General Comments: This manuscript describes the investigation of the fluorescence properties of water-soluble organic compounds (WSOC) in size-segregated atmospheric particles collected in a rural area of Beijing. To attain these goals, the authors applied different data analysis tools to the excitation-emission matrix (EEM) fluorescence spectra of the WSOC fractions from each particle size-range. The WSOC is well recognized as an important driving factor in climate change due to its light absorption properties. However, our understanding of the chemical and optical properties of the WSOC is still inadequate, particularly when dealing with size-segregated aerosol WSOC fractions. The topic dealt in this manuscript has relevance to the atmospheric research community by providing new insights into the optical properties of size-segregated WSOC, but also by suggesting the impact of photochemical reactions on the fluorescence properties of atmospheric WSOC, which were previously unnoticed in studies focusing only on WSOC from fine atmospheric air particles (PM2.5). Nevertheless, this study has also several shortcomings that need to be adequately addressed by the authors. These are described below, in Specific Comments.

Specific Comments:

Lines 124-127: The text in lines 124-125 is repeating the same information as that provided in lines 115-120. Furthermore, the sentence in lines 126-127 makes more sense in a Introduction section, rather than in a Data analysis section.

Section 2.4.3. Grey relational analysis (GRA): The authors make a strong focus on the novelty of GRA applied to the analysis of EEM fluorescence data. Nevertheless, this is the most obscure section of this study, particularly to those potential readers not familiarized with this analytical tool. The authors should provide a thorough explanation regarding the meaning of each variable in equations (5) and (6) and their relation to the EEM fluorescence data. Furthermore, it is unclear to which factors are the authors referring to when stating that the “fluorescence intensity is highly affected by WSOC concentrations and many other factors (…)” and that “their relations are not clear”. Please, be more clear regarding these issues, and explain why you are considering the particles < 0.26 µm as “the references” (only mentioned in line 256)? Moreover, when referring to the “references”, do you mean the EEM fluorescence data of WSOC from particles < 0.26 µm? All these issues need to be adequately addressed and thoroughly explained in the manuscript.

Lines 166-167: The authors state that other researchers also verified a bimodal distribution for the organic matter in other locations within the same region of this study. Firstly, it would be important to clarify whether this bimodal distribution followed the same size distribution as that reported in this study for the summer samples. Secondly, the authors should be aware that the concept of “organic matter” is different from the concept of “WSOC”, because in the former you must consider the contribution of atoms (e.g., H, N, S and O) other than carbon to this fraction. This is why it is common to use an aerosol organic mass-to-organic carbon ratio (OM/ OC) to assess the content of organic matter in the air particles in order to achieve a mass closure. Even though it was not possible to estimate the OM/OC ratio in this specific study, it would be interesting to assess whether the organic matter also follows a similar bimodal distribution (see, for example, the work of Li et al. (2020), Science of The Total Environment, 703, 134937, https://doi.org/10.1016/j.scitotenv.2019.134937, for OM/OC ratios for primary and secondary organic aerosols).

Line 168: Is it possible to include some explanations for the fact that the WSOC/OC ratios are higher in winter than in summer. Could this difference be associated to the prevalence of biomass burning emissions in winter?

Lines 175-176: The authors state that “The bulk fluorescence features of WSOC showed evident distinctions among fine particles and coarse mode particles on EEM spectra”. In this Reviewer’s opinion, these differences between the EEM spectra of fine and coarse mode particles are more evident in terms of the fluorescence intensity rather than in terms of different fluorescence peaks.

Lines 197-199: The authors state that “FRI Ⅲ and FRI Ⅴ (HULIS) were the most abundant two fluorophores rich in fine particles.” The authors are considering the total fluorescence intensity of these two regions? Figure 4 suggests that FRII is the most abundant fluorophore in fine particles for both summer and winter samples.

Furthermore, the authors also state that “FRI Ⅳ (microbial related species) peaked between 1.4 to 2.5 μm and showed little variations with particle size increase.” However, Figure 4 depicts different results: for the winter samples, FRIV accounts for 23% for particles between 0.26 and 2.5 μm, whereas for the summer samples, the lowest percentage of FRIV (15%) is reported for particles between 1.4 to 2.5 μm. The authors should correct these inconsistencies in their assessment of the results.

Lines 238-239: If component C3 (assigned to HULIS-2, in line 237) has no physical significance and is considered as a “noise signal”, why it is quantified in Figure 6, for the Summer samples? Does it means that 17 to 46% of the fluorescence intensity of PARAFC components for each particle size, in summer samples, is due to “noise signal”? This should be clarified in the manuscript, alongside with a reference to the variance of the model and the core consistency value for each particle size, for the winter and summer samples.

Section 3.5: As above mentioned, the lack of explanations regarding the GRD analysis applied to the EEM fluorescence data is the main issue of this work. For example, in line 256, which are the comparing factors (and why) and why the particles below 0.26 μm were used as references? In lines 257-258, what do you mean with the statement “The GRD of WSOC, AFI, and UV between particle sizes were basically well among both seasons.”?

Furthermore, in lines 267-268, the authors state that “GRD were strongly negatively correlated with estimated secondary organic carbon (SOC) concentrations with correlation efficient r at -0.64 (p<0.000) in winter and -0.63 in summer.” Where is the data regarding the estimate of SOC in the collected air particles samples? What was the method followed by the authors to estimate the amount of SOC in the collected samples? Additional data and explanations are required here for a better understanding of how fluorescent WSOC is highly affected by secondary processes, and that GRD between WSOC and AFI could serve as an indicator of secondary formation.

Technical Corrections:

1. In this Reviewer opinion, the English language needs extensive revision throughout the manuscript in order to improve not only its reading, but also to clarify the structure and discussion of the scientific results and conclusions.
2. Line 117: where it reads “Roman unit” it should read “Raman unit”.
3. Line 211: where it reads “p-conjected” it should read “p-conjugated”.
4. Line 219: The reference “(Valeur and Berberan-Santos, 2012)” is not accurately listed in the reference list (see line 373).
5. Line 222: In my opinion, “Figure 3” should appear as “Figure S3” in the text, because the authors are referring to Figure S3 of the Support Information. Please, also see my comment below on Figures S1, S2, S4, S5 and Table S1, in Supporting Information.
6. Line 271: where it reads “indicter” it should read “indicator”.
7. Figure 1 is not mentioned nor discussed in the manuscript, although it is presented at the end as being part of the manuscript.
8. Figure 2: please, clarify which axis correspond to the Emission and Excitation wavelengths in order to facilitate the analysis of the EEM spectra by the potential reader.
9. Figure 3 caption: where it reads “Roman unit” it should read “Raman unit”.
10. Table 1: please, include the units of the WSOC and WSIN concentrations (micrograms per cubic meter?). Moreover, in Table’s caption, where it reads “standard divisions” it should read “standard deviations”.
11. Please, update the year of the reference Almeida et al. (Environ. Sci. Technol. 54, 1082-1091), since it was published in 2020.
12. Supporting information: the organization and cross-reference, in the main text, of the data presented in the Supporting Information needs to be better addressed. For example, there is no reference in the main text to Figures S1, S2, S4, S5 and Table S1. The authors should also clarify the purpose of these figures and table and how these data were obtained and how it is being used to support the main results and discussion presented in the manuscript. In this regard, as an example, in Figure S2 caption, it is unclear to which particles size correspond the EEM spectra in Figure S2(a), as well as to which stidies are the authors referring to in Figure S2(b) and S2(c).

Comments: In the present research, an informative excitation-emission matrix (EEM) of fluorescence method is applied to estimate the fluorescence properties of water-soluble organic compounds (WSOC) for different particle sizes, in a rural site of Beijing, China. The light absorption properties of WSOC and its significance on secondary transformation of aerosol has been long researched. A sort of data analysis methods is combined to investigate the fluorescence properties of size-segregated WSOC, and the possible aging processes of WSOC with particle size increase is unveiled. This research is well arranged indeed. However, there are still some questions that need to be further modified. I hope that after the author has carefully improved the manuscript, it is recommended to publish it to Atmospheric Chemistry and Physics.

Special comments

1. Line 93: Itseems unnecessary to have another repeated “were” at the end of this line.
2. Line 101: It should be “adapted” rather than “adopted”.
3. Line 115 to 116: Are these parameters for EEM sampling? Please make it clear what are they refers to.
4. Line 155: A verb is missing after “ξ”
5. Line 181-185: The sentences of this paragraph are hard to read because of lacking main logic board. Try to describe the seasonality and the size distribution of SFI separately, rather than mixing them together.
6. Line 196-200: Line 196 to line 198 were mainly about the size distributions of FRI Ⅰ to Ⅴ, however, the description of “FRI Ⅲ and FRI Ⅴ (HULIS) were the most abundant two fluorophores rich in fine particles.” Seemsincongruent with the context. Moreover, what is the purpose of adding the reference of Huang et al., (2020) found similar size distribution of protein and HULIS by isotopic method at the end of this paragraph?
7. Line 243-246: Similar to former issue, the sentences were uncombined with each other. So the intention of each description is confused. Why do you propose a HULIS1/HULIS2 ratio for winter results? If HULIS1 (or 2) implies different oxidation state of HULIS, the last sentence should be brought forward.
8. Section 3.5: If it is just as my comprehension,the GRD is a factor of reflecting relations between two factors, why does the author use grey relational analysis rather than correlation analysis?
9. Line 191: The author state that “Our unpublished research found that the AFI/WSOC ratios were lower than 0.2 for anthropogenic source samples, indicating that this ratio might be higher in oxidized fluorescent WSOC.” This “indicating” may not be easily deduced here, and I noticed that these inductions are discussed in line 282 to line 292, so the description in line 191 can be saving for later paragraph.
10. Line 293: The first sentence shows weak leadership for this paragraph, it also shows little connections with later context
11. Line 315 to 318: The conjectured sources of HULIS is not closely related to former context.

Minor issues:

1. The tense form should be unified
2. Check the abbreviations and capitalized letter throughout the article, some of them are in wrong form.
3. Some of definite or indefinite articles are missing.

General Comments: This manuscript titled "Particle size-dependent fluorescence properties of water-soluble organic compounds (WSOC) and their atmospheric implications on the aging of WSOC" describes fluorescence properties of size-segregated WSOC aerosols in a rural area of Beijing. To attain the study objectives, the authors applied different data analysis tools to the excitation-emission matrix (EEM) fluorescence spectra of the size segregated WSOC. The WSOC aerosols represent a significant fraction of organic aerosols and one of the driving factors in climate change due to their light absorption properties. The topic of the study is within the scope of the journal and has relevance to the atmospheric research community. Although the technique used (i.e., EEM fluorescence spectra) in this study might have some limitations (such as difficulties in segregating anthropogenic and bioaerosols WSOC fractions etc) if used alone. However, size-segregated EEM fluorescence spectra (this study) can be helpful in mitigating many of these limitations and understanding anthropogenic and natural sources of WSOC, their atmospheric evolution, and optical properties. Even so, this study has many shortcomings in its current version given below:

Major Comments:

Introduction: Lacks organization and continuity? The reviewer suggests the revision of introduction section to make it more organized and in tandem with the objective of the study.

Line 161: The size distribution of WSOC/OC and WSOC concentration doesn't follow similar trend. Although several studies in the past (Dasari et al., 2019 science advances; Choudhary et al., 2021 environmental pollution) as well as this study (in introduction) have stated that majority of WSOC are secondary (oxidized) in nature. The author can elaborate possible rationales briefly?

Line 174: Figure 1 is not discussed in the manuscript. Either delete it or add some relevant discussion about same?

Line 186-193 and 285-290: The author stated that "The AFI/WSOC ratios ranged from 0.22 to 0.57 in winter and 0.18 to 0.34 in summer, respectively." "Our unpublished research found that the AFI/WSOC ratios were lower than 0.2 for anthropogenic source samples, indicating that this ratio might be higher in oxidized fluorescent WSOC." If that is the case, size distribution of AFI/WSOC should have follow the distribution trend of WSOC/OC (a tracer for photochemical oxidation), but this is not the case in this paper (Figure 3). Explain the rationale/s behind this behaviour?

Line 212: The author stated that Stokes shift (SS) of 1.2 μm-1 is an important border of hydrophobic and hydrophilic components. And later used Stokes shift of 1.1 to determine ratios of fluorescence intensity in high SS. Elaborate the possible reason/s?

Line 205:HIX (aromaticity) and WSOC/OC (oxidation) ratio following same size distribution trend. How come? This could be an important finding of the manuscript.  Add some discussion about same in  Discussion and Implication sections.

Line 209, 210, 243: The author categorized Protein-like compounds into biogenic origin. But aerosols partitioned from VOCs (isoprene etc.) emitted from plants also categorized into biogenic aerosols. Does the author also incorporating these aerosols produced from VOCs in Protein-like compounds or it is just bioaerosols? Please clarify?

Line 256: Why did author used particles <0.26 μm as references for Grey relational analysis (GRA)? Why not use size bins where WSOC, UV and AFI are maximum?

Minor Corrections:

In the Reviewer's opinion, the English language needs significant revision throughout the manuscript before acceptance. The some of English-related corrections and other minor

comments are suggested below:

Line 33: Replace “mysterious” with  either “Unknown” or "Uncharacterized".

Line 34-35: The sentence lacks continuity. Revise the sentence "incorporating with different highly oxidized functional groups or heteroatoms like" with may be something like "WSOC mixture contains both aromatic nuclei and aliphatic chains (Decesari et al., 2001; Dasari et al., 2019), with functional groups or heteroatoms like hydroxyl, carboxyl, aldehyde, ketone, amino, and other nitrogencontaining groups (Duarte et al., 2007; Cai et al., 2020)".

Line 37: Is the reference “(ParkSeungShik et al., 2017)” is correctly cited and listed in the reference list (also see line 485).

Line 39: Revise "Nuclear magnetic resonance (NMR) and mass spectrometry (MS) are two remarkable analytical methods using to structurally  unravel the complex WSOC (Duarte et al., 2020)."

line 46: It is "Accelerator" not "accelerate".

Line 46: Revise the sentence to something like "Isotopic ratio mass spectroscopy (IRMS) and accelerator mass spectroscopy (AMS) are widely used to distinguish organic emissions from fossil combustion sources and biogenic sources using carbon isotopic characteristics (Masalaite et al., 2018; Zhao et al., 2019; Huang et al., 2020)."

Line 50-56: Whole paragraph lacks organization and continuity. The reviewer suggests the revision of the paragraph.

Line 57: Replace "3-Dimensional fluorescence of excitation-emission matrix (EEM)" to "3-Dimensional excitation–emission matrix (EEM) fluorescence spectroscopy"

Line 59: it should be "mainly helpful in investigating"

Line 62: what does author mean by "in early years"? Does author mean "earlier studies", if so, revise the sentense.

Line 65: It should be "analyse" not "analysis"

Line 69: "(great parts of WSOC)"? It should be something like "significant fraction of WSOC"

Line 70: "reversely"?

Line 82: "neighbor particle sizes" should "adjacent particle size bins"

Line 83: The use of "But" is not perfect here. The reviewer suggests to use "and" instead.

Line 94: confusing sentence "All samples were collected by quartz filters (Whatman) were prebaked for 5 hours (500°C) and wrapped by aluminum foil stored at -20°C after sampling." May be revised to "All samples collected on quartz filters (Whatman), prebaked for 5 hours (500°C) before sample collection, were wrapped by aluminum foil after sampling and stored at -20°C."

Line 95: Need clarification? Total 20 groups for 2 seasons or 20 groups each for every season?

Line 106: Should be "The extract was then filtered through a 0.22 μm membrane filter to remove impurities."

Line 113: Confusing? The sentence may be written like "The extraction procedure of samples subjected to fluorescence and ultraviolet-visible (UV-Vis) measurements were same as WSOC detection."

Line 117: Should be "Raman Unit"

Line 124: Revise the sentence "The EEM data were spectrally corrected by blank sample for instrument bias, inner filter effects, Rayleigh scattering, and

most of Raman scatter had been removed" to "The EEM data were spectrally corrected by blank sample to remove interferences from instrument bias,  inner filter effects, Rayleigh scattering, and Raman scatter."

Line 133-134: Equations number is not matching? Example: "equation (3)" should be "equation (2)" and "equation (4)" should be "equation (3)"

Line 218: Revise "On a large scale of a π-conjugated system, the...."

Line 222: "Supporting information Figure 3, and Figure 5(c)." shoud be Figure S3 and Figure S5(c). Do same thing for Figures S1, S2, S4 and Table S1, in Supporting Information.

Line 87 and 228: The full form of PARAFAC is already mentioned on Line 87. No need to repeat it again. Follow same comment for others as well (e.g. GRA on line 249 etc.).