Review to "Measurement report: Formation and brownness of aqueous secondary organic aerosol from the aged biomass-burning emissions in the Sichuan Basin, China" by Peng et al.

This manuscript by Peng et al. uses ambient measurements of organic aerosols (OA) chemical composition and aerosol light absorption to understand the relation between the sources of OA and their absorption properties. The authors claim, using positive matrix factorization on OA, that a significant fraction of secondary OA during pollution episodes in the Sichuan Basin originate from the aqueous processing of biomass burning OA in aerosol water. These secondary OA formed in the aqueous phase are further argued to contribute largely to the light absorption by secondary brown carbon.

While this study makes great use of different datasets and statistical methods to understand the relation between OA sources and aerosol light absorption during clean and polluted periods, the text contains a lot of repetitive information that are often not clearly expressed. I also do not understand why this manuscript has been submitted as a measurement report. Indeed, the scientific conclusions from this manuscript are not limited to a description of observations made during a measurement campaign, but are based on parameters calculated/inferred from these observations (e.g., using positive matrix factorization, or a multiple linear regression method).

Below is a list of major and minor comments on the manuscript that I believe should be considered by the authors to facilitate publication into ACP. A number of technical corrections, mostly regarding language, are also suggested for improved clarity.

Major comments:

• The paper often uses terms such as “field observations”, “direct observations”, or “ambient observations” (e.g., lines 24, 266, 526, 551, 559, 575) to refer to aqSOA, its formation and its properties. This is misleading, as aqSOA was not “observed” but is the result of a statistical model (positive matrix factorization). The same goes for the light absorption by secondary brown carbon, which is a value derived from statistical methods (minimum R-squared method and multiple linear regression method). This needs to be clarified in the manuscript.
• I find that a justification and a discussion of the PMF factors is missing in the manuscript. A lot of the discussion is linked to the PMF factors (mostly aqSOA and BBOA), but apart from a short paragraph in the SI where a couple of correlations to external tracers are given, there is little justification and description of the PMF factors. For instance, it is mentioned at lines 45-50 of the SI that the factors were constrained based on factors from the literature, but the description of the way the a-values were dealt with is confusing (values from 0.7 to 1 deleted for HOA and CCOA, but what about BBOA?). Overall, showing figures for the 3-6 factors solutions, as well as the evolution of the Q/Q_exp for the different number of factors and a-values would help in justifying the choice of the factors.
• For the absorption values, the authors often refer to measurements of Abs_BrC, but it is never clearly stated that this value is calculated from Abs - Abs_BC, where the latter is calculated based on assumptions stated at lines 183-184. A more thorough description of this is needed in the manuscript or in the SI, as it currently missing.
• The manuscript contains a lot of redundant sentences and grammatically incorrect sentences that perturb the reading. I have tried listing most of these issues below in the “minor comments and technical corrections” section.

Minor comments and technical corrections:

• Lines 40-42: I would suggest changing to present tenses when you make a general statement not necessarily related to one particular study. This comment is valid elsewhere in the manuscript (e.g., lines 49, 76, 85, etc.). Please check.
• Lines 52-56: This sentence is very difficult to understand as it is. Can you maybe split it in two and reformulate it?
• Line 94: What do you mean by “typical city in SCB”? This comment is also valid for the statement at line 106. For the general reader, it is not clear what “typical” means in that sense. Is it typical in terms of aerosol pollution, in terms of population, in terms of meteorological conditions?
• Lines 104-105: Please rephrase (e.g., “… was conducted at a measurement site in a city affected by severe aerosol pollution”).
• Line 111: What do you mean by “interfered by neighboring buildings”? Do you mean in terms of dynamics or emission sources?
• Lines 120-125: I would suggest moving this part to Sect. 2.3.1 where it discussed in more details at lines 146-150.
• Line 121: Please change to “… were regularly calibrated using a scanning…”.
• Line 124: Please change “could” to “can”.
• Line 127: The Aethalometer AE33 measures equivalent black carbon, not black carbon. Please clarify that in the text.
• Line 135: Please change “could” to “can”.
• Line 157: Please spell out the “SoFi” acronym (i.e., Source Finder).
• Line 163: I would suggest adding the reference of Paatero and Hopke (2003), as they originally presented this downweighting scheme.
• Lines 183-184: The formulation “Abs880 was sole from BC” is not clear. Do you mean that it is assumed that, at 880 nm, only BC is contributing? Please rephrase.
• Lines 195-198: If I understand correctly, the constant a-e are wavelength-dependent right? If so, can you clarify that?
• Line 209: Similar to a comment I made above, please use present or present perfect tense when referring to a figure shown in the main or SI (i.e., “… during the campaign is shown in Fig. 1.”). This comment is valid elsewhere in the manuscript. Please check.
• Lines 216-217: There is an alternate use of “Org” and “OA” in this manuscript. While I understand that “Org” refers to the species measured by the ACSM, and “OA” is more general, the distinction is not always clear. I would suggest sticking to one or another.
• Lines 220-222: As it is written, giving a range of mass concentrations does not justify that the “OA composition were substantially different”. Either refer to Fig. 2 in this sentence, or move this sentence to the next paragraph and merge it with the sentence where you actually discuss the differences in chemical composition.
• Line 227: Since you refer to the method as “PMF” before, stick to “PMF” and not “PMF-ME2”.
• Line 229: How can you tell for sure it is C₂H₄O₂⁺ contributing at m/z 60, when you only have unit mass resolution with the ACSM? There are other ion fragments at this m/z. Please change to m/z 60, since you also just mention m/z 73 just after. You can add the ion fragment that is most likely contributing in parenthesis. This is valid elsewhere in the manuscript (e.g., line 251, 447, 448).
• Line 229: This is the first time you mention a “correlation”. Please define the metric you use. You mention “r²”, is it a Pearson correlation squared, a Spearman correlation squared, a coefficient of determination?
• Line 228-234: For someone who is not an absolute expert in PMF, this whole paragraph gives very little information on the justification of the choice of factor. At least give references for why the correlation mentioned justify the factor names.
• Figure 2: The legends are very confusing, as the elements of the legend in (a) also apply for (b) and vice-versa. The left y-axis are generally weird, can you make them start from zero and have constant increments?
• Lines 242-245: Please reformulate this sentence, it is grammatically incorrect.
• Line 257-258: Do you rather mean here the chemical processing from BBOA to aqSOA? Since they don’t peak at the same time the argument was more that one leads to the other, not that there are processed at the same time.
• Line 261-265: This sentence is a repetition of the sentence at the end of the last paragraph. Also, you now mention fog events, but you mentioned before that the aqueous processing was related to ALWC, so aerosol water, not fog water. Regarding the last point, you mention this difference at lines 305-308, so you could move it here.
• Lines 281-283: Please reformulate this sentence, it is grammatically incorrect.
• Figure 3: Is the f29 coming from the ambient ACSM data? This distinction was never made and it is a bit confusing, as you sometimes also refer to the f29 for each induvial factor separately (which are constant values and not variables). Also, why is the x-axis the percentage of aqSOA to BC+NR-PM2.5 and not the percentage of aqSOA to total PMF OA as shown in Fig. 1d.
• Line 288: What do you mean by “located in a similar region”? Which figure are you referring to here?
• Lines 292-296: The correlations mentioned are not shown on any figure, please add “(not shown)” in the sentence.
• Line 295: Change “production” to “produced”.
• Lines 301-305: Please reformulate this sentence, it is grammatically incorrect.
• Line 311: This is the first time you make a distinction between POA and SOA. What exactly do you consider as POA and SOA for you PMF factors? You need to make this clear here as the reader doesn’t know what you are referring to otherwise. At line 361, you explain what you mean by “SOA” but still not for POA.
• Lines 341-342: This sentence feels like a repetition of what was said just before with Fig. 4.
• Lines 342-345: The comparison is difficult to make, as you have much less datapoints during polluted periods. Can you check the statistical significance of your statement of “stronger positive correlation”?
• Line 345: The correlations mentioned here are different than the ones given at lines 254-255 for the same variables. Why are they different? If these are supposed to be the same correlations, why repeat it here?
• Lines 345-347: Such statements that BBOA leads to aqSOA formation are repeated in almost every paragraph and it feels redundant (e.g., lines 290-291, 326-328, 350-352, 406-407), can you remove it in some places to avoid redundancy?
• Lines 347-348: Figure 5b and d don’t show any correlation between f29 and aqSOA, at least not using circle size for f29.
• Lines 348-350: If you mention that the correlation is “strong” can you give numbers? It also seems to be based on a subset of a subset of data, which doesn’t seem so fair.
• Lines 355-359: Please reformulate this sentence, it is grammatically incorrect.
• Figure 6: The colors and the legend of the figure are very confusing. Especially because the color bar for ALWC has the same colors as for the PMF factors. It is also not clear what are the data points from other studies and what are the data points from this study since the legend only describe the squares but not the circles.
• Lines 361-363: The conclusion of this sentence is too far-fetched. How does a larger f44 for SOA compared to POA justifies the statement that aging of OOA leads to formation of aqSOA?
• Line 365: Do you mean Fig. 6b or maybe 6d, instead of Fig. 4c?
• Lines 365-367: Is 0.011 “much higher” than 0.009 or 0.008?
• Line 367: Insert comma after 0.002.
• Lines 369-371: The information in parenthesis “f44 (representation of aged OA)” and “f60 (presence of anhydrosugars)” is important to understand Fig. 6. This information should come much before when introducing Fig. 6 (i.e., between line 353 and 359).
• Lines 373-376: You mention “previous research” but don’t cite papers. Also, the second part of the sentence after the comma is unrelated to the first part. I.e., the fact that the observations are within the triangle space does not imply that POA are fresh and aqSOA are more oxidized and formed from BBOA.
• Lines 379-382: What are the numbers in parenthesis after CP? Please reformulate the sentence, at present it is not possible to understand it.
• Lines 383-387: I would argue that not “some” points are outside the bottom boundary of the triangle, but most are. Also, please rephrase the sentence, it is not grammatically correct.
• Lines 387-389: In general, it also just seems that there is much less ALWC during CP (which makes sense since aerosol concentrations are lower during CP). I am not convinced that this is an argument to justify the sentence that follows.
• Lines 389-391: Please reformulate this sentence, it is grammatically incorrect.
• Lines 394-398: This sentence is already in the caption of Fig. 6. No need to duplicate the information, remove it here.
• Lines 398-399: This sentence should go in the caption and be remove here.
• Lines 399-401: It is not clear if you are referring to the PMF factors or the datapoints. If you are referring to the PMF factors then this sentence is in contradiction with the sentence at lines 367-368, where it is said that OOA has an f60 below the 0.003 threshold.
• Lines 401-403: The part “while the mean f44 value was significantly higher” was already stated at lines 377-378.
• Lines 403-405: I am bit skeptical about this sentence since the schematic space is indeed a schematic, so you cannot quantify how many of your datapoints are within this region. Perhaps formulating it differently would help.
• Lines 425-427: Are you making a general statement or an observation based on your data? Also, please reformulate the sentence, it is grammatically incorrect.
• Line 431: Please remove “As shown in Fig. S6”. The average value of Abs_370,BrC is not shown in Fig. S6 and cannot be guessed since BrC is split between primary and secondary contribution.
• Lines 434-436: The distinction between POA and primary brown carbon is unclear here. These are not the same thing and should not be interchanged.
• Line 438: Change “Here we showed” to “Hereafter, we show”.
• Lines 443-446: Where do the values in parenthesis come from?
• Figure 7: Are the “MAC” values the a-e constants in the MLR method? If yes, then please explain this in the text clearly. Same comment for lines 459-460.
• Lines 452-454: Please reformulate this sentence, it is grammatically incorrect.
• Lines 462-465: Please reformulate this sentence, it is grammatically incorrect.
• Lines 465-467: What the values in parenthesis? Are they from the literature? Have you calculated them? If so why mentioning at wavelength 370-660 nm while it is later introduced at 370-880 nm at line 505.
• Lines 467-469: Same comment as for lines 443-445, where do the numbers come from?
• Lines 472-482: This entire paragraph is based on discussion of an SI figure (S9). I would suggest moving this figure to the main. Eventually, Fig. 7 could go in the SI as it does not provide much information and is not discussed much.
• Line 487: Change “but no OOA” to “this was not the case for OOA”.
• Lines 488-489: Please reformulate this sentence, it is grammatically incorrect.
• Line 496: The relation between Abs_370,BrC,pri/ΔCO looks more like a cloud of point to me.
• Line 501: Change “should” to “could”.
• Line 508: I would define a correlation of 0.49 as strong. Even though it remains unclear if r² is really the correlation or the coefficient of determination.
• Lines 513-515: I might be mistaken but isn’t this relation circular? Since AAE is calculated based on a log relation of Abs values and BC is linearly dependent on Abs, don’t we expect the relation we see on Fig. 9?
• Lines 526-529: It appreciated to have a concluding statement here, but feels redundant since a conclusion is provided just a few lines after.
• Line 540: I would call this section “Conclusion” and not “Implications” as the content of the section is about summarizing and repeating the content of the manuscript.
• Line 541-542: This sentence is a copy-paste of the first sentence in the intro. Please reformulate or delete.
• Line 545: What do you mean by “an increasing laboratory research”?
• Lines 551-552: You mention that your study highlighted the importance of aqSOA on air quality. However, this is the first time in the paper that air quality is mentioned.
• Lines 552-557: As shown in Fig. 7, isn’t it BBOA that has the strongest absorption across UV to visible, rather than aqSOA?
• Lines 558-559: This sentence is redundant.
• Line 560: Saying “considerable” implies that you can quantify it. However, in this manuscript the amount of aqSOA that originates from BBOA is not quantified. Please rephrase.
• Line 564: Not sure how Fig. 6 justifies that the formation of aqSOA was through aqueous-phase reactions instead of photo-chemical reactions.
• Lines 571-574: Please reformulate this sentence, it is grammatically incorrect.
• Lines 575-577: Please reformulate this sentence, it is grammatically incorrect. Seems like part of the sentence is missing.

References:

Paatero, P. and Hopke, P. K.: Discarding or downweighting high-noise variables in factor analytic models, Analytica Chimica Acta, 490, 277–289, https://doi.org/10.1016/S0003-2670(02)01643-4, 2003.

This study reported the direct ambient observation of SOA formation and absorption properties in the aqueous phase from the Sichuan Basin, China. The results showed considerable aqSOA was originated from the aged biomass-burning emissions via aqueous-phase reactions instead of photo-chemical reactions under high aerosol liquid water content (ALWC) conditions, especially during the polluted period. This study revealed the aqSOA formation and brownness from aged biomass-burning emissions and highlighted the importance of aqueous-phase reactions on air quality and climate. This is a manuscript with potential. I recommend the manuscript be revised before being accepted for publication.

1. Lines 83-87. The authors mention the geographic variability of the North China Plain amount the Sichuan Basin, but there is a study on aqSOA in these two regions is not described in detail, it is recommended to add a comparative analysis to better highlight the regional uniqueness of this study.
2. Section 2.1. As a field measurement report, this paper lacks a map of the geographic location of the observatory sites, which does not allow us to visualize the geographic information about the Sichuan Basin, so we suggest that a map of the observatory sites and the surrounding terrain be added.
3. Line 151. Further details are required on the basis for setting the CE value of ToF-ACSM to 0.5.
4. Section 2.3.2. The ISORROPIA-II model estimates ALWC from these inorganic constituents only, but will the organic constituents affect the estimates, especially at higher OA conditions.
5. Lines 220-222. There is a need for further clarification on the judgment of clean-up and contamination periods, the standards are different in each country and it is recommended to add comparisons with other standards.
6. Lines 294-296. The authors suggest that aqSOA may also be produced by aqueous phase reactions of fossil fuel-related OA components, but this is only speculative and a more convincing basis is needed.
7. Lines 350-352. These results indicated that considerable aqSOA might be formed from BBOA, which was more intensive than OOA at high ALWC levels during PP. What are the authors' judgments about the magnitude of their intensity, and what are the differences in "intensive"?
8. Lines 372-373. These results indicated that considerable aqSOA might be formed from BBOA, which was more intensive than OOA at high ALWC levels during PP. Any details on what the mechanism is for this result?
9. Lines 406-407. The authors emphasize the importance of aqueous-phase reactions, but how can the contribution of photochemical pathways be judged?
10. Lines 497-500. How was this result obtained and please provide further clarification on the mechanism?
11. The conclusion section needs to be further condensed; it seems that parts of the conclusion have been emphasized several times, and there is a need for the author to reorganize the structure of the conclusion to give a complete and clear conclusion.