In this manuscript, the authors provide quantum chemical and theoretical kinetics data that demonstrate the diversity of monoterpene + acylperoxy radical reactions.  The authors present a very clear exposition of the ring-opening, hydrogen-shift, and beta-scission reactions to expect for each of seven terpenes considered in the manuscript.  The manuscript makes frequent, relevant references to previous mechanistic studies and provides solid evidence for which terpenes may be expected to generate significant secondary organic aerosol due to initial attack by acylperoxy.  This is a valuable contribution to the atmospheric chemistry literature.

I have three minor scientific issues for the authors to consider.  First, as presented in Table 1, the rates of ring-opening rearrangements as predicted by RRKM theory are higher than those predicted by transition state theory for beta-pinene, sabinene, and alpha-thujene.  Clearly, then, these acylperoxy-terpene adducts are chemically activated.  Thus, there is the possibility that the yields of ring-opened products, as depicted in Figure 4, may be pressure-dependent.  The authors should address this possibility.  Second, on p. 11, the authors compare the rates of 5-membered-ring vs. 6-membered-ring endoperoxide cyclizations.  The fact that the 5-membered-ring process involves the formation of a presumably more stable tertiary carbon-centered radical should be considered as a reason for the lower activation barrier.  Finally, the focus on the conversion of peroxy radicals to alkoxy radicals, starting on p. 11, is understandable and justified, but it would be good to acknowledge the competing pathway of RO₂ + HO₂ -> ROOH + O₂.

Technical corrections: (1) In Figure 9, the rightmost structure should have two C=O double bonds, but the C=O next to the radical center is hard to discern.  (2) In Figure 14, the rightmost block of text is almost too small to be legible.

The manuscript describes a theoretical study of the very complex reaction mechanisms arising from addition of the CH₃C(O)OO radical (APR) to in total seven different monoterpenes (MT) in presence of O₂. Particular emphasis is laid on the kinetics of the competition between unimolecular reactions (bond dissociation, hydrogen shift, and ring opening) of the intermediarily formed alkyl-type radicals and their addition of O₂. The work seems in part to supplement and extend an experimental study published very recently by three of the authors jointly with three other researchers (Pasik 2024a). In the current study, advanced quantum chemical methods were used to calculate molecular and transition state properties, and statistical rate theory was applied to derive rate constants and branching ratios. In general, all the methods used are state-of-the-art and appear adequate. The mechanisms derived and the calculated kinetic data are discussed in great detail, including even stereochemical aspects. Conclusions about the atmospheric relevance of these oxidation pathways are also discussed.

So, all in all, this is a very laborious work exploring a special branch of atmospheric monoterpene oxidation. Whether this branch is really important has still to be shown and depends essentially on the actual atmospheric APR concentration, a point which is also mentioned by the authors. Because also important general mechanistic and kinetic information on atmospheric terpene oxidation is provided, the manuscript can generally be recommended for publication in ACP.

There is however one important point that should be addressed by the authors before final acceptance: The presentation of the kinetics calculations should be improved. From the current version of the manuscript, it is very difficult to detect, which method was applied to which reaction step: Obviously, the bimolecular reaction APR + MT was characterized by MC-TST, eq. (1). The rate constant of the subsequent ring-opening reaction was calculated by both TST-2, eq. (2), and RRKM (not specified, probably within MESMER). Here the APR-addition reaction (APR + MT?) was simulated using ILT (line 108). Does this rate coefficient grossly agree with the value obtained from eq. (1)? What was the rationale for choosing 1x10^-12 cm³ molecule^-1 s^-1 for O₂ addition? What does k_TST-1 (Table 1) characterize: APR + MT or alkyl + O2 or ring-opening product + O₂? This is difficult to find out. It is reasonable that k_RRKM (should it not better read k_MESMER?) in Table 1 is larger than k_TST-2. But this is not always the case, why? In Figure 4, species profiles are plotted; what is the difference between RO-APR and ring-opened RO-APR?

All these points (which can also not be resolved by looking into the supplemental material or into reference Pasik 2024a) should be presented a bit more clearly and more conclusively, maybe within a graphical scheme. This would help the reader of this otherwise convincing paper very much.

Some minor, more formal points are:

general: The authors should consider using the term ‘acetyl peroxyl” instead of ‘acyl peroxyl’ because the latter may suggest RC(O)OO whereas only CH₃C(O)OO was studied in the present work.

line 37: ‘unique’ should probably better read ‘specific’

line 44: insert [X] ‘… and [this] reaction … to lead [to] epoxides …’

line 45 and elsewhere: for elementary reactions, the term ‘accretion’ is very unusual, it should better read ‘association’ or ‘addition’

line 73: What was done with ‘squared rotational constant values’?

line 86: Different conformers of the acyl peroxyl radical were accounted for. What about possible conformers of the monoterpenes (alpha-thujene, sabinene)?

line 104: ‘reaction dynamics’ should read ‘reaction kinetics’

Figure 2: please check the meaning of RO₂ above the third arrow.