This work studied nocturnal oxidation of alpha-pinene synergistically by O3, NO3, and OH. The manuscript reports that in the synergistic O3 + NO3 regime, CHO-HOM production is substantially suppressed compared to O3-only regime, due to rapid termination reactions between RO2 formed from alpha-pinene + NO3 and those formed from ozonolysis and OH oxidation, which is 10-100 times faster. This effect also leads to a reduction in ultralow and extremely low-volatility organic compounds. The work is solid and well written. However, there are a few issues and unclear details that need to be addressed before published at ACP.

Scientific comments:

1. Line 23 in Abstract. Stating that termination reactions are “10-100 times more efficient” is vague. In the kinetic model later, does it assume that the difference is only about RO2 + RO2 reaction rate constant, but not about dimer yields from these reactions?
2. Line 117. A reaction time of 25 seconds is long enough to form particles in precursors’ concentrations are high. Was particle measurement performed for this?
3. Section 2.1. A few important details should be provided in this section: (1) under the mixed O3/NO3 condition, how much of alpha-pinene was oxidized by either oxidant? (2) Was NO2 also present when alpha-pinene was oxidized? (3) What was the typically reacted alpha-pinene concentrations? (4) A model-based estimation of RO2 bimolecular lifetime under these conditions should be provided. And (5) Did the authors assume that in NO3-CIMS, all HOM species have the same sensitivity?
4. Line 185-189. Besides these two reasons, it is also possible that the presence of NO2 scavenged all acyl RO2, which may be key to forming dimers. Earlier in the text, the authors stated that RO2 + NO2 reactions are considered. How about acylRO2 + NO2 specifically to remove acylRO2s out of the system? CIRO2 contain more aldehydes and thus its product RO2s are more likely acylRO2 than the OHRO2. This could make sense if NO2 has a major impact on the termination reactions for the CIRO2 pathways.
5. Figure 1. For (a) and (b), I suggest further clarifying what fractions of the RO2, monomers, and dimers are made of compounds containing nitrogen. For (c), I suggest including CHO compounds as well, but using a different color. It might be also nice to show a mass spectrum with O3 only, so that the comparison can be more clarified. In Line 207, the authors claimed “substantial formation of these dimeric ONs”; having a direct comparison can support this. In (c), C10H17NO8 is the largest peak. Its formation should be briefly discussed. How does it form if C10H16NO5 does not autoxidize rapidly, and the RO from RO2+RO2 reactions mainly release NO2 and produce pinonaldehyde? Besides these suggestions, I wonder if the relative changes can be affected if the sensitivities are different from different species. This is such a major assumption, but it was not discussed in the manuscript.
6. Line 249. This is related to comment #1. It is not true if the different RO2 cross reactions could also change branching ratios of ROOR. This possibility needs to be discussed.
7. Line 290-294. Can these findings be explained by the kinetic model?
8. Line 326-328. However, the C* distribution in Figure 5 does not show higher abundance for the SVOC & IVOC range under NO3/O3 mixed oxidation conditions. How come the SOA mass loading is higher?
9. Section 3.4. It is nice to expand the chemistry into real-world conditions. The authors considered boreal forest conditions where monoterpenes are high. But they also mentioned southeast US conditions, where isoprene is high. Can the southeast US scenario be modeled? I think this is doable as the same authors published a paper on mixed isoprene/monoterpene oxidation.

Technical comments:

1. Line 109. Change “their” to “its”.
2. Line 164. Does NO3RO2 represent only the primary RO2 from NO3 + alpha-pinene (i.e., C10H16O5-RO2) throughout the manuscript? It should be clarified if that is the case.
3. Line 233. Change “strong” to “stronger”.
4. Figure 4. On the y-axis, “conc” is not accurate. Should be intensity or signal. Also, does “CA” mean cyclohexane? It should be clarified.

Zang and coworkers investigated synergistic effects on the reduction of low-volatile organic compounds during nighttime oxidation of a-pinene. Through laboratory flow tube experiments, the authors found that NO3-RO2 reacts with CI-RO2/OH-RO2 and impedes the formation of low-volatile HOMs that would form a secondary organic aerosol. The results robustly show the synergistic effect on low-volatile organic compound reduction via well-designed experiments under conditions with and without NO3 radicals. The findings in this study would improve our understanding of complex and more realistic environments where different atmospheric radicals present and affect the oxidation chemistry of biogenic volatile organic compounds.

However, there are drawbacks in this study that need to be improved. My main concern is that the experimental conditions would not successfully represent the ambient atmosphere conditions. In Section 3.4., the authors commented on the input conditions of the model they ran, which were similar to the ambient atmosphere conditions of boreal forests reported in previous studies. While the authors ran the model under humid conditions, lab experiments in this study were performed only under dry conditions. The humidity condition would affect RO2/ozonolysis reaction chemistry as well as the fate of criegee intermediates and the other oxidation products. I suggest conducting additional experiments and validating if the authors would get the same experimental results between dry and humid conditions, and then applying such results to the model to understand if the findings in this study can be applied to the actual ambient environment.

Scientific comments:

• Line 180 - 183: What was the RO2 fate like at each experiment? Might be helpful if providing figures in the SI
• Line 192: Why did you normalize by Δ[a-pinene]O3? Please add a more detailed explanation.
• Line 209 - 213: Would the low signal of NO3-RO2(C10H16NOx) be indeed because of less autoxidation? Or could it be due to NO3-CIMS's limitation on sensitivity over such compounds? Were there possibilities that unidentified compounds were being lost to the wall or particles?
• Line 225: What are the "other reactions" in Figure S3? Please specify in the legend or embed those reactions in the figure. Also, would H-abstraction by NO3 be small?
• Line 233: Do you expect the predominant type of RO2 would be different among CI-RO2, NO3-RO2, and OH-RO2 (i.e. if they are primary, secondary, tertiary, or acyl-RO2)? Could you add more discussion on the NO3-RO2's termination effect? 
• Cyclohexane experiment: Why haven't you run any SOA experiments for this condition? If this experiment was just for a sanity check, I suggest moving it to SI. Also, is there a reason why some of OH-RO2 and HOMs monomer species in Figure 2 are not shown in Figure 4 (i.e., C10H17O10, C10H18O11)? Additionally, if you labeled specific ON-HOM compounds in Figure 1c, you should have shown how they changed in Figure 4c as well.   
• Line 301: Weren't the results up to this line showing that CHO-HOMs were terminated via NO3-RO2 and CI-RO2 reactions? Little via OH-RO2?
• Line 328: I suggest the authors shall add more discussion on particle formation and growth. What is the main factor that drives larger mass SOA concentration? Did you identify more numbers of compounds showing higher signals over certain thresholds? Was the entire sum of CPS different by reaction conditions?
• Figure 6: At least in SI, I would like to see how size distribution is different between the experimental conditions, and how they vary. That comparison may give some insights into the observation in Figure 6.  
• Line 338: How well do the experiments reflect the given ambient condition? How were NO and NO2 concentrations in the experiments? How would RH variation affect NO3/N2O5? How would the aqueous-phase reaction affect RO2 formation and fate? Also, high RH would have hydrolysis of ON-HOMs and the reaction mechanism/products would not be the same as what you explored in your experiments. I think you should validate from additional humid condition experiments if your experimental results can be applied to the atmospheric models regardless of the humidity conditions.
• Line 374: Do these HOM monomers and dimers have high numbers of oxygen as what you observed from the lab experiments?
• Line 388: How about under very low NO2, NO3, and N2O5 environments? Would NO3 still suppress CHO-HOMs during nighttime?
• I think you should add a discussion on the role of CI-RO2 on dimer & ULVOC formation as well. Additional discussion on this based on the comparison with previous studies would help readers learn about nighttime oxidation chemistry and would help emphasize why your findings are important.

Technical comments:

• Line 178: Please add a more detailed explanation on the y-axis of Figure 1.   
• Line 180 & Table S1: How about adding a footnote of experimental conditions that are compared to each other?
• Line 198: Could you also specify that these monomers & dimers are CHO-HOMs? Because the next figure focuses on ON-HOMs, it would be better to make it clear to avoid confusion.
• Line 200: Add "among experiments with same initial a-pinene concentration" before "(Exps 1-10)"
• Line 215: Were you trying to say that the instrument's resolution is not good enough to separate these? If so, I would say "the instrument's resolution is not enough to differentiate the mass closure between NO3-RO2 and CHO-HOMs (Table S3), limiting the detection of NO3-RO2 species."  
• Line 257: Please add a statement in general words and specify what this reaction efficiency means to the observations in Figure 1 and/or 2 results.   
• Figure 4: What is "CA" on the right axis?
• Line 307 & 311: Figure 5 only has one figure, not any subfigures
• Line 349: I think it would be better to have a pie chart showing RO2 fate in SI (both from your experiments and model application)
• Line 369 - 371: Please check the grammar in this sentence. 

This manuscript presents measurements of gas-phase organic peroxy radicals (RO₂), highly oxygenated organic molecules (HOMs), and dimeric compounds formed from oxidation of α-pinene by either O₃ or NO₃ + O₃ in a flow tube reactor made using a nitrate chemical ionization mass spectrometer (NO₃-CIMS), together with kinetic model simulations. The authors find that the formation of ultra-low and extremely low volatility organic compounds (ULVOC and ELVOC) measurable by NO₃-CIMS is significantly reduced in the NO₃ + O₃ system and further conclude that “the formation of new particles in the synergistic oxidation regime is substantially inhibited compared to the O₃-only regime.” However, aerosol mass concentrations in the NO₃ + O₃ system were observed to be a factor-of-two higher than in the O₃-only system, directly contradicting this conclusion. Although the manuscript is well written, in many respects it replicates the work of Li et al. 2024 and Bates et al. 2022. For these reasons, I recommend that publication be considered only after the comments detailed below are addressed.

1. Table S1. Please specify how the initial α-pinene, cyclohexane, and O₃ concentrations were determined (i.e., measured, modeled, or estimated). Please add columns that report the modeled fractions of α-pinene that reacted with each oxidant (i.e., O₃, OH, and NO₃) as well as the modeled initial NO₂ concentrations.
2. Figure 1. Figures 1a and 1b are redundant. Please replace Figure 1a with one that shows the signals of total RO₂, total monomer, and total dimer normalized by the total α-pinene reacted for both the O₃ + NO₃ and O₃-only systems, with the bars subdivided to indicate the fractions of CHO and CHON species. Please include a discussion of this figure (e.g., were normalized signals of total monomers and dimers higher in the O₃ + NO₃ or O₃-only system?) and revise L176–196 accordingly. Please also include a CIMS spectrum of an O₃-only experiment for comparison to Figure 1c.
3. CHON Dimers. Both Bates et al. 2022 and Li et al. 2024 observe significant (and often dominant) contributions of CHON₂ dimers to total (CHO + CHON) dimer signals, yet in this work “HOM-ONs mainly consist of…C₂₀ dimers that only contain one nitrogen atom.” Please include a discussion of potential explanations for these differences.
4. Trends in O₃- and OH-Derived RO₂. L226–229 report a larger decrease in the normalized signals of C₁₀H₁₅O_x-RO₂ than C₁₀H₁₇O_x-RO₂ in the O₃ + NO₃ vs. O₃-only system. Conversely, Li et al. 2024 report that “the measured C₁₀H₁₅O_xrose with NO₃ radicals” while “C₁₀H₁₇O_5,7 radicals from OH chemistry decreased by a factor of 9.” Please include a discussion of these discrepancies and potential explanations.
5. Figure 3. How/why were these particular RO₂ and HOM species selected? Why not report simulated ratios for all RO₂and HOMs in Figure 2 as well as for total CI-RO₂, OH-RO₂, CI-HOM, and OH-HOM? Please reformat figure to make radicals open symbols and HOMs closed symbols.
6. RO₂ Rate Constants and Branching Ratios. This work sets the rate constant for ^NO3RO₂ + ^CIRO₂ to 1 x 10^-12 cm³ molec.^-1 s^-1 and then constrains the rate constant for ^NO3RO₂ + ^OHRO₂ to be 1 x 10^-13-14 cm³ molec.^-1 s^-1. Bates et al. 2022 constrains the bulk rate constant for ^NO3RO₂ self/cross-reactions to be 1 x 10^-13 cm³ molec.^-1 s^-1 with an upper limit of 1 x 10^-12 cm³ molec.^-1 s^-1. Please include a discussion that justifies and compares the chosen rate constants. Additionally, Bates et al. 2022 report a branching fraction to the ROOR for ^NO3RO₂ + ^NO3RO₂ self/cross-reactions of 16% while the ROOR branching fraction for the self-reaction of ethene-derived RO₂ was recently shown by Murphy et al. 2023 (DOI: 10.1039/D3EA00020F) to be over an order of magnitude higher than previously assumed (23% vs. 1%). What branching fraction to the ROOR was assumed for the kinetic modeling? Did it vary depending on the identity of the RO₂ (i.e., ^NO3RO₂ vs. ^OHRO₂ vs. ^CIRO₂)? Please include a sensitivity analysis that explores the impact of the assumed ROOR branching ratio(s) on the modeling results.
7. OH Scavenger Experiments. Based on results from the OH scavenger experiments, it is suggested that “the cross-reaction of ^CIRO₂ + ^NO3RO₂ is fast compared to that of ^CIRO₂ + ^CIRO₂ and ^CIRO₂ + ^OHRO₂.” However, the observed trends are determined by the relative reactivities (concentrations ´ rate constants) of the ^NO3RO₂, ^CIRO₂, and ^OHRO₂toward reaction with ^CIRO₂. As such, without knowledge of the RO₂ concentrations, an assessment of the relative magnitudes of the rate constants cannot be made. That said, in order to observe both C₂₀H₃₀O_x and C₂₀H₃₁NO_x signals, the ^CIRO₂ + ^CIRO₂ and ^CIRO₂ + ^NO3RO₂ reactions must competitive. As such, the qualitative statement in L292–294 is valid.
8. Figure 4. Please report ratios for all RO₂, HOMs, and dimers in Figure 2. The vertical line in panel b is misplaced. The x-axis labels in the total column of panel c are mislabeled. The y-axis labels should be signals not concentrations. Please use the same color/labeling schemes in Figures 2 and 4.
9. Figure 5. Analogous to Figures 3b and 3c in Li et al. 2024, please include pie charts showing the fractional contributions of total (CHO + CHON) IVOC, SVOC, LVOC, ELVOC, and ULVOC to the total normalized signals measured in the O₃ + NO₃ and O₃-only systems. Please use the same color/labeling schemes in Figures 5 and 7.
10. Compound Abundances. It is important to note that “abundances” (e.g., L311–313) are measured CIMS signals and that different compounds could potentially have different CIMS sensitivities.
11. Figure 6. Please include measured particle-size distributions for the O₃ + NO₃ and O₃-only systems.
12. New Particle Formation. Consistent with Li et al. 2024, this work finds that the presence of NO₃ radicals during α-pinene ozonolysis reduces the abundance of ELVOC and ULVOC measured in the gas phase. However, in contrast to Li et al. 2024, this work observes a factor-of-two increase in aerosol mass concentrations in the O₃ + NO₃ vs. O₃-only system. Given that these experiments were conducted in the absence of seed aerosol, the higher aerosol mass loadings in the O₃ + NO₃ system indicate more efficient particle nucleation and growth, despite the reduced signals of gas-phase ELVOC and ULVOC measurable by NO₃-CIMS. The reduced particle number concentrations in the O₃ + NO₃ system are “ascribed to the suppressed formation of ULVOCs,” however, enhanced coagulation seems more likely given the differences in mass loading. These results are also in contrast to Bates et al. 2022, which found that O₃ + NO₃ oxidation of α-pinene does not nucleate. However, they align with the seeded chamber experiments in Bates et al. 2022, which demonstrate that “high ^NO3RO₂ + RO₂ contributions without any ozonolysis exhibited some of the highest measured SOA yields, suggesting perhaps that the ^NO3RO₂ + ^NO3RO₂ pathway on its own results in even higher SOA yields while ^NO3RO₂ + other RO₂ pathways have lower yields.” Please include a discussion of these discrepancies and potential explanations (e.g., efficient formation of ELVOCs and ULVOCs in O₃ + NO₃ system that are not measurable by NO₃-CIMS).
13. Termination Reactions. The term “termination reaction” is used throughout the manuscript to refer to RO₂ self/cross-reactions. In addition to radical termination to either alcohols and carbonyls or ROOR accretion products, however, alkoxy radical propagation is also possible. As such, please replace instances of “termination reaction” with RO₂self/cross-reaction.
14. Atmospherically Relevant Simulations. Are the stated reductions in L351 compared to simulations with the same initial conditions but with NO₃ concentrations and formation rates set to zero? Is the same amount of α-pinene consumed in the simulations with and without NO₃? Please clarify. Please also compare with the atmospherically relevant modeling results in Bates et al. 2022.