General Comments:

This study investigated the spatial variations of the concentration of different types of organosulfates in cities in southern and northern China. The manuscript further analyzed the factors leading to significant regional differences in organosulfate concentration and composition between the southern and northern cities. Based on principal component analysis, correlation analysis, organosulfate formation mechanism analysis, and field simulation combustion experiments, the authors concluded that the emissions from biomass burning, rather than those from gasoline, diesel, and coal combustion, can play a significant role in the formation of isoprene-derived organosulfates. This is a significant and valuable finding. It thus follows that when attempting to predict winter isoprene emissions in China, it is essential that the influence of biomass combustion emissions is duly taken into account, with due consideration given to the regional differences in question. Overall, the manuscript presents high quality research data based on field observations and simulation experiments. The content presented here will be of interest to the readership of Atmospheric Chemistry and Physics. After the authors address the following minor comments, this manuscript would be suitable for publication.

Specific comments:

• Lines 142-143: I suggest that the authors add specific information about the columns used here or in the supplemental file. It is important to consider this when comparing the analysis method from this study with those from previous or future research.
• Lines 157-158: Why did the authors choose 111 of the OSs for further quantification? I have checked your previously published paper showing quantification of 106 OS species. Is there any difference here? Please clarify.
• Lines 172-174: How is this insignificance determined or evaluated?
• Line 196: Please remove any possible extra spaces between 'sulfate' and 'atmospheric oxidation capacity'.
• Line 217: Please change 'an' to 'the'.
• Lines 224-225: Was the correlation analysis presented in Figure S3 using all the data from this study? In addition, I suggest adding 'further' before 'given'.
• Line 248: Please remove any possible extra spaces before 'considering'.
• Line 261: Please change ' precisely' to 'further'.
• Line 269: I suggest the author delete 'fully' or replace it with a more appropriate word.
• I know that those isoprene-derived organosulfates were detected in the samples from the simulated combustion experiments, however, I would like to know how you think about their formation. This could be a critical question for future research in this area, although it does not affect the results of this study.
• I suggest the authors add a space after Ali. in Figure S3. Furthermore, how was the relative intensity calculated in Figure S5? Please clarify it.

General comments

The present study employed a systematic approach to characterize organosulfates in PM_2.5 samples from both southern and northern Chinese cities during the winter months, complemented by the analysis of smoke particle samples obtained from simulated combustion experiments. The analysis indicated that biomass burning, in contradistinction to the combustion of gasoline, diesel, and coal, exerted a significant influence on the increased levels of particulate isoprene-derived organosulfates observed in northern urban areas. The authors pointed out that stronger biomass and fossil fuel combustion activities in the northern cities resulted in the formation of a greater number of anthropogenic organosulfates. In general, this work offers a compelling example and novel insights into understanding organosulfate pollution in Chinese cities.  The manuscript is well organized and its topic is very interesting. Thus, I think it can be accepted after a minor revision.  

Detailed comments:

1. Lines 72-74: It is recommended that the author rephrase the sentence to enhance clarity. For example: This complicates our understanding of how aerosol OS pollution is formed and what limits it in a complex polluted atmosphere across different cities in China.
2. Line 102-107: it seems to me that about ten samples were collected in each city, so at least 40 samples were studied by this study, why here stated " a total of four PM2.5 samples were collected and stored at a.....? please clarify. 
3. Line 146: Please change ‘a optimized solution’ to ‘an optimized solution’.
4. Line149-150: the authors mentioned that the two references (Brüggemann et al. 2020a; Kristensen et al. 2016) emphasize the impact of the sampling process on the quantitative results of OSs. However, Lines 156-157: the authors also mentioned that the possible consequences of sampling without denuding SO₂ for the quantification of OSs were not taken into account in our studies (Brüggemann et al. 2020a; Kristensen et al. 2016). It is strange to quote the same reference in sentences with different meanings. Please clarify.
5. Line 158: What are the surrogate standards? How were they obtained? What are the recoveries of the surrogated standards?
6. Line 159-164，here I suggest to give a brief description on how OSm and OSi were defined, which is helpful for readers to quickly understand what the author have done in this study, although more details can be found in SI．
7. Line 167: Please remove any extra spaces between K+ and Mg2+.
8. Line 172-174: here should give a reference to support this conclusion.
9. Line 233: Does the spatial variation of OS_i concentration have temperature dependence?
10. Line 239: For the statement ‘indicator (CL× CT) of biogenic VOC emission rate’, please add some references to support it.
11. Line 281: what are the "N-base compounds" ? please give more explanation.
12. Section 3.3: Are the OSi species detected in smoke particles directly emitted or are they produced secondarily?
13. Figure 5: Based on my understanding, OSi-BB can not only come from biomass combustion, but also from atmospheric transformation of isoprene derived from biological sources. Therefore, although these OS species were indeed detected in the particulate matter released from biomass burning, in order to avoid misleading readers into thinking that these OS species were all from biomass burning release, I recommend the author to add relevant explanations in the caption.

The primary objective of this study is to elucidate the role of non-biogenic emissions (e.g., rice straw, pine branch, gasoline, diesel, and coal combustion) in the formation of isoprene-derived organosulfates in aerosols in China during winter. The authors synthesized data from large-scale observational studies (comparing northern and southern Chinese cities) with data from simulated combustion experiments. They demonstrate that biomass burning emissions are a significant contributor to aerosol organosulfates in northern cities, rather than fossil fuel combustion emissions. The overall results provide valuable insights into the formation of aerosol organosulfates associated with biomass burning, making this a noteworthy and meaningful finding. Generally, the manuscript is well-structured and presents a robust experimental approach with clear results. I recommend that this paper be published in Atmospheric Chemistry and Physics once the authors address the following comments.

Major Comments:

The identification of over 100 organosulfates is impressive. While the use of surrogate standards is not ideal, it is currently the best solution in the absence of authentic standards. Therefore, please provide a detailed explanation of the criteria used for selecting surrogate standards for quantifying organosulfate species in this section or supplementary information. Additionally, the recoveries of individual organosulfate surrogate standards should be included in the manuscript or supplementary information, as this is crucial for ensuring data quality.

Additional Comments:

Keywords: I recommend adding "biomass burning" to the list of keywords.

Line 140: The rationale for centrifuging is unclear, especially since a syringe filter was used earlier. It would be helpful to clarify whether the centrifuge was employed later due to solid precipitate formation in the extracts after adding water.

Table S5 and Figure S5: Please describe the methodology used to calculate the relative intensity.

Line 392: In the caption of Figure S6, please clarify what panels a and b represent. Also, indicate which data were used for the correlation analysis in Figure S8.

It would be beneficial to determine whether the authors can provide a quantitative estimation of the contribution of biomass combustion emissions to aerosol isoprene-derived organosulfates, even though this may be a challenging task.

Technical Corrections:

Line 146: Change "an optimized solution" to "the optimized solution."

Lines 149 and 153: Please correct the two spelling errors present.

Line 364: Amend "… be also supported…" to "…also be supported…" S2. Quantification of OSs.

Line 116: Ensure the correct citation format: (Hettiya… Ding et al. 2022a).

Figures S3 and S5: I suggest removing the hyphen in "Ali.-OSs" and "Aro.-OSs."