From the journal Environmental Science: Atmospheres Peer review history

12-Jul-2023

Dear Dr Ijaz:

Manuscript ID: EA-ART-06-2023-000085
TITLE: Molecular and Physical Composition of Tar Balls in Wildfire Smoke: An Investigation with Complementary Ionisation Methods and 15-Tesla FT-ICR Mass Spectrometry

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Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by companion journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci

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Reviewer 1

The authors have used two ionization method: LDI(-) and ESI(-), combined with FT-ICR MS to characterize the molecular composition of refractory OA from wildfire smoke. Interestingly, by complementing the ESI analysis, the LDI method presents molecule details that provide a more apt reflection of the physical properties of TB. This work demonstrates the usefulness of LDI-FT-ICR MS in analyzing OA and elucidates individual molecular species in TBs. Meanwhile, there are still some parts that confuse me. For example, why the authors chose the negative ionization model instead of the positive ionization model? In addition, some unnecessary mistakes in the manuscript suggest the manuscript was roughly prepared. Therefore, we recommend this manuscript can be accepted after careful revision.
The detailed comments are listed as below. Because the manuscript PDF file does not provide the line numbers, we specify the place by “subtitle + paragraphs number + sentence”:

1. Table 1. About the sample ID. What do the acronyms “DT” and “NT” mean, respectively. “DT” may be mean daytime, and “NT” may mean nighttime according to the sampling duration. The author should note the full name at the first time of an acronym.

2. “Experimental” section - “sample collection and preparation”: Why were the particulate matters collected by full size but not segments?

3. “Experimental” section - “sample collection and preparation”: How much volume of solvent did the author used for a filter extraction? The author should also provide more information on the dilution of the samples by H2O:ACN.

4. Fig 1. “Red dots indicate fires and thermal anomalies during day and night”. No red dots were found in Fig. 1 A to C, Did the author mean the orange dots here?

5. “Experimental” section - “Laser desorption ionization” - the second paragraph – “Laser desorption ionisation was also applied to aerosol samples extracted in 100% ACN aerosol, dried, and reconstituted in 50:50 H2O:ACN.”.
More details should be given in the volume of ACN for a filter and the volume of H2O:ACN to reconstitute sample. In addition, we notice that the sample preparation process in this section was difference from the process for the ESI method: no filtration was applied after extraction with ACN, which was opposite to the description at “Results and discussion” section – “The molecular composition of TB-rich aerosol relative to non-TB aerosol” – the fourth paragraph – “Samples were prepared for (-)LDI extract in the same way as for ESI,”

6. “Results and discussion” section – “The molecular composition of TB-rich aerosol relative to non-TB aerosol” – the third paragraph – “Amongst the aerosol samples of this study, the average O/C of TB-specific and non-TB species from (-)ESI analysis was 0.54 and 0.81, respectively”. The author explained this by “TBs were somehow prevented from oxidative atmospheric ageing” and “leaving behind E/LVOCs of high viscosity on the surface that may slow further diffusion of oxidants into the interior as proposed by previous studies for high-viscosity aerosol”. While, “On the contrary, average O/C of 0.72 and 0.27 were observed for TB-specific and non-TB species with (-)LDI” showed the high oxygenation of the TB-specific species.
The data (i.e., the average O/C of TB-specific and non-TB species) obtained by the two ionization modes are significantly different. These findings were used separately to explain the atmospheric oxidative aging state of the same aerosol samples, and thus contradictory and in consistent conclusions were obtained.

7. “Results and discussion” section – “The molecular composition of TB-rich aerosol relative to non-TB aerosol” – the fourth paragraph – “(-)LDI may be a more suitable choice for refractory particles like TBs as it can cover a broader spectrum of high to low-oxygenation species”:
How to understand the high to low-oxygenation species? Appropriate definitions or descriptions in the preceding text may be necessary.

8. “Results and discussion” section – “Single-particle micro-spectroscopic analysis of wildfire smoke” – “However, TCA/AED ratios indicated only BBAug1314 to have >90% of its single particles in a solid or semi-solid state”.
What does the acronym “AED” mean? “AED” seems to means “area eq. diameter” according to Figure S4. In addition, how did the index - TCA/AED ratios indicate the state of a single particle?

9. “Experimental” section - “Data processing, formula assignment, and estimations of physicochemical parameters”:
Author translated the molecular formula to physical parameters (C0, Tg,dry and Tg,RH, phase state) by referencing methods. We suggest that the parameters should be given in Table S1. So that we can access them directly, instead of accessing them by checking the references.

10. “Results and discussion” section – “Functional group analysis” – “To gain insight into the distribution of COO and other functional units from high resolution mass spectrometric data, the frequency of all possible differences in measured masses of ions (∆m) were calculated in the mass spectra”.
Since both ESI and LDI are soft ionization techniques, little to no fragmentation would occur during the ionization process. Then, the ∆m from the mass spectra obtained by ESI or LDI would not provide functional information of a molecule, but only the mass difference between two individual molecules. Therefore, it makes no sense to connect the ∆m with characterizations for functional groups.

Reviewer 2

I find this to be a very well organized and thought out paper that assesses the chemical differences between TB aerosols from wildfires and non-TB aerosols. It utilizes standard negative ESI approaches and LDI (-) approaches to make the differentiations. The conclusions reached are supported with other techniques as well as physical microscopic data. There are two issues that need more explanation in my mind because they jumped out at me right away. The first is the large predominance of molecules with high H/C ratios. I would have expected that being derived from burning, the molecules would have a significant and overwhelming distribution of condensed aromatic materials like one observed from wood biochars. At the very least, the authors should explain this. The second point deals with the higher O/C ratios from LDI vs ESI for TB. The authors attempt to explain this as a spatial fractionation within TB but it may simply reflect the solubility of high O/C molecules in acetonitrile and spray dynamics used in ESI while LDI does not depend upon solubilization or spraying.
The figures were all well done and explained and the paper was very logically expressed.

Referee: 1

The authors have used two ionization method: LDI(-) and ESI(-), combined with FT-ICR MS to characterize the molecular composition of refractory OA from wildfire smoke. Interestingly, by complementing the ESI analysis, the LDI method presents molecule details that provide a more apt reflection of the physical properties of TB. This work demonstrates the usefulness of LDI-FT-ICR MS in analyzing OA and elucidates individual molecular species in TBs. Meanwhile, there are still some parts that confuse me. For example, why the authors chose the negative ionization model instead of the positive ionization model? In addition, some unnecessary mistakes in the manuscript suggest the manuscript was roughly prepared. Therefore, we recommend this manuscript can be accepted after careful revision.

The detailed comments are listed as below. Because the manuscript PDF file does not provide the line numbers, we specify the place by “subtitle + paragraphs number + sentence”:

Author’s response:

We thank the reviewer for their careful assessment of our manuscript. They highlight several key aspects requiring improvement, which we have addressed below. We are sorry for the comprised readability due to the lack of line numbers. We have added line numbers to the revised manuscript and hope that it is now more accessible and easier to read.

We understand the reviewer’s confusion regarding the choice of a single ionisation model, i.e., negative ionisation mode. Indeed, the results presented herein are not universally representative. The proof-of-evidence nature of this study to assess the potential of LDI on tar ball-rich aerosol necessitated using the most suitable and relevant mode of ionisation. Negative-mode ionisation is sensitive to molecules with acidic functional groups, and thus, it is ideal for the analysis of processed organic aerosol, which is generally acidic in nature. In addition, the spectra obtained from negative-mode ionisation tend to be less complicated than positive-mode, as the latter is often more prone to adduct formation (especially (+)ESI).

We agree with the reviewer that the readers can benefit from a justification for this choice. Therefore, the following text has been added to the revised manuscript:

Lines 326-332, page 05, cyan highlight – “Negative-mode ionisation was chosen in this study as it is sensitive to molecules with polar acidic functional groups, rendering it ideal for the analysis of processed organic aerosol, which is generally acidic in nature20, 47. It also offers simplicity of interpretation as deprotonation is the primary ionisation mechanism, rather than the more variable adduct formation48.”

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1. Table 1. About the sample ID. What do the acronyms “DT” and “NT” mean, respectively. “DT” may be mean daytime, and “NT” may mean nighttime according to the sampling duration. The author should note the full name at the first time of an acronym.

Author’s response:

All sample acronyms were summarised in Table 1, which was cited at the beginning of the methodology (line 124, page 02). We agree with the reviewer that DT and NT were not defined. This oversight has been corrected by adding a footnote to Table 1 as follows:

Line 125, page 02, green highlight – “BB, biomass burning; DT, daytime; NT, night-time; PST, Pacific Standard Time; TB, tar balls”

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2. “Experimental” section - “sample collection and preparation”: Why were the particulate matters collected by full size but not segments?

Author’s response:

The specific aim of this study was to explore the overall composition of aerosol rich in tar balls, which exist in a range of sizes <500 nm (100–300 nm, on average); size-resolved sampling was not conducted to collect tar balls across all possible sizes. Indeed, the next logical step would be to test the potential of LDI to delineate the composition of size-resolved TB-rich organic aerosol. We believe that this attempt was beyond the aims and scope of the current study, therefore, its pursuit is best warranted as part of future work. We have highlighted this possibility by modifying the Conclusion as follows:

Lines 588–592, page 09, teal highlight – “Considering the excellent performance of (-)LDI in delineating the composition of TBs compared to non-TB aerosol in this study, its potential to examine other aerosol, such as fresh emissions or size-resolved fractions of SOA, merits evaluation in the future.”

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3. “Experimental” section - “sample collection and preparation”: How much volume of solvent did the author used for a filter extraction? The author should also provide more information on the dilution of the samples by H2O:ACN.

Author’s response:

We appreciate that has reviewer has drawn our attention to this oversight. We have now specified the necessary information in the revised manuscript as follows:

Lines 126–128, page 02, fuchsia highlight – “A quarter of each filter was immersed in 30 mL of LC-MS-grade acetonitrile (ACN; 100%) and shaken at 60 rpm for 15 minutes.”

Lines 131–133, page 02, fuchsia highlight – “All samples were diluted as required in 50:50 H2O:ACN immediately before analysis until a stable and optimised signal could be obtained.”

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4. Fig 1. “Red dots indicate fires and thermal anomalies during day and night”. No red dots were found in Fig. 1 A to C, Did the author mean the orange dots here?

Author’s response:

Yes, we meant the orange dots. The necessary correction has been made as follows:

Line 178, page 03, red highlight – “Orange dots indicate fires and thermal anomalies during day and night at 375 m captured by Aqua / MODIS, Terra / MODIS, and Suomi NPP / VIIRS.”

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5. “Experimental” section - “Laser desorption ionization” - the second paragraph – “Laser desorption ionisation was also applied to aerosol samples extracted in 100% ACN aerosol, dried, and reconstituted in 50:50 H2O:ACN.”.
More details should be given in the volume of ACN for a filter and the volume of H2O:ACN to reconstitute sample. In addition, we notice that the sample preparation process in this section was difference from the process for the ESI method: no filtration was applied after extraction with ACN, which was opposite to the description at “Results and discussion” section – “The molecular composition of TB-rich aerosol relative to non-TB aerosol” – the fourth paragraph – “Samples were prepared for (-)LDI extract in the same way as for ESI,”

Author’s response:

We agree with the reviewer that there appears to be a discrepancy here. Basically, the (-)LDI extract experiments were conducted on the extracts that were prepared previously for (-)ESI analyses. We agree that this does not come across clearly in the original text. Therefore, we have re-phrased the relevant section as follows to clarify this point.

Lines 168–170, page 03, yellow highlight – “Laser desorption ionisation was also applied to aerosol samples prepared for ESI analyses in LC-MS-grade ACN, after drying and reconstituting them in 50:50 H2O:ACN.”

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6. “Results and discussion” section – “The molecular composition of TB-rich aerosol relative to non-TB aerosol” – the third paragraph – “Amongst the aerosol samples of this study, the average O/C of TB-specific and non-TB species from (-)ESI analysis was 0.54 and 0.81, respectively”. The author explained this by “TBs were somehow prevented from oxidative atmospheric ageing” and “leaving behind E/LVOCs of high viscosity on the surface that may slow further diffusion of oxidants into the interior as proposed by previous studies for high-viscosity aerosol”. While, “On the contrary, average O/C of 0.72 and 0.27 were observed for TB-specific and non-TB species with (-)LDI” showed the high oxygenation of the TB-specific species.
The data (i.e., the average O/C of TB-specific and non-TB species) obtained by the two ionization modes are significantly different. These findings were used separately to explain the atmospheric oxidative aging state of the same aerosol samples, and thus contradictory and in consistent conclusions were obtained.

Author’s response:

We appreciate that the reviewer shares an understanding of the enigma we faced here! We agree that the results presented herein from (-)ESI and (-)LDI provide not just “significantly different” observations – as the reviewer has pointed out – but provide almost opposing information on the composition and estimated physicochemical traits of TB and non-TB aerosol samples from the two ionisation methods. We agree with the reviewer that explaining them separately would be contradictory and rather misleading. In the original manuscript, we strived to thoroughly evaluate the accuracy of these differences by comparison with non-mass spectrometric techniques to see which ionisation method may be obtaining a more representative composition of TBs. Additionally, we attempted to reconcile and/or hypothesise the possible reasons for these different observations from (-)ESI and (-)LDI as much as possible. To ensure that this came across to the readers properly, the following additions and changes have been made to the text.

Lines 423–469, pages 6–7, teal highlight – “On the contrary, average O/C of 0.72 and 0.27 were observed for TB-specific and non-TB species with (-)LDI (Table 3), where the very high oxygenation of TB is indicative of their aged nature as compared to non-TB aerosol. It is critical to understand whether these contrasting observations from (-)ESI and (-)LDI originate from their ionisation selectivity or are an artefact of the unique morphology and physicochemical traits of TBs. Most importantly, it must be assessed which ionisation method could obtain a more representative composition of TB.”

Line 481–489, page 7, purple highlight – “Samples were prepared for (-)LDIextract in the same way as for ESI, but it revealed trends in TB versus non-TB differences that were similar to those seen with (-)LDI; this may be indicative of sample preparation (for instance, extraction/solubilisation in ACN or any other suitable solvent) for (-)ESI analysis being a minor factor in determining outcome observed here. Rather, it is more likely the unique morphology and physicochemical nature of TBs renders (-)LDI to be better suited to measure their composition.”

Additionally, Figure 5 (along with Figures S6–S9) and the discussions on them in the text aim to compare the contrasting compositions obtained from (-)ESI and (-)LDI and present possible explanations. While they are unexpected, we are confident in our observations due to the comprehensive (e.g., we also test LDI on extracts and use multiple non-mass spectrometric techniques to evaluate the qualitative mass spectrometric findings) and replicative nature (e.g., the spectra for each sample were averaged from triplicates or duplicates for all ionisation methods) of this investigation.

Tar balls are a unique form of aerosol, and thus, it is important to report these discrepancies in their measured composition from the two different ionisation methods. We believe that our findings provide great impetus to the atmospheric chemistry community for detailed investigations into why these significant differences occurred with the two ionisation methods, whether such observations could be limited to refractory fractions of aerosol or non-refractory fractions as well and whether greater care is required in choosing ionisation methods based on the predominant fractions in aerosol.

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7. “Results and discussion” section – “The molecular composition of TB-rich aerosol relative to non-TB aerosol” – the fourth paragraph – “(-)LDI may be a more suitable choice for refractory particles like TBs as it can cover a broader spectrum of high to low-oxygenation species”:
How to understand the high to low-oxygenation species? Appropriate definitions or descriptions in the preceding text may be necessary.

Author’s response:

We agree with the author that the readers could benefit from a better description here. We have now rephrased the sentence as follows:

Lines 493–469, page 07, green highlight – “We speculate that although neither ionisation method might be fully representative of the molecular composition of TBs, the distribution of ion abundances of molecular species shown in Figures S6-7 suggest that (-)LDI may cover a comparatively broader spectrum of high (e.g., #O > 7) to low-oxygenation species (e.g., CH and CHN species with #O = 0), especially those with extremely low volatilities, as compared to (-)ESI, making it a more suitable choice for refractory particles, such as TBs that may have a unique spatial fractionation of aged and non-aged molecular species.10, 46”

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8. “Results and discussion” section – “Single-particle micro-spectroscopic analysis of wildfire smoke” – “However, TCA/AED ratios indicated only BBAug1314 to have >90% of its single particles in a solid or semi-solid state”.
What does the acronym “AED” mean? “AED” seems to means “area eq. diameter” according to Figure S4. In addition, how did the index - TCA/AED ratios indicate the state of a single particle?

Author’s response:

We agree that this aspect could benefit from further elaboration in the main text. We have updated the text as follows:

Lines 256-261, page 04, golden highlight – “Total carbon absorbance (TCA) was calculated from STXM/NEXAFS data using the optical density.42 TCA is proportional to the particle thickness, and thus, when modelled as a function of particle size, i.e., the area equivalent diameter (AED), it can indicate phase state as described previously.42-44”

Lines 311-315, page 05, golden highlight – “BBAug1314 featured a greater frequency of particles with higher TCA values but the same AED as in other samples indicating >90% of its single particles exist in a solid or semi-solid state, while the majority of single particles in the remaining samples were liquid or semi-solids.”


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9. “Experimental” section - “Data processing, formula assignment, and estimations of physicochemical parameters”:
Author translated the molecular formula to physical parameters (C0, Tg,dry and Tg,RH, phase state) by referencing methods. We suggest that the parameters should be given in Table S1. So that we can access them directly, instead of accessing them by checking the references.

Author’s response:

The point is well taken. We have now included these parameters with the appropriate citations as table insets in Table S1 as required.

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10. “Results and discussion” section – “Functional group analysis” – “To gain insight into the distribution of COO and other functional units from high resolution mass spectrometric data, the frequency of all possible differences in measured masses of ions (∆m) were calculated in the mass spectra”.
Since both ESI and LDI are soft ionization techniques, little to no fragmentation would occur during the ionization process. Then, the ∆m from the mass spectra obtained by ESI or LDI would not provide functional information of a molecule, but only the mass difference between two individual molecules. Therefore, it makes no sense to connect the ∆m with characterizations for functional groups.

Author’s response:

We agree with the reviewer that decisive information on the functional groups present cannot be derived from the experimental design used here. We justify using the ∆m approach considering that in highly complex mixtures, such as the aerosol samples here, molecular species are related to one another with commonly repeating units, such as CH2, H2, or O, etc. Calculating all possible ∆m helps identify other such repeating patterns. Although these may not be unambiguously indicative of the functional groups present, they do provide a good estimation. In line with this, we have remained wary of relying solely on the ∆m approach and used other techniques to confirm that ∆m indeed corresponded to the functional groups measured with other techniques.

We agree with the reviewer that better justification is required in the manuscript for using this approach. Therefore, we have clarified our intentions by making the following additions to the main text.

Lines 568–573, page 08, gray highlight – “Considering that molecular species are related to one another with commonly repeating units, such as CH2, H2, or O, in complex mixtures35 like the aerosol samples studied here, predominant ∆m values help delineate other such repeating units, which in turn, provide insight into the possible functional groups, albeit not decisively.”

Lines 561–566, page 08, gray highlight – “We focused on them by Kendrick mass defect (KMD) analysis with C6H5O, C7H6O, and C6H5NO3 as base units; this approach makes it possible to recognise a series of compounds belonging to the class or type with the corresponding base unit, but different extents of base unit addition.58”

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Referee: 2

Comments to the Author

I find this to be a very well organized and thought out paper that assesses the chemical differences between TB aerosols from wildfires and non-TB aerosols. It utilizes standard negative ESI approaches and LDI (-) approaches to make the differentiations. The conclusions reached are supported with other techniques as well as physical microscopic data. There are two issues that need more explanation in my mind because they jumped out at me right away.

Author’s response:

We thank the reviewer for their positive assessment of our manuscript. They additionally highlight several key areas for improvement and improved clarity of communication, which we have addressed below.

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1- The first is the large predominance of molecules with high H/C ratios. I would have expected that being derived from burning, the molecules would have a significant and overwhelming distribution of condensed aromatic materials like one observed from wood biochars. At the very least, the authors should explain this.

Author’s response:

We appreciate the reviewer for bringing up this point, which was not adequately discussed in the manuscript.
We agree with the reviewer that the H/C ratios are indeed very high with both (-)LDI and (-)ESI. We reassert that the results presented herein are for ambient samples that were only majorly influenced and did not originate solely from wildfire smoke. In addition, these H/C ratios may be a characteristic of TB-rich samples as well; similar values were reported for TB-rich aerosol in a previous study by Brege et al. (2021) in ACS Earth Space Chem. We agree that these must be discussed better, therefore, the following additions were made to the main text:

Lines 408–414, page 06, yellow highlight – “These molecules presented high average H/C of >1.0 with both ionisation methods, which remained in a narrow range (1.16–1.25 with (-)ESI and 1.01–1.06 with (-)LDI) for all samples as summarised in Tables 2-3 and S2-3; these values are consistent with H/C reported for long-range transported or aged BBOA.13, 20, 21”

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2- The second point deals with the higher O/C ratios from LDI vs ESI for TB. The authors attempt to explain this as a spatial fractionation within TB but it may simply reflect the solubility of high O/C molecules in acetonitrile and spray dynamics used in ESI while LDI does not depend upon solubilization or spraying.

Author’s response:

We agree with the reviewer that this is a crucial aspect of this study, and it could most certainly benefit from further elaboration in the main text.

Lines 481-489, page 07, purple highlight – “Samples were prepared for (-)LDIextract in the same way as for ESI, but it revealed trends in TB versus non-TB differences that were similar to those seen with (-)LDI; this may be indicative of sample preparation (for instance, extraction/solubilisation in ACN or any other suitable solvent) for (-)ESI analysis being a minor factor in determining outcome observed here. Rather, it is more likely the unique morphology and physicochemical nature of TBs renders (-)LDI to be better suited to measure their composition.”

The figures were all well done and explained and the paper was very logically expressed.

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07-Sep-2023

Dear Dr Ijaz:

Manuscript ID: EA-ART-06-2023-000085.R1
TITLE: Molecular and Physical Composition of Tar Balls in Wildfire Smoke: An Investigation with Complementary Ionisation Methods and 15-Tesla FT-ICR Mass Spectrometry

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Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by companion journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci

Reviewer 1

In this revised manuscript, the authors provide detailed responses to our comments. The necessary information on the details of experimental procedures have been specified in the revised text. Some errors in the original manuscript (i.e., incorrect color markings in the original Figure 1 and forgotten definition of abbreviations in the text) also have been carefully revised.

A significant improvement in this revised manuscript is that the authors have now clearly discussed the difference results between the two observations by (-)ESI and (-)LDI, which provide almost opposing information on the composition and estimated physicochemical traits of TB and non-TB aerosol samples. It is deemed important to report these discrepancies to readers, because it reminds readers to take a cautious view of the findings from a sole technique (even the commonly used ESI method). The authors reconcile these different observations and give some possible reasons, such as ionization selectivity and artefact of the unique morphology and physicochemical traits of TBs. I also agree with the conclusion that the (-)LDI is better suited to characterize the OA composition in TBs as this method directly present the surface chemistry, which is the most effective fraction in aerosol particles of impacting on climate and health.

In summary, the revised version is fully well-organized and unambiguous. Therefore, I recommend acceptance of this manuscript without additional revision.