the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Nov 2024
New insights into the nonlinear effects of NOx on SOA formation from isoprene photo-oxidation
Abstract. Atmospheric isoprene can be oxidizene SOA yield on NOx concentrations was investigated by performing a series of batch chamber experiments; both the gas and aerosol phase chemical species were characterized using High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-TOF-CIMS) and High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-TOF-AMS), along with an Observation-Based Model (OBM) incorporated with the Master Chemical Mechanism (OBM-MCM model) simulation. We found that NOx could influence the formation of the ultralow volatility organic compounds (ULVOCs, log10 C* < −8.5), low volatility organic compounds (LVOCs, −4.5 < log10 C* < −0.5) and extremely low volatility organic compounds (ELVOCs, −8.5 < log10 C* < −4.5) by changing the RO2 fate, which are the critical compounds in nucleation and condensation in particle phase respectively. The SOA of isoprene photooxidation was mainly from RO2+HO2 and RO2+NO pathways. When RO2+HO2 was the dominant RO2 fate, the SOA yield increased with the fraction of RO2+HO2 and RO2+NO increasing. While when NO is the major sink for RO2, RO2+NO would inhibit the formation low volatile VOCs and affect the SOA yield. The branching ratio term (β) is used to denote the competitive relationship between the two RO2 fates (RO2+HO2 and RO2+NO). The loss rate of RO2+HO2 pathway was maximized at a branching ratio β of 0.5 ([NOx]/[Isoprene]=0.77), when more low volatiles were produced and the SOA yield reached maximum. The branching rate term (β) can be used as a reference for field campaign and modeling.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-3046', Anonymous Referee #2, 23 Nov 2024
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RC2: 'Comment on egusphere-2024-3046', Anonymous Referee #1, 28 Nov 2024
Review of “New insights into the nonlinear effects of NOx on SOA formation from isoprene photo-oxidation” by Xu et al.
In this work, the authors examine the effects of NOx on isoprene SOA yields. The authors varied NOX from 10 ppb up to 2 ppm. The starting concentration of isoprene was about 1 ppm for all experiments. The authors then discuss the effects of oxidant identity and RO2 fate on the observed SOA yields. The main RO2 fates in these experiments were RO2+NO and RO2+HO2 and the maximum SOA yield is observed when the ration between these two is 1:1.
It is unclear to me what the goal of these experiments is based on the current presentation of the data. For example, lines 34-37 “We found that NOx could influence the formation of the ultralow volatility organic compounds(ULVOCs, log10 C* < −8.5), low volatility organic compounds (LVOCs, −4.5 < log10 C* < −0.5) and extremely low volatility organic compounds (ELVOCs, −8.5 < log10 C* < −4.5) by changing the RO2 fate” is something that is already known so the expectation would be that there is a more quantitative analysis of the NOX dependence for these species. However, the article only shows that more of these species form at 712 ppb than at 2 ppm (Figure 3) with no other context or information. How was the yield of these species affected by NOX and how did that in turn affect the overall SOA yields? Does the updated model predict these products? No questions were answered.
If the goal is to study the effect of NOX on SOA yields from isoprene, then why use such atmospherically irrelevant starting concentrations? In fact, there is so much NOX in this system that there is a significant amount of NO3 oxidation occurring. Considering the NO3 concentrations reported in figure S3 it is likely that many of the species that contribute to SOA in this experiment are organic nitrates from multiple generations of NO3 oxidation. Was the goal to study nighttime isoprene SOA yields? Was the goal to study multi-generation oxidation in the isoprene system? In Figure 1 SOA does not peak until much after the isoprene is consumed.
Even the 𝛽 obtained in this work is hard to interpret. In the real atmosphere, RO2 from isoprene have NO, HO2, RO2 and isomerization as reaction pathways, the latter two which are not represented here at all but would be major pathways during nighttime oxidation. Also, considering the oxidant identity is likely a mixture of OH and NO3 there is no guarantee that the 𝛽 would work when applied to daytime oxidation where the bulk of the isoprene is consumed. It would not apply to nighttime oxidation either.
I do not believe the current article should be accepted. If the authors wish to use the data from these experiments then they should focus on the observed products as a function of NOX and accurately identify the oxidant identities per experiment along with the RO2 lifetimes and fates.
Small notes:
Line 28: “oxidezene”
Line 152: The use of HOM in the context of this paper is inadequate. HOMs refer to molecules that incorporate a significant number of O2 through unimolecular isomerization which does not happen in this work according to the presented RO2 fates. The large number of oxygens are from nitrate and peroxynitrate functional groups and peroxides is from RO2+HO2.
Line 192: “Figure S1 illustrates the simplified formation mechanism of four monomers in this system. C5H8N2O8 is formed form the H shift and unimolecular autoxidation of the C5H8NO5-RO2.” This is an incomplete description of the reactions required for this product to form although it is correctly depicted in the SI.
Figure S3: The color scale for the data points in panel b is likely incorrect.
Citation: https://doi.org/10.5194/egusphere-2024-3046-RC2
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