This reviewer enjoyed reading this manuscript, which presents a quite systematic investigation of the aqueous-phase photo-oxidation of eugenol. Indeed, these authors conducted a comprehensive analysis of the degradation kinetics of eugenol, along with the chemical-, optical-properties as well as toxicity (oxidative potential) of the products under direct photolysis, OH- and 3C*-initiated oxidation in the bulk aqueous-phase.

The manuscript is well structured and each subsection comes with a thoughtful discussion. I would therefore recommend its publication subject  to minor corrections.

One of the key messages here is that the 3C*-initiated oxidation dominates over the OH one. This is interesting and in line with other reports. However, it should nice to state whether this is only valid for the actual laboratory conditions selected here, or if it can be also extrapolated to atmospheric conditions. In other words, how does the ratio of the 3C* to OH radical concentrations compare to realistic atmospheric conditions? Can the authors comment on that and therefore justify their choices of concentrations for the oxidant precursors?

The use of acronyms for the selected compounds is justified but it does not ease the reading of the manuscript. It would be nice to state the actual of the compounds when firstly mentioned in the result section (and not just in the experimental one).

While this manuscript reads well, a final polishing of the English would improve it further (but this is a very minor comment).

In this study the authors investigated the degradation of eugenol by three ways i.e. photolysis, excited triplet states, and OH radicals. The obtained results suggest that the excited triplet states are most reactive toward eugenol followed by OH radical induced degradation and photodegradation. The fluorescent spectra indicated formation of HULIS. The use of dithiothreitol indicated formation of harmful species during the oxidation process.

I think that this topic could be interesting for the readers of ACP and the manuscript should be reconsidered upon major revision. I have some comments that I hope can be helpful for the authors to improve the quality of the manuscript. My main comments are mostly related to the experimental details.

The authors used mercury lamps with discontinuous emission spectrum at 313 nm, 365nm, 419nm, and 436 nm to irradiate the aqueous solution consisting of eugenol, which can potentially lead to misleading conclusions. For this kind of experiments, it is more appropriate to use Xenon lamp (solar simulator) with continuous emission spectrum from 300 to 700 nm. It will be useful in Figure S1 to compare the spectral irradiance from the mercury lamp with the sunlight spectral irradiance.

Figure 5a: How it is caused the photolysis of eugenol that absorbs light at 280 nm with mercury lamp irradiating at 313, 365, 419 and 465 nm?

The changes of pH values and the levels of dissolved oxygen before and after the reactions in the aqueous phase are not reported.

Line 88: it should state “several” instead of “sever”.

Figure S3: On X axis should be “magnetic field” instead of “magntic field”.

Figure 3: It is not correct to state “under OH system”. Please revise it.

Line 251: Which chemical bond?

Line 255: In a real world environment the photon energy of the sunlight does not have two peaks at 313 nm and 365 nm.

Lines 375-376: To state that the “role of OH is weak” is not appropriate term here. Moreover, the statement is somewhat misleading as the continuous emission spectrum would probably induce different effect.

Line 389: “the formation of “brown carbon”. Did authors observed change of the colour of the solution? Did really becomes brownish?

Please change the title of section 2.3.5. “Products analysis of products” is not right.

Lines 421-422: Please rewrite the sentence starting with “This is likely due……”. It is not clear at all the meaning of this sentence.

My other comments are related to the references. The authors used many self-citations and at first glance the paper looks like that previously not so many groups studied the reactions of phenols and methoxyphenols in the aqueous phase.

For example, lines 57-61: Many recent papers related to the photodegradation of phenolic substances are not cited here.

Lines 66-67: There are other more appropriate references related to OH radical concentrations in the atmospheric aqueous phase. See for example: Chem. Rev. 2015, 115, 24, 13051–13092; Chem. Rev. 2003, 103, 4691-4716.

Line 93: In which sense these compounds can damage human body? Also reference is here needed about the health implications of quinones and PAHs.

Lines 251-255: Please give the reference for the reported bond dissociation energies (BDE) which is by the way more appropriate term than chemical bonds energies.

Lines 406-409: There are many previous studies that have shown the formation of fluorescent compounds associated with HULIS at wavelengths 400-500nm. For example: http://dx.doi.org/10.1016/j.atmosenv.2013.09.036, doi.org/10.1016/j.atmosenv.2019.03.005)

The manuscript presents an interesting investigation on the aqueous phase photooxidation of eugenol, and its impacts on light-absorption and oxidative potential. However, the entire manuscript is presented as a report, very far from the standard requests of a scientific publication.

• It lacks organization and a logical follow up : instead of clearly providing UV-vis spectra of eugenol and comparing it to the solar actinic fluxes and to their lamp’s actinic fluxes to show the potential importance of the photolysis reaction, the authors directly start with a complex comparison between the kinetics of photolysis vs OH-oxidation vs reactivity towards ³C*, with no appropriate discussion (see below)
• The use of SI is not appropriate: the most interesting figures are provided in SI, and redundant figures are provided in the text with less information (see for example Figure 2 compared to Figure S2)
• The authors provide a systematic and very convincing study of quenching reactions during the photolysis of eugenol, but they forget to discuss on their analytical uncertainties when comparing the influence of the various quenchers. This is particularly critical for the discussions on Fig S4 and S5a
• Too many results are presented with no clear links, and sometimes with no appropriate discussion, but only in reference to other papers, mostly from their own group. For example, the comparison between the kinetics of photolysis vs OH-oxidation vs reactivity towards ³C* where no discussion is performed on the amount of OH and ³C* precursors, neither the lamp’s actinic flux.
• Many small errors can be mentioned. See for example confusion between water solubility and water miscibility, confusion between a radical and triplet state species, …

There are too many English errors and typos;