SOA enthalpies in CMAQ

CMAQ Chemistry and Aerosols
1

Hi, all
I am currently conducting SOA simulations using AERO7 in CMAQv5.3.2.
In this mechanism, the VBS approach is applied to simulate SOA from aromatic precursors, categorized as AAVB1-4 based on their volatility(C*).
I am curious why the enthalpy values of AAVB1-4, despite their differing volatilities, are all uniformly set to 18 kJ/mol.
(This contrasts with AMT1-7 assigns different enthalpy values across a total of 7 bins.)

Thanks
Jaeeun

2

Hi Jaeeun,
The lower enthalpies for the AAVB series are a hold over from previous methods. Individual semivolatile species should generally have C* on the order of 100 kJ/mol (e.g., Epstein et al. 2009). The large range of concentration data in the monoterpene system supported constraints on a large range of C* and thus enthalpies were set consistent with individual compounds (see Xu et al. 2018). Coarse volatility binning (approximating a multicomponent system with few species) has been shown to require lower enthalpies of vaporization to reproduce observed behavior (Riipinen et al., 2010). Since the AVB series was based on data that goes back to pre-2010 (Pye et al. 2010) and has a limited span of concentrations resulting in limited C* bins, we retained the older enthalpy values introduced in CMAQv4.7 for aromatic systems (Carlton et al. 2010).
Havala

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3

Hi Pye,
Thank you so much your responses.

And I have one more questions about enthalpy in CMAQ.
If I wish to apply a new enthalpy for each individual SVOC(e.g. BENZ, TOL), is it possible to use the VBS approach (or can the 2-product approach be used)?

Thanks
Jaeeun

4

Hi Jaeeun, Both VBS and Odum 2-product approaches work with enthalpies of vaporization. If you need to track the aromatics as separate systems, you would need to modify aero7 to do that as the benzene, toluene, and xylene systems are not distinguishable once placed in the AAVB series (yields are system specific but products have the same name). The coarser the volatility bins (e.g. Odum vs VBS), generally the lower the enthalpy to achieve the same temperature sensitivity of OA. Ideally, the enthalpies should be determined based on the C* to ensure the combination of enthalpy and C* reproduce observed data, if available. If you have temperature dependent OA data, you could fit the AAVB enthalpies across all 3 aromatics and see how well that does. The fact that different precursors populate different C* in different amounts means that each precursor’s OA will have a different sensitivity to T within the current AVB lumping. Havala