This article presents a complete study of the evolution of DOM under irradiation using different techniques: aerosol mass spectrometry, ESI-Orbitrap, UV-vis absorption and volatility measurements. The manuscript is globally well written and the study is interesting and up to date. Nevertheless I have some concerns that need to be addressed before publication.

• The sample was collected 15 years ago! How do you prove that DOM is not degraded at -20°C? For instance, standard solutions with micro and nanomolar concentrations of certain amino acids or carboxylic acids are not stable in the freezer for 2 years. I suppose that DOM composition was already changed compared to the composition in 2010. This hypothesis is supported by the fact that no short chain carboxylic acids or aldehydes are detected in this study (line 195).
• The authors froze the sample before irradiation: have you measured H2O2 before and after freezing? H2O2 is not stable in frozen samples and maybe what you observed is just direct photolysis, which would explain the predominant oligomerization on fragmentation. Please, state on this consideration in the manuscript, and maybe plan a future experiment where you have both direct photolysis and OH mediated.
• I perfectly understand the difficulty of sampling enough fog (in volume) to conduct the experiment, but I still think that drawing general conclusions with the analysis of only one sample is risky. Especially considering that the sample was frozen for 15 years.
• Do you have information about the pH of the sample, its ionic composition, and the total DOC?
• The spectrum of the lamps reported in Jing et al. 2021 shows emission at 280 nm, which is in the UVB-UVC region and in contrast with the statement line 65. Nevertheless, the reactor is a Pyrex one, which should stop wavelength below 300 nm. Please, clarify this point and specify if other filters were used. Could you also clarify if the optical depth of the irradiated solution decreased during irradiation? I actually don’t think so, but I prefer to have no doubt about it.
• I’m quite surprised that the authors analysed the samples in ESI +, while almost all the works cited about HR-MS of atmospheric aqueous samples used ESI – (except Altieri for CHNO-CHN). Could you please justify this choice?
• Paragraph 3.2 is just ESI-Orbitrap? Because it seems to be a mixture of Orbitrap and AMS results.
• The figures presented in the article are particularly complex and merge a lot of information, especially Figs 1, 2 and 3. Is there a way to simplify the graphical representation of the information?
• Line 178: what are HxO1+?
• Discussion lines 178-185: is the intensity of the signal kept into account for the discussion?
• Line 182: “Lightly… molecules” I’m probably tired, but I really don’t understand the sentence, especially “variability among individual ions indicates”. Could you please rephrase?
• Line 270: is the variation of N/C from 0.036 to 0.038 significant? Since I don’t think so, the authors should not conclude about the incorporation of nitrogen compounds into DOM.
• Line 307: “in contrast with the assumption of photobleaching” please add a reference to this assumption. Other papers suggest the formation of brown carbon instead of photobleaching (Bianco et al. Tryptophan, 2016; Paglione et al, 2014)
• Conclusions: please, in the last few lines, remember that the study is performed on one sample collected more than 10 years ago. This article is good and well written, but more evidence is needed to confirm that fog-driven aqueous phase processes regulate the climate-relevant properties of organic carbon and nitrogen.

The study by Jiang et al. presents very interesting results on photochemical aging of dissolved organic matter (DOM) in fog water. The concept of the experiments is to illuminate fog samples while continuosly atomize the solutions and analyze the particles using a HR-ToF-AMS. This a very interesting concept because it allows to study the photochemical processing in real time. The scope of the study fits well to ACP, so that it should be published. However, the consequences of the long time between sampling the fog water and the analysis should be discussed (see major comment below).

Major comments:

I agee with referee #1 that the main problem is the age of the samples. Actually, the data of the laboratory analysis is not given, but it is to assume that this was only done recently.

Maybe it is not possible now to prove that the samples have not aged during the many years of storage, but this issue should be discussed. Are there no newer, more recent samples that can be/were studies with the same technique? If they show the same result, it will be an important indication that the presented data of the 2010 samples are valid.

Minor comments:

- lines 174-176: You mention the formation of organic nitrate:

There is the AMS metric of NO2+/NO+ to obtain information on organic nitrates as opposed to inorganic nitrates (e.g. Day et al., AMT 2022): Did you look at this? Did the ratio NO2/NO change over time? 

- Figure 2: Please explain f_43, f_44, etc. for the non-AMS experts. Especially the difference between f_44 and f_CO2 etc. Adjust caption for d) (f_44 vs f_43, f_CO2 vs f_C2H3O+). Same for e). Is f_60 the same as f_C2H4O2+? Most likely not, because f_60 is UMR and f_C2H4O2+ is HR. Also, please add motivation of showing the "f60" plot. f60 is typically used a biomass burning marker. Explain the line labeld "0.003". In the caption and explanation of shaded region in Fig 2d, you should reference Ng et al., 2011. Are Figs 2d,e,g mentioned in the main text all? I think not. If that is so, why showing them?

- It might be interesting to show the fractional mass contribution of all main AMS compounds over time (in the supplement). Fig 1d shows it only for t_0 and t_end.

Technical remarks:

Figure 1:

In Figure 1d the color legend is missing. Please state that it is the same as in a-c

In figure caption to (d): I suggest adding "ion categories":  Mass fractions of major inorganic species and DOM_OA ion categories at t_0 and t_end.

References

Day, D. A., Campuzano-Jost, P., Nault, B. A., Palm, B. B., Hu, W., Guo, H., Wooldridge, P. J., Cohen, R. C., Docherty, K. S., Huffman, J. A., de Sá, S. S., Martin, S. T., and Jimenez, J. L.: A systematic re-evaluation of methods for quantification of bulk particle-phase organic nitrates using real-time aerosol mass spectrometry, Atmos. Meas. Tech., 15, 459-483, 10.5194/amt-15-459-2022, 2022.

Ng, N. L., Canagaratna, M. R., Zhang, Q., Jimenez, J. L., Tian, J., Ulbrich, I. M., Kroll, J. H., Docherty, K. S., Chhabra, P. S., Bahreini, R., Murphy, S. M., Seinfeld, J. H., Hildebrandt, L., Donahue, N. M., DeCarlo, P. F., Lanz, V. A., Prévôt, A. S. H., Dinar, E., Rudich, Y., and Worsnop, D. R.: Organic aerosol components observed in Northern Hemispheric datasets from Aerosol Mass Spectrometry, Atmospheric Chemistry and Physics, 10, 4625-4641, 10.5194/acp-10-4625-2010, 2010.

The authors studied how fog water containing DOM can generate indirect photochemical changes in organic matter. The authors use a nice suite of instruments and characterizations to show this photochemical oxidation process. For example, the authors show photo-functionalization through the addition of oxygen and nitrogen-containing groups. The PMF analysis is a good addition data analysis tool to showcase the progression of the aging. This manuscript can be considered for publication once the authors have considered the following points.

Experimental setup:

• The authors should add the irradiance spectra (could be added in the SI) for the irradiation setup. Adding the sum of output spectra of the different bulbs at different wavelengths will help in the comparison with the solar spectrum.
• The equivalence to “~56 hours of winter solstice sunlight” is mentioned but would have required actinometry and conversion factors. Could the authors report these experimental details and equations?

Sample representativeness:

• Only three fog samples from a single day (Jan 9, 2010) were pooled. These samples represent a very limited dataset for generalization. There could be aging occurring overtime that needs to be specifically addressed. Referring to "Resch et al, Environ. Sci. Technol. 2024, 58, 32, 14318–14328" could also be useful.
• The implications for variability (seasonal, meteorological, source-related) can be added in order to contextualize the results. Considering the body of research on fog samples from the Po Valley, Italy, I recommend that the authors extend their discussion to include references like Mattsson et al, ACP, 25, 7973–7989, 2025.  Including more explicit connections to ambient implications, beyond Fresno, can strength the atmospheric relevance.

Definition of DOM_OA:

• The definition “low-volatility fraction of DOM that forms OA upon water evaporation” remains vague. The cutoff for volatility and how it was operationally determined should be better explained. Does it come from literature? If yes, could the authors add these references?

Light absorption:

• The literature reports photobleaching more often after irradiation. The authors instead report photo browning, so they should address this discrepancy with the literature and contextualize better the results. Why are the authors not seeing photobleaching? Because of the length of irradiation? Are there other studies of similar samples showing the same property?

Technical points

• The authors should refer and describe all the figures (figure 2d, 2e, 2g are not mentioned in the text)
• The authors should label the species in the figures and in the text (for example line 207 and 210 page 8 and figure 2b shows different acronyms)