I am curious what the state-of-the-art understanding is regarding the abundance (ratio) of different SOA products of different volatilities for each precursor like monoterpene or isoprene? I guess it will be different for the Western US and Southeastern US?
Hello,
I’ll highlight some papers that can provide more detail as the SOA system and our understanding continues to evolve. Volatility and yields of all species are available in SOA_DEFN* or mech*.def in the CMAQ code. The following information is current as of CMAQv5.3.
Isoprene SOA is abundant only in summer and can account for ~20% of total SOA in the southeastern/eastern US (based on AMS data, 2-methyltetrol and organosulfate observations, and CMAQ predictions). The current isoprene SOA treatment is described in Pye et al. 2017 ACP. The IEPOX SOA (subset of isoprene SOA, species AISO3J) was implemented in Pye et al. 2013 ES&T, with a Henry’s law coefficient update in the 2017 paper. IEPOX SOA is implemented as nonvolatile, but a portion of it, specifically the 2-methyltetrols should be semivolatile. Isoprene +NO3 makes semivolatile SOA, but the IEPOX SOA dominates. A figure in Pye et al. 2017 illustrates a comparison with AMS Isoprene SOA for IEPOX SOA as well as IEPOX+glyoxal and IEPOX+semivolatile SOA. Roughly, the isoprene SOA as whole seems to be well represented compared to (limited) measurements, but certainly volatility and the role of phase separation continue to be areas of research.
Monoterpene SOA dominates all types of SOA in the eastern US (Zhang et al. 2018 PNAS, Lee and D’Ambro et al. 2020 ACS ESC) and is abundant at all times of year (Xu et al. 2018 ACP). In v5.3, monoterpene SOA it is much less volatile than in previous versions of CMAQ. We are currently using a VBS parameterization that promptly forms low volatility material as documented in Xu et al. 2018 ACP. The prompt low-volatility SOA is a stand-in for HOM/autoxidation SOA, but is not a great representation of that process.
Isoprene and monoterpene SOA are more abundant in the eastern US. See Baker et al. 2015 ACP for CalNex evaluation of CMAQ v5.0.2. Isoprene and monoterpene SOA have not been individually evaluated for CalNex since then, but total SOA has been evaluated.
Anthropogenic SOA is abundant in CMAQ if you use semivolatile POA and pcSOA as implemented by Murphy et al. 2017 ACP (without that treatment or some other empirical SOA, anthropogenic SOA is modest-see Baker et al. 2015 ACP; Woody et al. 2016 ACP). pcSOA is low volatility (~entirely condenses) and designed to reproduce measurements. We think it nominally covers anthropogenic SOA from vehicles as well as VCPs. Qin et al. 2020 Nat Sustain showed that SOA from VCPs shows correlation just like pcSOA.
Full citations for publications mentioned here can be found on the CMAQ EPA website:
Thanks @Havala.Pye for the detailed response. I had one additional question about the (water) solubility of the semivolatile oxidation products of BVOCs and anthropogenic VOCs like AAVB1-4, AISO1-3, AMT1-6: are their solubility properties very high and therefore prone to washout by rain in Southwest US and in California?
In the latest versions of CMAQ, we have generally tried to specify unique deposition properties for all gas-phase organic species . You can find the values unique to the species in hlconst.f. You can also refer to the papers where the algorithms were developed. For SOA species, the relative role of solubility and gas-phase deposition depends on volatility (see Figure 5 of Pye et al. 2017) as well as the solubility of the gas-phase species (refer to hlconst values).
