This study provides many valuable information on the molecular-level PM2.5 components during different levels of haze events with high-time resolution. It analyzed a wide range ofindividual components of PM2.5, which allows a detailed study of various sources at the same time. The radio carbon measurements suggest a greater contribution from fossil fuels to WSOC, while the contribution of non-fossil fuels increased with increasing haze pollution, coinciding with elevated biomass burning (BB) during that time. This new finding highlight BB may be an important driver for heavy haze formation, despite great contributions of fossil fuel sources. This manuscript presents many interesting results that will deepen our understanding of haze evolution. This work is worth being published in the journal of ACP.



Here are some minor comments below:



In Figure 4, I noticed that sugars and sugar alcohols have high concentrations during the last two periods, with levels relatively higher than those of anhydrosugars. Could you explain why this is the case?



This study conducted 14C analysis on WSOC. Why did the authors choose WSOC over other PM2.5 components?



The sampling period is from December 31 to January 2. Why was this specific period selected? Does it overlap with the Spring Festival or any holidays?

Publisher’s note: this comment was edited on 12 September 2024. The following text is not identical to the original comment, but the adjustments were minor without effect on the scientific meaning.

This publication aims to determine the molecular- level of PM_2.5 components and source contributions of PM_2.5 during hazy days in winter at Nanjing University of Information Science and Technology. The study has significance for assessing the air quality during the research period. Moreover, before to accepting this publication in the ACP Journal, we need to tackle significant editing concerns. Pay attention to the accompanying comments for more information.

1. Line 128: Before the samples, the authors baked the quartz filter. What is the temperature for 5 hours of baking? Is there a standard procedure for this temperature and baking duration? 
2. Following from the previous question, do you have any standard for controlling the weight of the filter?
3. Line 130 – 131: The authors mentioned collected the filed blank. Are you able to provide the chemical data in the blank field? If yes, you used the results of the field blank to calculate.
4. Line 132 – 133: Please explain the reason for divided into three episodes.
5. Following from the previous question, do you have any standard for controlling the weight of the filter?
6. Line 178 – 182: In Table 1, the authors discovered a high concentration of NO₃^-. However, you reported that NO₃^- was the second dominant species. What is the primary species from your studies? What is the source of NO₃^- from ambient air? Please add more references.
7. Line 199: Please explain the nss-SO₄^2- and add the references in this point “suggesting the may share similar formation pathways”.
8. Line 205: What are the three SIA components.
9. Line 298 – 299: Please explain the reason for this phenomenon in this section.
10. All figures are not clear, please redraw and change the alphabet as the same in the texts.
11. I recommend that the conclusions be evaluated and revised instead of duplicating the content from the results and statements section.

This study explored the molecular levels of inorganic and organic components in wintertime PM2.5 using intensive sampling and a range of techniques. It also assessed the contributions of primary and secondary sources with tracer-based methods. The finding that BB may significantly influence haze formation is notable, especially compared to the fossil-dominated conditions typically observed in winter. In this regard, the biomass-burning contribution to haze can more clearly be identified compared to some recent studies conducted in India, where BB is consistently prominent, particularly during haze events. Overall, the whole manuscript is well organized. I recommend the manuscript for publication on ACP after the following comments have been well addressed.

Major comments:

In lines 75-77: The authors categorized biomass burning as an anthropogenic source. While biomass burning refers to the combustion of organic materials like wood, crop residues, and other plant matter, which releases VOCs into the atmosphere. In this context, these VOCs are biogenic. Could you explain this?

In lines 136-145: Was the extraction procedure and measurement process for sugar compounds the same as for ions when using ion chromatography?

In lines 161-162: Please rewrite this sentence.

In lines 228-229: High OC/EC ratios larger than about 2.0 mean high SOC formation, why? The authors should rephrase this sentence to better reflect this relationship and provide supporting evidence.

In lines 243-245: Is the high average WISOC/OC ratio observed during the last two periods related to changes in weather conditions, such as wind speed?

In lines 250-252: This study used radiocarbon measurements to quantify the contributions of fossil and non-fossil sources to WSOC. I am curious why only WSOC was chosen for this analysis instead of other PM2.5 components?

In lines 259-261: A high contribution from fossil fuel sources is also likely associated with low temperatures during cold times. Rising coal combustion for cooking and heating may be a result of cold weather. Hence, I suggest that the authors refine this discussion to address the potential link between cold weather and elevated fossil fuel contributions.

In lines 275-279: This sentence is too long. Please rewrite it to make it more readable.

In lines 303-305: Change it to “…surface soil and fertilizers containing abundant potassium…”.

In lines 326: Syringic acid should be the most abundant among lignin and resin acids during heavy haze. Modify this sentence.

In lines 346-347: Remove “beneficial meteorological parameters” as it is improper to use “beneficial” in this context.

In lines 353: Enhanced biogenic emissions might be also attributable to increased wind speeds.

In lines 408-409: Anthropogenic emissions may be more appropriate here.

In lines 417-419: How does biomass burning promote the formation of biogenic SOA tracers? Is it through increasing radical concentration?

In lines 419-423: The whole sentence is too long. Besides, low levels of pinic acid during the first episode may be due to high relative humidity as well.

In lines 460-463: This is a bit confusing. The authors need to provide more evidences or references for support this conclusion. The pronounced correlations might indicate that they share similar sources, formation mechanisms, or atmospheric processing pathways.

In lines 497-498: Changing “primary vehicle exhaust emissions” to “primary emissions” may be more reasonable. The result also suggests that they are probably mainly produced in the atmosphere by the photochemical oxidation of various organic precursors.

In line 500: The authors can cite some references here. For example, the papers by Kawamura et al., 2016, Atmospheric Research. DOI: 10.1016/j.atmosres.2015.11.018

In lines 503-504: Change “secondary sources (e.g., PAHs and biogenic VOCs including …)” to “secondary sources (e.g., oxidation reactions of PAHs and biogenic VOCs …”.

In lines 510-513: The authors can find more references to support BB’s significant impact on fine particle formation. Some reports found that species released by BB can enhance radical concentration and atmospheric oxidation capacity.

In line 525: Replace “produced by …” with “from”.

In lines 542-543: This may also indicate fossil fuel sources make a dominate contribution to SOC formation during urban haze events in winter.

In lines 544-546: A large amount of fossil fuel combustion during cold periods could also contribute.

In lines 553-555: Rewrite.

This manuscript reports on the molecular-level characterization of primary and secondary constituents in PM2.5 at high-time resolution in Nanjing City, China. Biomass burning (BB) was found to be the most significant contributor to organic carbon (OC). Results are supported by the presented data, and the findings are well contextualized in light of other current source tracking studies. The results are timely and will be of great interest to the readership of Atmospheric Chemistry and Physics. 

Here are some minor suggestions

1. Table 1 is too large. Is it possible to move it into the supplementary materials? Maybe the authors could use 1-3 boxplot figures to replace Table 1?  
2. Section 2.1: Is there a reason to choose the sampling period from 12/31/2017 to 1/2/2018. This period was the new year holiday but fireworks use was forbidden. Will that impact the conclusion of this paper?
3. Nanjing is a megacity in China and the major energy sources are hydropower and power plants. Where did the biomass burning come from? Is it possible to generate a figure, which includes HYSPLIT backward trajectories + FINN fire points? 

The report studied molecular composition and source contributions of PM2.5 during winter hazy days. The ¹⁴C measurements of WSOC and further characterization of organic matters reveal that biomass burning is an important driver for the haze formation, in contrast to the fossil-dominated “normal” situation in Nanjing in winter. The findings are interesting to the atmospheric chemistry community. The manuscript is generally well-written and presented with solid evidence. It has both values regarding molecular-level characterization and source tracking. I would like to recommend accepting this manuscript for publication in ACP after addressing the following comments.

Major:

1. The title can be changed to “Significant role of biomass burning in heavy haze

 formation in Nanjing, a megacity in China: Molecular-level insights from intensive PM2.5 sampling on winter hazy days”.

1. The main contribution of biomass burning is likely related to significant chlorine emissions, which was reported by Chang et al. 2024 (National Science Review, nwae285, https://doi.org/10.1093/nsr/nwae285) to affect the long-term atmospheric chemistry and air quality in Asia. The authors can compare this work with the study by Chang et al. to support these findings in the manuscript.

Minor:

1. The pie charts in Figure 1 are a little blurry.
2. Keep the font size in the figure legend consistent (e.g., Fig 4)