This study examines the evolution of chemical and optical properties of phenolic aqSOA generated via ^•OH- or ³C* oxidation during photoaging, as well as the effects of increased concentration of oxidants. The changes in the chemical composition and light absorption of aqSOA were tracked using HR-AMS and UV-Vis spectroscopy. The findings for the ³C*-aqSOA have been reported in a previous study by the same group (Jiang et al., 2021). Although this paper attempted to explain the differences between 3C*-aqSOA thoroughly and ^•OH-aqSOA, several statements appear unclear or not clearly supported by the results. The paper is well organized, but several points need clarification.

1. Could the authors give examples of products likely resistant to fragmentation?

1. Were the 4 aliquots prepared for further aging also continuously stirred?

1. Section 3.2: why would ^•OH production from ³C* be more important during prolonged photoaging? is there any evidence for this?

1. Section 3.2: Does higher light absorptivity always correlate with faster direct photodegradation? It seems not as, for example, Smith et al. (2016) attributed the essentially equal reactivity of syringaldehyde and acetosyringone against direct photodegradation to the greater light absorption by syringaldehyde and higher quantum efficiency for loss for acetosyringone. Also, how does this reconcile with the statement in section 3.1 about ^•OH-aqSOA being more vulnerable to photodegradation than ³C*-aqSOA?

1. Section 3.2: Could the authors explain why acid formation is more pronounced in the aging of ³C*-aqSOA? In section 3.1, it was mentioned that ^•OH-aqSOA has a greater tendency to form volatile and semi-volatile compounds that evaporate from the condensed phase

1. Section 3.2: What are the differences between the highly oxidized products from ³C* and ^•OH oxidation? Those from ³C* are stated to be resistant to degradation, while those from ^•OH are mentioned to degrade over long aging times. Also, why are some of the highly oxidized products less reactive with ³C* than with ^•OH, and that they can persist in the aqueous phase?

1. Section 3.3: Why would the higher oxidation degree of ^•OH-aqSOA lead to the destruction of chromophores?

1. Section 3.4: Why would ^•OH be more reactive with non-phenolic organic compounds? For example, a study on the oxidation of green leaf volatiles (Richards-Henderson et al., 2015) comprising both non-phenolic and phenolic compounds did not show a general trend in the reactivity with ³C* and ^•OH.

1. Section 3.4: Could the authors give some examples of these unique low-volatility, light-absorbing products that cannot be generated via ^•OH oxidation?

1. Section 3.4: What do the authors mean by ^•OH only accounts for a small fraction of the total oxidant amount in the ³C*-initiated reaction system?

Minor comments and questions:

1. Intro: Is hydroxylation an example of functionalization?

1. Please provide more information on the HPLC method used to determine the concentration of GA and DMB.

1. Please correct what [Org]t and [GA]t refer to

1. There are other recently published articles regarding the formation of aqSOA by ³C* chemistry and the corresponding light absorption by reaction products (e.g., Li et al., 2022, https://doi.org/10.5194/acp-22-7793-2022; Mabato et al., 2023, https://doi.org/10.5194/acp-23-2859-2023)

References:

Jiang, W., Misovich, M. V., Hettiyadura, A. P. S., Laskin, A., McFall, A. S., Anastasio, C., and Zhang, Q.: Photosensitized reactions of a phenolic carbonyl from wood combustion in the aqueous phase—chemical evolution and light absorption properties of aqSOA, Environ. Sci. Technol., 55, 5199−5211, https://doi.org/10.1021/acs.est.0c07581, 2021.

Richards-Henderson, N. K., Pham, A. T., Kirk, B. B., and Anastasio, C.: Secondary organic aerosol from aqueous reactions of green leaf volatiles with organic triplet excited states and singlet molecular oxygen, Environ. Sci. Technol., 49, 268–276, https://doi.org/10.1021/es503656m, 2015. 

Smith, J. D., Kinney, H., and Anastasio, C.: Phenolic carbonyls undergo rapid aqueous photodegradation to form low-volatility, light-absorbing products, Atmos. Environ., 126, 36−44, https://doi.org/10.1016/j.atmosenv.2015.11.035, 2016.

Review comments on the manuscript "Photoaging of Phenolic Secondary Organic Aerosol in the Aqueous Phase: Evolution of Chemical and Optical Properties and Effects of Oxidants" in EGUsphere. The manuscript addresses the long-term aqueous aging of aqSOA formed from the photooxidation of guaiacyl acetone (GA) by OH radicals or the photosensitizer model compound 3,4-dimethoxy-benzaldehyde (DMB) using Pyrex tube experiments and performing positive matrix factorization (PMF) analysis of combined HR-AMS and UV-vis spectral data.

Questions and remarks:

Could the authors elaborate more the description of the experimental and analytical method, as it is a bit vague at the moment. It is not clear to the reader if the experiment is performed in a closed system or if it is, open to the atmosphere, if oxygen is present or not? Could the authors describe the potential contribution of singlet oxygen to the conversion within the system when the photosensitizer dimethoxybenzaldehyde and oxygen are present? Could the guaiacylacetone itself act as a photosensitizer? Would the authors expect a similar oligomer formation yield and conversion rate if 4-propylguaiacol had been used? What would be the general reaction mechanism in the photosensitizer system in the presence of GA and DMB after the first step of H-atom abstraction or electron transfer? Is addition of the photosensitizer possible? What would be the difference in the first oxidation products formed by the photosensitizers compared to OH radicals? What would be the result of the involvement of oxygen?  Would the resulting peroxyl radicals lead to the formation of oligomers, what is the authors' opinion? If not, could this explain the lower yield of oligomers in OH radical-induced oxidation? How likely is alkyl/phenoxy-like radical recombination in the presence of oxygen at the steady-state concentrations used? In the authors' opinion, what are the main oxidation products of the OH radical reaction with GA? On page 7, line 166, it is mentioned that the products formed can evaporate more easily. Later (line 177), the formation of carboxylic acids and compounds formed by the degradation of aromatic rings is mentioned. How likely is it that these compounds will evaporate? Could the authors indicate how long the H2O2 (100 uM) as well as the DMB is present in the solution before it decays by the photochemistry? How many times is DMB involved in a reaction as a photosensitizer before it is degraded? Is it possible that GA acts as a photosensitizer in the GA + H2O2 system used and reacts with H2O2, which is subsequently more important than the production of OH radicals by H2O2 photolysis, since the absorption of GA is somewhat greater compared to H2O2 in the specific wavelength range? Did the authors do any experiments with GA in the absence of an oxidant? How justified here is the statement (on page 10 line 215) that the steady-state concentration of OH is the same or similar?

The end of section 3.4 is difficult to follow and is full of speculation. Could the authors sharpen the end of the section for clarity and with concrete numbers, e.g., how likely evaporation is? This brings me back to the experimental description, where it is not clear whether evaporation may or may not play a role in this study, so the experimental design is not well described within the manuscript.

In summary, this manuscript, which certainly has its merits and is quite interesting in its present form, might be improved more in terms of the clarity and coherence of its scientific basis to enable a recommendation for acceptance.