|First of all, I would like to thank the authors for their thorough responses and revisions to the referee comments. Upon reading the manuscript after the authors’ substantial improvements to clarity and background, I am able to better understand the very interesting content of the article and the comprehensive scope of the dataset. The main referee suggestion to add additional context and interpretation based on prior literature has been well addressed, as has the concern about proper attribution. While the organization of the paper has improved, I have two additional suggestions for the results and discussion. Further consolidation of the many small figures would enhance the clarity of the story, but I will leave that to the discretion of the authors. I have also added some final comments on the clarity of the added content.|
The authors’ revisions substantially improve the quality of the paper in terms of the communication of ideas, references to prior work that is relevant, and overall scientific value of the content. I still have some suggestions, but I recommend this paper for publication after minor revisions.
Lines 370-396: I find this section to be the most fascinating in the paper. This work expands upon that of Wu et al., 2018 by applying this scenario to the aq-OOA concentration. However, I have two concerns:
1. I found that the description presented in the authors’ response to the referee was more clearly laid out than that described in the article: describing the summertime observations (ALWC dependence) and hypothesis (aq-OOA formation from bulk aqueous reactions), followed by the contrasted wintertime observations (SIA dependence) and hypothesis (heterogeneous surface reactions).
2. Even given the clearer explanation, it looks in Figure 6 as though there are simply more data points at higher aq-OOA concentrations in the winter dataset, and, based on the figure and current discussion, I’m not entirely compelled to agree with the authors’ hypothesis (that there is a difference in summer versus winter aq-OOA relationships with ALWC and SIA concentrations, respectively). Please directly explain how the dependence on ALWC implies a bulk reaction scheme, while SIA mass dependence implies a surface area scheme, and relate that to the data. Including example values and directly discussing the arrows in Figure 6 would also be helpful to support the hypothesis. In addition, please consider the supposition by Wu et al. that wet deposition over 80 % RH impacts the observed relationship. Does this work support that finding?
Van Krevelen discussion (lines 310-312, Section 3.4, Abstract, Conclusion):
1. I agree with the authors that the Van Krevelen analysis is useful to demonstrate the similarity of chemical pathways in different regions/studies (as in Heald et al., 2010) or the chemical pathway in higher resolution data (as in Ng et., 2011). I applaud the very thorough and interesting analysis in Section 3.4, and I agree that discussing which molecular pathways could be at play makes sense with the higher time resolution datasets. However, this expanded analysis does not really fit, and could be a starting point for an entirely new paper in terms of content. Please clarify how the expanded Van Krevelen discussions support the story this paper tells, and add an explanation in the text and/or save some of the analysis for another manuscript. Since a slope over multiple field campaigns and lab studies is already available in the literature (Heald et al., 2010 and possibly others), it might be sufficient for this paper to contrast the current field campaign values with those from the literature.
2. The chemical pathways that the authors give do not seem to align with the slopes calculated from the data. For example, the slopes of -0.39 and -0.33 shown in Figure 7b are attributed to addition of alcohol or peroxide groups, which would be a slope of 0 (assuming no fragmentation). Some combination of moieties that could add to the slope that is found (i.e., -0.39 and -0.33) should be given. Please revisit, and perhaps more clearly explain the reasoning, for the slopes in the Conclusion and Section 3.4 in particular.
3. If Figures 7c and 7d are kept, please list the references for all of the studies included as data points (or cite where the references can be found if they are already included as a list elsewhere).
4. Lines 428 onward: Please describe the difference between “the O/C ratio of bulk OA” (line 430) and “the average O/C ratio” (line 429). The values listed in this paragraph don’t seem to match those in Figures 7c and 7d. Please verify that the values in the text are accurate.
While the Conclusions align well with the major findings of the paper, the Abstract is not as representative. Please consider updating the Abstract to better align with the content of the paper.
Please write out the objectives and/or hypotheses of the paper in a statement at the end of the Introduction section. Several sentences throughout the Introduction give hints (lines 74-75 “...found the contribution of SOA increased…”, lines 76-77 “...it is expected that…”, lines 89-90 “these studies indicated…”), but there should be a clear statement. It seems to me that the two main objectives are along the lines of: (1) finding which variables control aq-OOA formation and how; and (2) quantifying the changing contributions to OOA between seasons and years at Xi’an.
Please consider moving the paragraph at line 224 (“Six OA sources were resolved…”) to before the paragraph at line 166 (“A continuous and large increase…”). The paragraph at line 224 is more related to the theme of changes over time in OOA sources, which is discussed above line 166.
Line 258: Please add takeaways about the paragraph: is the key addition to the paper’s story that the SIA_P2 represents a period of greater oxidation?
Line 303: “In comparison, the MO-OOA resolved in Baoji…” - should this be the LO-OOA? The
LO-OOA looks closer to the biomass burning region in the figure.
In addressing the referee comment 5a regarding the expectations for the PMF factors, the following background was provided for the referee. Please consider adding this perspective to the section of the manuscript discussing POA factors.
“Agricultural burning was an important contributor to OA in harvest season before 2013. However, agricultural burning in harvest season has been banned after 2013 and BBOA source becomes a negligible contributor to OA in summertime, especially in urban city in recent years (Huang et al., 2021).”
Lines 161-165: Please add concentrations here as well (the authors include contributions) to demonstrate whether concentrations also decreased between Xi’an 2013 and 2018 winters.
Lines 210-212: Thank you to the authors for clarifying the NOR and SOR relationships. Please also specify what “n” indicates in these equations.
Line 88: Is “RH/ALWC” a ratio or specifying that either could be a factor? In Duan et al., 2021, it seems like it is the latter (not a ratio); if that is the case, please use “RH and ALWC” or similar.