Comments on acp-2021-201

In this manuscript, Cheng et al., investigated the oxidation products of benzene and toluene using a PAM flow tube reactor under lowand high-NOx conditions. The authors used a nitrate-based tof-CIMS to measure the oxidation products (HOMs), and some unmeasured species, such as HOx and ROx, were quantified with a chemical model designed for the PAM. By investigating the behavior of different HOM classes under different OH dose and NOx level, the authors suggested that 1) their system is more favorable for highly oxygenated products, 2) multi-generation OH oxidation is likely via the H-abstract route, and 3) Many even the majority of N-containing HOMs are likely peroxyacyl nitrates. Based on these results, some reasonable atmospheric implications were given.

In this manuscript, Cheng et al., investigated the oxidation products of benzene and toluene using a PAM flow tube reactor under low-and high-NOx conditions. The authors used a nitrate-based tof-CIMS to measure the oxidation products (HOMs), and some unmeasured species, such as HOx and ROx, were quantified with a chemical model designed for the PAM. By investigating the behavior of different HOM classes under different OH dose and NOx level, the authors suggested that 1) their system is more favorable for highly oxygenated products, 2) multi-generation OH oxidation is likely via the H-abstract route, and 3) Many even the majority of N-containing HOMs are likely peroxyacyl nitrates. Based on these results, some reasonable atmospheric implications were given.
In general, I found this manuscript interesting and provided enough insights into the oxidation scheme for benzene and toluene. However, I do have some concerns, which should be addressed before it can be accepted for publication in ACP.

General comments:
How much the oxidation system of PAM with high OH concentration can be extrapolated to presented the real atmospheric conditions is always a major concern of the community. The lifetime of many effectively non-volatile HOMs is in the order of minutes, but the minimum equivalent OH exposure time in this study is 0.8 days. Also, at low OH concentrations, it is to be demonstrated if the ppm-level O3 dominates the oxidation of the first-generation HOM products, which likely have an endocyclic double bond. Likewise, the NO3-initated oxidation at high-NOx levels may also play an important role. These should be at least mentioned in the manuscript.
The authors investigated the oxidation of benzene and toluene, and clear differences in the products and their response to different oxidation conditions were observed. However, readers might hope to see some more detailed explanation of these differences, rather than only some descriptions. However, it is ok if the authors show more dedicated studies in their follow-up manuscript. L171-172. The authors attribute the more steps of auto-oxidation in their experiments to the longer residence time. This is one possible reason. Can the authors exclude the possibility that the concentrations HO2, RO2, NOx in Garmash et al., (2020) were higher than those in this study, so that the auto-oxidation was suppressed to a greater extent? L187-188. It is interesting to see that the dimeric HOMs decrease when OH exposure increases for toluene HOMs. It deserves a bit more explanation/speculation than just say "Whether this phenomenon is related to the substituted methyl group or not needs further investigations".
L255-260. The yield is probably one key message that readers would like to fetch from these studies. Thus, the big differences in the yields reported by different studies should be better explained. As the authors concluded that multi-generation oxidation is important in benzene and toluene oxidation, the highest yield by Garmash et al., (2020) and the lowest yield by Monteni et al., (2018) cannot be explained by the residence time, because the OH exposure in Monteni et al., (2018) is the highest among these studies. It may point to either the auto-oxidation is more important, which are different among these studies controlled by the termination reactions, or the peaks counted for "HOMs" are different. I can read from , because all of them lead to termination reactions that control HOM formation. Here, k1, k2, k3 is the average first-order rate constant of the bi-molecular reactions with RO2.
L294-295. Could NO3 radical also be important in the formation of other HOMs, particularly for high-generation products?