Cheng et al. present in this study a systemic investigation of the oxidation of multiple aromatic VOCs using an oxidation flow reactor. To start with, the authors performed detailed analyses on the oxidation products measured by a nitrate CIMS, by which they showed the importance of both multi-generation oxidation and autoxidation in producing OOMs and the significant influence of steric hindrance in intra-molecular H-shift and dimer formation. The authors further estimated the accretion reaction rate constants between RO2 radicals, which are consistent with the values in previous literature. In the end, the authors estimated the contribution of OOMs to SOA via condensation and equilibrium partitioning, which appeared to be much lower than the value estimated from ambient measurement in a recent study (Nie et al., 2022). In this regard, the inconsistency points out the substantially incomplete understanding of the role of OOMs in SOA formation.

In general, I think this topic is of high importance, and this manuscript is well-structured and easy to follow. However, I do have some concerns that need to be addressed before it can be accepted for publication.

Major concerns:

I appreciate that the authors mentioned the weak representativeness of OFR to atmospheric conditions (Line 265). However, I worry that this message is not clear enough and can be easily overlooked. In Line 264, the authors say “Large uncertainties remain in the estimation”, which is just handwaving. The authors should explicitly list possible sources of uncertainties, which help navigate the knowledge gap for future research.

Some specific comments are listed below:

(Line 65-66) The OH and HO2 concentration is disproportionally high in the experiment, which affects the competition among different reaction channels of OOM formation. First, the RO2 termination reaction is dominated by RO2+HO2 reactions; Second and more importantly, the fast RO2+HO2 reaction (due to high HO2 concentration) could lead to a very short lifetime of RO2 radicals that limits the RO2 autoxidation. This should be clearly discussed (at least mentioned) in Sect. 2.1.

(Line 103-104) Besides a general calibration factor, do the authors consider the mass-dependence transmission efficiency of the instrument (Heinritzi et al., 2016)? A steep transmission curve can significantly affect the signal strength, affecting the concentration estimation (SOA calculation) and the determination of accretion reaction rate constants.

(Line 108-110) Is the steady state a good assumption for OFR conditions? The stable concentration at each individual experimental condition could also be interpreted as that the chemical reactions are stable in the OFR, so the formation and loss of OOMs at a constant residence time yields a stable concentration (not necessarily at the steady state). Can the authors provide data or calculations to support this assumption, or is there any previous literature discussing this?

Also, there is evidence that ROOR’ could further react with OH, forming different ROOR’’ (Wang et al., 2020). Did the authors consider this reaction as a loss/source term of ROOR’ when deriving the kR,i?

(Line 126-127, and the corresponding text in SI) The parameterization by Mohr et al., (2019) are more suitable for OOMs from monoterpene oxidation, which contains several hydroperoxyl groups, which may not apply to OOMs from monoterpene oxidation. In fact, Wang et al., (2020) showed that this is not suitable for naphthalene products and provided a new parameterization. I suggest adopting the one by Wang et al., (2020). Also, it seems that the temperature-dependence of dHvap (in eq. S3) is different than the one used in e.g., Stolzenburg et al., (2018). The authors need to reference this equation. These will affect the volatility distribution of OOMs and the estimation of the contribution to SOA.

(Line 186-188) As the authors stated and consistent with tradition knowledge, BPRs are the central radicals. Assuming that dimers are formed from RO2+RO2 -> ROOR + O2, the least oxidized C2x dimer would be CxHyO8. Given this assumption, the observation of abundant O4-7 dimers is interesting. Can the authors speculate the formation pathway?

(Line 240) I am confused about this. Isn’t that C12H14O15 can form via different combinations of RO2 radicals (as in Table 2). Then how can the k_R,i be derived with only one exclusive combination?

Minor comment

(Line 105) Cheng et al., 2021a is missing from the reference list.

(1)        Heinritzi, M.; Simon, M.; Steiner, G.; Wagner, A. C.; Kürten, A.; Hansel, A.; Curtius, J. Characterization of the Mass-Dependent Transmission Efficiency of a CIMS. Atmospheric Meas. Tech. 2016, 9 (4), 1449–1460. https://doi.org/10.5194/amt-9-1449-2016.

(2)        Wang, M.; Chen, D.; Xiao, M.; Ye, Q.; Stolzenburg, D.; Hofbauer, V.; Ye, P.; Vogel, A. L.; Mauldin, R. L.; Amorim, A.; Baccarini, A.; Baumgartner, B.; Brilke, S.; Dada, L.; Dias, A.; Duplissy, J.; Finkenzeller, H.; Garmash, O.; He, X.; Hoyle, C. R.; Kim, C.; Kvashnin, A.; Lehtipalo, K.; Fischer, L.; Molteni, U.; Petäjä, T.; Pospisilova, V.; Quéléver, L. L. J.; Rissanen, M. P.; Simon, M.; Tauber, C.; Tomé, A.; Wagner, A. C.; Weitz, L.; Volkamer, R.; Winkler, P. M.; Kirkby,  jasper; Worsnop, D. R.; Kulmala, M.; Baltensperger, U.; Dommen, J.; El Haddad, I.; Donahue, N. M. Photo-Oxidation of Aromatic Hydrocarbons Produces Low-Volatility Organic Compounds. Environ. Sci. Technol. 2020. https://doi.org/10.1021/acs.est.0c02100.

(3)        Stolzenburg, D.; Fischer, L.; Vogel, A. L.; Heinritzi, M.; Schervish, M.; Simon, M.; Wagner, A. C.; Dada, L.; Ahonen, L. R.; Amorim, A.; Baccarini, A.; Bauer, P. S.; Baumgartner, B.; Bergen, A.; Bianchi, F.; Breitenlechner, M.; Brilke, S.; Buenrostro Mazon, S.; Chen, D.; Dias, A.; Draper, D. C.; Duplissy, J.; El Haddad, I.; Finkenzeller, H.; Frege, C.; Fuchs, C.; Garmash, O.; Gordon, H.; He, X.; Helm, J.; Hofbauer, V.; Hoyle, C. R.; Kim, C.; Kirkby, J.; Kontkanen, J.; Kurten, A.; Lampilahti, J.; Lawler, M.; Lehtipalo, K.; Leiminger, M.; Mai, H.; Mathot, S.; Mentler, B.; Molteni, U.; Nie, W.; Nieminen, T.; Nowak, J. B.; Ojdanic, A.; Onnela, A.; Passananti, M.; Petaja, T.; Quelever, L. L. J.; Rissanen, M. P.; Sarnela, N.; Schallhart, S.; Tauber, C.; Tome, A.; Wagner, R.; Wang, M.; Weitz, L.; Wimmer, D.; Xiao, M.; Yan, C.; Ye, P.; Zha, Q.; Baltensperger, U.; Curtius, J.; Dommen, J.; Flagan, R. C.; Kulmala, M.; Smith, J. N.; Worsnop, D. R.; Hansel, A.; Donahue, N. M.; Winkler, P. M. Rapid Growth of Organic Aerosol Nanoparticles over a Wide Tropospheric Temperature Range. Proc Natl Acad Sci U A 2018, 115 (37), 9122–9127. https://doi.org/10.1073/pnas.1807604115.

Comments to the “Oxygenated organic molecules produced by low-NOx photooxidation of aromatic compounds and their contributions to secondary organic aerosol.“ by Cheng et al.

The paper presents a set of experiments conducted in an oxidation flow reactor (OFR) targeted at speciating and quantifying oxygenated organic molecules (OOM) in a reaction of aromatic VOCs with OH. Six common aromatic VOCs and 4 to 5 OH exposures per precursor were sampled. Oxidation products are detected by a nitrate-scheme chemical ionization mass spectrometry. The study is limited to low-NO_x regime and high precursor conditions adding to previous experimental studies a wider range of conditions, precursors as well as extending the research question to the contribution of the detected species to secondary organic aerosol (SOA). Unfortunately, quantification of molar yields and SOA contribution is only available for 3 out of 5 VOCs. The manuscript is well written and results are adequately discussed. However, there are few places in the paper, where I would like to see some clarifications regarding the uncertainties, methods and discussion. I present my comments below.

Major comments.

1. The paper lacks overall discussion on the limitations of the study in representing real atmosphere or in providing quantitative results.

a) The experiments are conducted at high VOC and OH concentrations, higher than some previous studies. How could this affect the observed OOM composition and yields?

b) What are the uncertainties associated with calculating OOM molar yield as well as the contribution of OOM condensation to SOA?

For instance, molar yield calculation includes correction due to losses, and that correction is likely large. K_OHloss is an approximation of the loss due to the reaction with OH. How sensitive is the yield to the uncertainty in K_OHloss or other loss parameters? Authors reference their previous study (Cheng et al. 2021) as well as Palm et al. (2016). However, those studies used a constant k_OH for a saturated C₁₀ molecule to approximate K_OHloss. Is that value still valid for compounds like naphthalene, products of which likely contain double bonds?

The authors also point to Cheng et al. (2021) to see how loss to aerosols is calculated. However, it is unclear if the same diffusion volume is being used for all VOCs or not. Authors should clarify specific methodology in the current manuscript and discuss uncertainties/biases associated with the choices made.

Could you provide short comment on how CIMS was calibrated and what is the uncertainty of the calibration factor? Were any corrections applied to the calibration factor for lower oxygenated OOMs that are not detected at collision limit in nitrate-CIMS? If no correction was done, is there a possibility for bias in interpretation of OOM contribution to SOA and/or molar yields?

2. As this paper presents quantitative results motivated by improving the available datasets, it would be reasonable that the authors would deposit the data (at least the data used for making figures) in a persistent repository.

3. Some aspects of SOA production in current experiments remain unclear.

a) It would be useful to see some description if SOA was produced in nucleation rather than aerosol seed experiments. It would help to understand the system if authors could provide details on how much of aerosol mass was produced and how SOA yields compared to previous nucleation studies. Also, to which sizes did the particles grow (at least in terms of understanding detection by AMS)?

b) HOM (a subset of OOM) are known to be most important for SOA growth at lower SOA concentrations (Ehn et al. 2014), exactly what one would expect in suburban or downwind low-NOx conditions. How the amount of SOA produced in current experiments could potentially bias the results (OOM to SOA contribution)? Some discussion on this should be included in the text.

c) If this study looked at nucleation experiments, is it possible to derive OOM importance at different particle growth stages (provided short residence times in OFR would allow for this)? This could help to further illustrate the relative importance of different VOCs in early particle growth.

Minor comments.

Author present OOM molar yields. How do new results compare to the previous study by the same author (Cheng et al. 2021) in terms of yield values and OH exposures?

From Figures S1 and S4, it is clear that the concentration of dimers for some compounds decreases at increasing irradiance in OFR (or increasing OH). It is also not consistent among different precursors, e.g. naphthalene vs 1-methylnaphthalene. Could some additional discussion be presented in C_2x section (section 3.2)?

Figures 3,5: when printed, the colors for toluene and benzene appear identical. Would be good to change the color as symbols are the same.

Figure 6 a,b: same axes limits would be useful.

Line 47. Some logical transition between sentences on multi-generation oxidation and dimer formation is needed.

Line 53: ‘subtraction’ – did you mean ‘abstraction’?

Lines 151-152: ‘suggesting significant hydrogen loss in the dimer formation’ – what do authors mean by this? Is it being suggested that hydrogen atoms are lost in RO₂+RO₂ reaction?

Line 166 and 176: ‘neutrals’ – as both open and closed shell products are electrically neutral, it is best to use just ‘closed shell’ in these sentences.

Line 229: ‘have high signals’ – here it would be useful to provide some numbers within the text.

Line 233: ‘They correlated well’ – from Figure 5, it seems xylene is an exception. Please clarify this in the text.

Line 234: ‘The slopes are lower for naphthalene and 1-methylnaphthalene’. It seems that the slope for naphthalene is identical to that of benzene. Would be good to clarify the text.

Line 248: ‘most of the O:C ratios are much greater while the H:C ratios are lower’. From Figure 6a, I can see that the H:C for naphthalene is on the same order as previous studies, while O:C is somewhere lower or higher. For toluene, O:C and H:C ratios are similar at highest OH exposure, while for benzene, H:C ratios are similar. This is in contrast with the text. Authors should be more specific when interpreting the comparison.

References:

Cheng, X., Chen, Q., Li, Y., Zheng, Y., Liao, K., and Huang, G.: Highly oxygenated organic molecules produced by the oxidation of benzene and toluene in a wide range of OH exposure and NOx conditions, Atmos. Chem. Phys., 21, 12005-12019, https://doi.org/10.5194/acp-21-12005-2021, 2021

Ehn, M., Thornton, J. A., Kleist, E., Sipilä, M.,..., and Mentel, T. F.: A large source of low-volatility secondary organic aerosol, Nature, 506, 476–479, https://doi.org/10.1038/nature13032, 2014

Palm, B. B., Campuzano-Jost, P., Ortega, …, and Jimenez, J. L.: In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor, Atmos. Chem. Phys., 16, 2943-2970, https://doi.org/10.5194/acp-16-2943-2016, 2016.