VSVPO2, VSVPO3, and VIVPO1) can evaporate and condense? Or can the emitted aerosol species also evaporate into their corresponding vapor species and therefore participate in reversible partitioning?

Best regards,

Darby

2

Hi Darby,

  1. Saturation concentrations, C*, indicate how volatile a species is. Species of C* 1, 10, 100 ug/m3 often have substantial mass in both phases. Species of higher and lower volatility still theoretically exist in both phases, but they are generally predominantly in one phase. The gas-particle abundance depends on the concentration of organics in the environment. See equation 1 of Donahue et al. 2006: https://doi.org/10.1021/es052297c.
  2. A/V prepended species are paired and partition between phases. Something emitted in the aerosol (ALVPO1, etc) will later dynamically partition between phases based on environmental conditions.

Havala