Hi, all
I have some questions regarding the oligomerization reaction in CMAQ.
I checked that koli (rate constant) was calculated from a half-life of 20 hours.(Carlton et al., 2010)
But, In CMAQv5.2, how were the oligomer fractions of each chemical species (e.g., AXYL1J = 0.8571 * AOLGAJ) determined?
Similarly, in CMAQv5.3, the VBS approach was used. I would like to know that how determined the fraction values applied in this case as well (e.g., AAVB2J = 0.907 * AOLGAJ).
Thanks
Jaeeun
Oligomerization was implemented to conserve carbon. For v5.2.1, the nC per species is the “decay” value (line 84) in CMAQ/CCTM/src/aero/aero6/SOA_DEFN.F at 5.2.1 · USEPA/CMAQ · GitHub.
In v5.3, the oligomerization reactions were moved out of SOA_DEFN.F and into the chemical mechanism (mech.def). Note that coefficients in mech.def are molar.
Thank you for your response!!
Through this mechanism, I confirmed that the molecular weights of AAVB2J and AOLGAJ are 179 and 206, respectively. However, I am curious about how the molar coefficient of 0.907 is derived (as my calculation resulted in 0.87).
Carbon is conserved with oligomerization. In CMAQv5.2, we used a number of carbon per species to track carbon content. In v5.3 and forward, we only have an OM/OC in the code (CMAQ/CCTM/src/aero/aero6/SOA_DEFN.F at main · USEPA/CMAQ · GitHub).
Hi Jaeeun, The AAVB series was implemented by Momei Qin, a former EPA ORISE postdoc. I pasted below some documentation she developed. Note that the nC, nO, etc for each species was rounded to an integer but it reflects a mixture of different nC species in each bin (which would result in fractional nC). Thus, there may be some inconsistencies in OM/OC, nC, molecular weight, etc depending on whether your allow fractional nC (to reflect an avereage) or not (as nC must be an integer for any real species). Her original values are different than the current code.
Development of AAVB properties for CMAQv5.3
Documentation and work by Momei Qin (ORISE postdoc at EPA mentored by Havala Pye), AAVB documentation also available in Qin et al. (2021)
Assign OM/OC ratio and the number of carbon (nC) to each anthropogenic SOA (SVOC) surrogate based on the properties of aromatic SOA (SVOC) in the original model. Using linear regression, OM/OC and nC for each volatility bin was determined.
Molecular weight (MW; g/mol) was derived from OM/OC and nC (Eq. 1).
MW=12 nC (OM/OC) Eq. 1
Gas-phase diffusion coefficient (cm2/s) was calculated as a function of molecular weight using Eq. 4., following Pye et al. (2017)
D_g=1.9 〖MW 〗^(-2/3) Eq. 4
LeBas molar volume was calculated assuming ring-opened products (Eq. 5), following Pye et al. (2017)
V_LeBas=14.8 nC+7.4nO+3.7nH Eq. 5
In Eq. 5, the number of oxygens (nH) and hydrogens (nO) are required. nO was calculated from nO/nC ratio, which is a function of mass-based OM/OC (Eq. 6 & Eq. 7).
nO/nC=12/15 (OM/OC)-14/15 Eq. 6
nO=nC (nO/nC) Eq. 7
The number of hydrogens, nH, was calculated from the molecular weight (MW) assuming only carbon, oxygen, and hydrogen (Eq. 8).
nH/nC=MW-12(nC)-16(nO) Eq. 8
Enthalpy for each SVOC surrogate used the old one (18 KJ/mol) in the original model, and the properties for the oligomer (AOLGAJ) were also unchanged
The volatility bin for C* of 0.1 was discarded as the yield is zero. Note that calculated nC, nO and nH could possibly not be an integer.
Mole-based yields (α_mole) for SVOC surrogates are derived from mass-based yields (α_mass) in Pye et al. (2010) based on molecular weight of the precursors and products (Eq. 9).
α_mole = α_mass×〖MW〗_precursor/〖MW〗_SOA Eq. 9
Oligomerization of the anthropogenic SVOCs (C* of 1,10 and 100 ug/m3). A yield was applied to keep carbon conserved, with the same reaction rate constant as of biogenic SVOCs.
A_AVB2J = 0.86 AOLGAJ
A_AVB3J = 0.86 AOLGAJ
A_AVB4J = 1.0 AOLGAJ
References
Hodzic, A., Aumont, B., Knote, C., Lee‐Taylor, J., Madronich, S., & Tyndall, G. (2014). Volatility dependence of Henry’s law constants of condensable organics: Application to estimate depositional loss of secondary organic aerosols. Geophysical Research Letters, 41(13), 4795-4804.
Pye, H. O. T., Chan, A. W. H., Barkley, M. P., & Seinfeld, J. H. (2010). Global modeling of organic aerosol: the importance of reactive nitrogen (NO x and NO 3). Atmospheric Chemistry and Physics, 10(22), 11261-11276.
Pye, H. O., Murphy, B. N., Xu, L., Ng, N. L., Carlton, A. G., Guo, H., … & Surratt, J. D. (2017). On the implications of aerosol liquid water and phase separation for organic aerosol mass. Atmospheric Chemistry and Physics, 17(1), 343-369.
Qin, M., Murphy, B., Isaacs, K., McDonald, B., Lu, Q., McKeen, S., Koval, L., Robinson, A., Efstathiou, C., Allen, C., & Pye, H.O.T. (2021). Criteria pollutant impacts of volatile chemical products informed by near-field modeling, Nat Sustain, 4, 129-137. Criteria pollutant impacts of volatile chemical products informed by near-field modelling | Nature Sustainability
Hi Pye,
Your response will be incredibly helpful for my research.
I sincerely appreciate it always, and I’m especially grateful for the detailed explanation.
Thanks
Jaeeun