the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Aug 2025
Photochemical Processing of Dissolved Organic Matter in Fog Water: Oxidation and Functionalization Pathways Driving Organic Aerosol Evolution
Abstract. Photochemical reactions of dissolved organic matter (DOM) in atmospheric waters can alter the composition and properties organic aerosols (OA), with implications for climate and air quality. In this study, we investigated the aqueous-phase transformation of fog DOM under simulated sunlight using online aerosol mass spectrometry (AMS), offline Orbitrap mass spectrometry with electrospray ionization, UV-vis spectroscopy, and aerosol volatility measurements. Irradiation increased the mass concentration of DOM-derived OA (DOMOA), defined as the low-volatility fraction of DOM that forms OA upon water evaporation. This increase was primarily driven by functionalization reactions that added oxygen- and nitrogen-containing groups, as indicated by a stable C mass, rising oxygen-to-carbon (O/C) and nitrogen-to-carbon (N/C) ratios, and enhanced signals of heteroatom-containing compounds over the course of irradiation. Despite evidence of fragmentation, spectral features associated with oligomerization, such as phenolic dimers, were also observed. To characterize chemical aging of fog DOM, we applied positive matrix factorization to the AMS spectra and identified three distinct factors representing progressive stages of aqueous-phase aging: initial DOMOA, more oxidized intermediates, and highly oxidized products, characterized by progressively increasing O/C and N/C ratios. These findings demonstrate that sunlight-induced aqueous-phase oxidation and functionalization of fog DOM drive the formation and aging of secondary OA, altering its composition, volatility, and light-absorbing properties with potential atmospheric consequences.
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Status: open (until 09 Oct 2025)
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RC1: 'Comment on egusphere-2025-3949', Anonymous Referee #1, 17 Sep 2025
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AC1: 'Reply on RC1', Qi Zhang, 03 Oct 2025
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Please find the author's responses to RC1 in the attached file.
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AC1: 'Reply on RC1', Qi Zhang, 03 Oct 2025
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RC2: 'Comment on egusphere-2025-3949', Anonymous Referee #2, 22 Sep 2025
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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.
Citation: https://doi.org/10.5194/egusphere-2025-3949-RC2 -
AC2: 'Reply on RC2', Qi Zhang, 03 Oct 2025
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Please find the author's responses to RC2 in the attached file.
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AC2: 'Reply on RC2', Qi Zhang, 03 Oct 2025
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RC3: 'Comment on egusphere-2025-3949', Anonymous Referee #3, 01 Oct 2025
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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 DOMOA:
- 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)
Citation: https://doi.org/10.5194/egusphere-2025-3949-RC3 -
AC3: 'Reply on RC3', Qi Zhang, 03 Oct 2025
reply
Please find the author's responses to RC3 in the attached file.
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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.