Ye Kuang et al. present an interesting study on the nighttime formation of secondary organic aerosol (Night-OA) from field observations of biomass burning emissions in the Pearl River Delta region of China. The study highlights the importance of nighttime chemistry in the evolution of biomass burning plumes, particularly the role of NO₃ radical chemistry and aqueous-phase processes in the formation of highly light-absorbing organic aerosols. The authors primarily use correlation coefficients to support their hypotheses. However, the study would benefit from additional analysis and discussion. For example, a more detailed investigation into the hypothesized reactions at play through box modeling would be valuable to assess the importance of the specific mechanisms involved in its formation. Within the paper, the authors also spend more time providing detailed explanations on key concepts. Finally, this article would strongly benefit from an English grammar review. I would not recommend this publication until my concerns are addressed. Below are my comments.



Major Comments:

1. This paper introduces a wide variety of scientific hypotheses and reasoning without a proper explanation to the audience (i.e., me).
   1. For example, on page 8, the authors test whether Night-OA was associated with transported plumes containing NO_x. They use diurnal variations of Night-OA/CO and Night-OA/NO_x ratios to conclude Night-OA is likely formed through secondary processes. Are these separate thoughts? How does the transported NO_x get tested and what do the ratios mean with regards to transported NO_x?
   2. Another example is provided in the minor comments section, line 176. It would benefit both the reader and the author to expand on these ideas and explain them.
   3. Another example is explaining to the reader how the carbon signals from SP-AMS were used to retrieve the shape of BC mass size distribution. The supplement of Luo et al. (2022) shows a simple formula (equation 8). Why not include it for the readers?
   4. What is the multivariate linear regression method for reporting mass absorption efficiencies? Please define mass absorption efficiency.
2. The authors spend much of their time in the second half of the paper discussing implications and suggestions as to how all these compounds interact with one another. It is not very convincing to me. I would like to see the field measurements, like those in Figure 5, put into a box model to see if these proposed reactions and mechanisms can reproduce these mixing ratios. Additionally, they can quantitatively compare the reaction rates of these different pathways.

Minor Comments:

Line 73: I would adjust this sentence to say, ”…Palm et al. (2020) observed that daytime oxidation of emitted phenolic compounds contributed a majority to BBSOA formation from direct gas emissions, with products…” In Figure 4 of Palm et al. (2020), he demonstrates that the majority of BBSOA (87%) is from evaporation of BBPOA.

In section 2.1, could you please describe what specific instruments were used to measure RH, temperature, and wind speed and direction?

Line 152: What are the chemicals composing the night-OA factor? The BBOA factor? Did other variables correlate in time with the night-OA factor? What other chemicals make up the aged BBOA factor? It sounds like it is a misnomer if nitrocatechol is the only “aged” compound. I don’t even think it is aged by that much. It is only 2 reactions away from the primary biomass burning compound catechol. Granted, you do discuss this misnomer in line 163 but continue to use this label throughout the paper. Perhaps oBBOA (oxidized BBOA) would be better. I also understand that at line 212, you decide to keep the labels for consistency with previous work. I would argue that the names should change since you know they are not completely appropriate.

Line 168: How does it make sense that the evening peak of aBBOA correlating with the aBBOA noon peak in the next day means it could be emitted from biomass burning? Or did you mean the BBOA noon peak? The wording is unclear.

Line 176: Please further explain why the hypothesis that aBBOA most likely originated from the gas-phase oxidation of biomass burning emitted VOC precursors is supported by aBBOA loading enhancing hygroscopicity. The connection is not clear in the text.

Figure 1: For the color of the scatter plots, is it the average time of the period of observation (e.g., 12 AM), or is it the average length of time of night-OA increase cases? The text implies the former, but it is not clear in the units (e.g., Local Time).

Line 182: Please define COA.

Line 205: Are the C_xH_y⁺ ions anticipated to be fragments of oxidized materials? Why?

Line 228: Why is the formula’s exponent AAE_BC,950-880 x R_AAE(λ), when the demoninator of R_AAE is already AAE_BC,950-880? Shouldn’t it be simplified to AAE_BC,λ-880?

Line 252: Please include the correlation coefficient figures in, at the very least, the supplement. However, wouldn’t we expect the relationship between BBOA and σ_BrC to decrease with wavelength increases? BrC is not very absorptive at larger wavelengths.

Line 254: Could you show us a plot of BBOA vs σ_BrC,370 colored in two colors representing daytime and nighttime?

Line 259: “and differ much at different wavelengths.” I do not think the figure demonstrates anything about different wavelengths. Please remove this phrase.

Line 290: How does the multivariate linear regression method translate to different OA factors, such as BBOA and Night-OA? I think this needs more of an explanation.

Line 304: Can you explain why you think Night-OA is evaporated during the daytime?

Line 319: What do you mean that “to be Lagrangian, air after midnight might differ with those before midnight?” This is a confusing sentence and I don’t understand why midnight is such a special time for air. Later on the dividing time for Figure 4a is 10 PM.

Line 351: There is no section 1.2 in your supplement.

Line 367: I don’t think Figure S6 shows that the HOA daytime loss is due to evaporation as it only shows that it’s not dilution.

Line 452: Biomass burning emissions are not the largest sources of primary aerosols. Sea salt emissions are.

Editorial Comments:

Throughout the manuscript: I would take some time to look at sentences that are more than three lines long. These sentences either need to be simplified or broken up into multiple sentences. I found it distracting.

Throughout the manuscript: There are a significant amount of grammar mistakes that need to be addressed. I would suggest having an editor read through the manuscript just for grammar improvements.

Throughout the manuscript: NOx should be NO_x.

Lines 32 and 33: “Our results demonstrate that the formation of Night-OA appeared high dependence on both…” should perhaps be “Our results demonstrate that the formation of Night-OA appeared to have high dependence on both…”

Line 44: “…water abundant pyroconvection cloud” should perhaps be “…water-abundant pyroconvective clouds.”

Line 61: Fires also emit methane, so you can just say “primary organic aerosols and volatile organic compounds.”

Line 72: “which found that the formed SOA” should be “and those investigations found that the formed SOA…”

Line 84: “…using NO₃ as oxidant…” should be “…using NO₃ as the oxidant…” and “…biomass burning emissions related SOA formations…” should be “… SOA formation from biomass burning…”

Line 92: “Nevertheless, field measurements that observed nighttime evolutions of biomass burning plumes… are highly in lack,” should be “Nevertheless, field measurements that observe nighttime evolutions of biomass burning plumes… are highly lacking.”

Line 97: Please break the following into two sentences. “…contributing greatly to light absorption (Lin et al., 2017;Bluvshtein et al., 2017). Based on this study, it was hypothesized…”

Line 101: “…most of previous laboratory…” should be “…most previous laboratory…”

Line 104: “…could not conclude what roles RH was…” should be “…could not conclude what role RH played.”

Line 107: “Therefore, how nighttime NO3 radical chemistry coordinates with aerosol aqueous or  heterogenous reactions under high nighttime RH conditions to affect SOA and BrC formations remains unexplored, which is a substantial knowledge gap in the research field of nighttime chemical transformation of biomass burning emissions and its role in SOA and secondary BrC formations,” should be “Therefore, how nighttime NO3 radical chemistry coordinates with heterogenous reactions under high RH conditions to affect SOA and BrC formations remains unexplored. Compounding this knowledge gap is how biomass burning emissions contribute to SOA and secondary BrC formation.”

Line 125: “…Peral River Delta…” to “Pearl River Delta.”

Line 125: “This site locates at the country side of Heshan county…” should be “…This site is located on the countryside in Heshan County…”

Line 129: Relative humidity was already defined as RH earlier.

Line 139: “More details on the site and set-up of instruments please…” should be “…For more details on the site and set-up of instruments, please…”

Line 143: “Source identification of organic aerosols was performed using the commonly used positive matrix factorization (PMF), two primary OA factors and four secondary OA factors are identified, and the determination of PMF factors are thoroughly discussed in Luo et al. (2022).” should be “Source identification of organic aerosols was performed using the commonly used positive matrix factorization (PMF). Two primary OA factors and four secondary OA factors were identified and determined from PMF as thoroughly discussed in Luo et al. (2022).”

Line 149: “The four SOA factors including more oxygenated…” should be “The four SOA factors include more oxygenated…”

Line 158: “until 1th of November” should be “until the 1^st of November.” Also, “until 18^th of November” should be “until the 18^th of November.”

Line 207: “Similar situation was previously found in study…” should be “A similar situation was previously found in a study…”

Line 211: “In summary, both aBBOA and Night-OA are not likely primary, while the naming of…” should be “In summary, both aBBOA and Night-OA are not likely primary. While the naming of…”

Line 216: “Therefore, the details about discussions of this method please refer to Luo et al. (2002), and we only introduce…” should be “The details of this method can be found in Luo et al. (2022). We only introduce…”

Line 229: “As the sophisticated discussions presented in Luo et al. (2022), variations of many factors… might influences the magnitudes…” should perhaps be “The sophisticated discussions presented in Luo et al. (2022) consider that variations… might influence the magnitudes…”

Line 249: “Timeseries of retrieved σ_BrC,370 is shown…” should be “Timeseries of retrieved σ_BrC,370and BBOA are shown…”

Line 254: “BBOA σ_BrC,370” should be “BBOA and σ_BrC,370”

Line 257: “Increase” should be “increases”

Line 269: “…low absorptivity of LOOA, thus MAE of LOOA is treated as zero.” should be “…low absorptivity of LOOA. Thus MAE of LOOA is treated as zero.”

Line 282: “nm are shown in Fig.S4, it tells that Night-OA…” should be “nm are shown in Fig. S4, demonstrating that Night-OA…”

Line 295: I can’t tell which order these four values are presented. Is it BBOA AAE_370-470, BBOA AAE_470-590, Night-OA AAE_370-470, Night-OA AAE_470-590? Or is it BBOA AAE_370-470, Night-OA AAE_370-470, BBOA AAE_470-590, Night-OA AAE_470-590?

Line 302: “increased during the nighttime, while usually decreased…” should be “increased during the nighttime and usually decreased…”

Line 323: “correlated with BBOA before the night…” should be “correlated with BBOA from the night before…”

Figure 4a: What are the units on the O_X,LT20-06? Please add ppb to the figure, not just the figure description.

Line 346: “diameter range of > 300 nm occurred even all other SOA factors…” should be “diameter range of > 300 nm occurred. Even all other SOA factors…”

Figure S7: What are the units of NO₂ + O₃? However, I don’t think you talked about NO₂ + O₃ in the main manuscript. Perhaps the color should be removed.

Line 379: “radical do not photolyze, NO reacts rapidly…” should be “radical do not photolyze. NO reacts rapidly…”

Line 386: “…this dilution effects…” should be “…this dilution effect…”

In this manuscript, the authors present evidence from field measurements that the nighttime formation of light-absorbing secondary organic aerosol from biomass burning emissions may involve coordinated gas- and aqueous-phase chemistry. This focus on nighttime formation of secondary brown carbon is a real strength of the work. The field measurements are extensive and appear sound, largely validated in previous reports from the same campaign. The results are presented in clear figures, and the paper is well structured at the paragraph and section levels for the most part. I do strongly recommend a revision for structure at the sentence level (grammar, etc.). The scope and implications of the manuscript are suitable for ACP. I have only minor comments for the authors to consider.  

180 - This paragraph includes a lot of rationale, which I usually associate with Results and Discussion, rather than Materials and Methods - should this passage be the first sub-section in the Results and Discussion?

218 - Replace "philosophy" for clarity.

278 - I think the observation that the contribution of Night-OA is briefly higher than that of BBOA is overstated here - in Figure 3b, the points at hour six are almost perfectly overlapping, surely within the experimental uncertainty. Of course, the observation that the peak in the Night-OA contribution coincides with the trough in the BBOA contribution is compelling evidence, regardless of the wording here.

Figure 3 - Can error bars be incorporated into panels a, b, and d, like they are in panel c?

298 - I think more detail here would be helpful, e.g., what specific values are being compared? Also, the change in wavelength dependence upon oxidative aging can be compared to trends reported in the literature, e.g., for laboratory studies of aging.

304 - This statement implies that evaporation is the only loss mechanism for the light absorption, although photodegradation is mentioned later on line 360. I recommend introducing photodegradation here or replacing evaporation with, more generally, decay or loss.

308 - I think a few representative past studies of aqSOA should be cited here.

311 - Rephrase this sentence for clarity.

319 - Capitalize Lagrangian.

338 - Similarly, I think a few representative past studies of gas-particle partitioning should be cited here.

344 - The discussion of these two size ranges could be clarified by adding a vertical dashed line or grey band, etc., at 300 nm in Figure 4b.

Figure 5 - The x-axis in these panels is shifted from that in Figure 3. Reformatting the time scale either here or there may help readers more quickly compare between the two figures.

391 - I recommend including a reaction scheme of this and the other gas-phase reactions discussed throughout this paragraph.

393 - From here to the end of the paragraph, additional references to specific features in Figure 5 would help.