Wang et al presents laboratory results where the authors oxidized thrimethylbenzene (TMB) in an oxidation flow reactor (OFR) with OH in the presence and absence of NOx to investigate the role of autoxidation. Using a combination of a Vocus proton transfer reaction mass spectrometer and nitrate chemical ionization mass spectrometer, the authors investigated the products produced from the oxidation of TMB. They argue that the highly oxidized material (HOMs), which has been observed and discussed to be potentially important for new particle formation and particle growth in both clean (e.g., Boreal forests) and polluted environments, is produced upon the second OH oxidation and subsequent reactions of the material instead of the first oxidation. They argue that the NOx products observed in the NO experiments provides evidence of this. 

This paper is of interest to the community and recommend publication after addressing the comments below and potentially restructuring/rephrasing some of the conclusions to address the concerns Matti provided (though disagree with the argument that the OFR is not a tool to be used to understand chemistry and its only role is regulatory).

Major

(1) Methods--how long is one condition sampled to ensure things have reached steady state for the calculations (e.g., lifetime of HOMs) to be true? This is also important for the assumption that is used to normalize the signal measured by HOMs divided by the TMB signal with Vocus (which is currently unclear why authors may not have used a signal from the nitrate CIMS instead that was constant).

(2) Were any experiments conducted at lower or higher TMB mixing ratios? E.g., this may help address some of the concerns of Matti as lower/higher TMB should change the RO2/HO2 ratios and provide insight into the chemistry that is occurring in the OFR.

(3) Discussion of the mechanism:  The purpose of the paper is to elucidate the mechanism of the production of HOMs. However, the authors only present the scheme from MCM without expanding the mechanism/scheme they believe they have observed, which makes the narrative very hard to follow. I strongly recommend expanding the schemes presented in the paper with the chemistry and products observed to improve the narrative and better understand how the second generation HOMs are being formed. To address the concerns of Matti, this can address both the chemistry that may be occurring in the OFR vs the chemistry that may be more prominent in urban atmosphere and the importance/products between the two regimes. Further, I think interspersing the results from the NOx chemistry into the discussion of the production of HOMs and which pathways occur would be beneficial instead of the NOx chemistry being a separate section. Right now, the NOx chemistry seems like a leftover section that is addressed to quickly instead of being used as a tool to verify the hypothesis that it is potentially a second OH attack is necessary to form the HOM.

(4) Units in the normalized relative molar yields: Currently, all figures that show the nominal relative molar yields are not intuitive to interpret and understand. E.g., the values in the y-axis are between 10^-10 to 10^-9, which would suggest that the HOMs are not important fates. It is not clear if it is due to taking the signal from the nitrate CIMS and normalizing to the signal from the Vocus may be the cause of this. Further, it is surprising the yields are apparently higher from the accretion (RO2+RO2) reactions compared to the monomer reactions. Due to the general lack of clarity and the concerns from reviewer number one, it may be better to focus on the fate of RO2 during these experiments instead of the yields, and which fates are more atmospherically relevant vs potentially related to the OFR. I would recommend also, to address the reviewers concerns, to include the estimated fate of the RO2 products due to fragmentation, photolysis, and wall loss in OFR.

Minor

Color scheme. Please avoid using red and green in the same plot, as that will be difficult to interpret for color blind people.

Ensure that SI figures are presented in same order discussed in paper (e.g., Fig. S4 is discussed after Fig. S5).

Line 335: declining instead of declination

Line 350 - 363: It is confusing which ratio is being discussed as it is switched from HO2:RO2 to OH:HO2. Please clarify (and may be addressed with the rephrasing of products/RO2 fates).