the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Mar 2025
New water-soluble, toxic tracers of wood burning identified in fine brown carbon aerosol using a non-target approach
Abstract. The molecular composition of water-soluble fine (PM3) brown carbon aerosol (BrCaq) generated by combustion of wood was studied with ultra-performance liquid chromatography coupled with electrospray ionization time-of-flight mass spectrometry (UPLC-ESI-ToF/MS) using a non-target analysis (NTA) workflow. The NTA analysis workflow based on MS-DIAL and MS-FINDER showed the best performance of the five software tested. Structures of 361 out of the 420 water-soluble organics in BrC were tentatively identified for the first time. The total emission of fine, water-soluble BrCaq was approx. 1 g per kg of wood burned, comparable with the emission factors of some semi-volatile organics from open biomass burning. Potential precursors of aqueous secondary organic aerosols (aqSOAs) and toxic molecules were selected among the newly identified molecules.
The newly identified harmful tracers of fine BrC included plant and wood care products, alkaloids, and fungal metabolites. Fungal metabolites were also identified among the potential precursors of aqSOAs with high Henry's law constants values, alongside natural compounds occurring in roots and leaves, diterpenoids, flavonoids, anthraquinones, and coumarins. The release of these natural and man-made compounds is possible during wildfires and domestic uses of biomass. The atmospheric lifetimes calculated for the newly identified precursors of aqSOAs showed that natural dyes, bacterial and fungal metabolites, and (aromatic) glucosides can undergo aqueous OH oxidation in cloud water. Such molecules can produce low-volatility products without decomposing due to their large carbon backbones. Many new potential chromophores were also identified in BrC, including natural dyes and molecules with conjugated double bonds and aromatic rings.
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RC1: 'Comment on egusphere-2025-1251', Anonymous Referee #1, 15 May 2025
General Comments:
This manuscript presents a thorough comparison of multiple NTA workflows for analysis of water-soluble brown carbon aerosol samples. This is a much-needed contribution to the field, and additionally, the best workflow was used to identify new potential precursors to aqueous SOA. My scientific comments are minor, and I have a few comments to address clarity and technical issues with the manuscript. Overall, this manuscript is worthy of publication in ACP.
Specific Comments:
It is not clear how model compounds were selected. Were these compounds chosen from studies focused on aqueous brown carbon, or on biomass burning in general? Or perhaps they were chosen because of commercial availability? Since workflow performance was assessed on the ability to correctly identify these compounds, it seems important to communicate how exactly these standards were chosen and cite references that informed the choice, if applicable.
I would welcome the addition of a comment from the authors about how well their combustion apparatus simulates real BB conditions, but do not feel it is absolutely necessary prior to publication. I am always a bit skeptical of these small-scale reactors in terms of their applicability to real conditions, and I am even more skeptical given this is a brand-new combustion system that has not been previously characterized. That being said, I don’t think the very nice NTA work done here is invalidated by the choice of combustor.
I found Figure 9 to be confusing – if the unlabeled areas correspond to unidentified molecules in each group, how are the authors determining that those molecules belong to that group? The information shown in Table S4 and S5 only added to my confusion. It seems like a reference to Tables S6 and S7 here would be more appropriate.
Technical Corrections:
I found Table 1 to be confusing in the sense that it implies no identifications were made at the highest level of confidence (e.g. Level 1). I suggest the authors add an example in the first row, since the SI shows that the method successfully reached Level 1 for a variety of compounds.
Several times throughout the manuscript, something to the effect of “level ≥ 4” is used. The context of these sentence conflicts with the literal meaning. In other words, what I think the authors are attempting to communicate is ‘a level of confidence higher than 4,’ which would correspond to a level with a numerical value ≤ 4. This occurs at lines 279, 344, 367, and 373 (and perhaps elsewhere). While a reader familiar with the Schymanski confidence level scheme will likely understand what is being implied, I suggest these be reworded to be more clear for those less familiar with NTA.
Line 290: missing a space between the words “abundant” and “in”
Line 293: I suggest spelling out molecular weights and organic aerosols to be more clear.
Commas in strange places in lines 299 and 301
Remove “identified” before the comma in line 303
Line 307: “characteristics” should be “characteristic”
Line 311: By the time I got here, I had totally forgotten what “STs” were. Consider not using an acronym all to refer to the surrogate standards.
Citation: https://doi.org/10.5194/egusphere-2025-1251-RC1 - AC1: 'Reply on RC1', Bartlomiej Witkowski, 22 Jul 2025
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RC2: 'Comment on egusphere-2025-1251', Anonymous Referee #2, 23 May 2025
Overall, the manuscript is timely and provides an important comparison of multiple NTA workflows. This work will provide a benchmark for others to work towards when undertaking NTA and highlights the type of optimising and testing which should take place. The link with toxicity is a great step forward, especially highlighting the lack of correlation with general aerosol composition (O:C, H:C etc).
However, while I agree with the approach of NTA optimisation and semi-quantification, I have some major comments regarding the LC-MS and semi-quantification methods used. Overall, the manuscript should be published in ACP once comments are addressed.
Specific comments:
L125- What mobile phase (A or B) does the 5% apply to?
L135 – How were the methods optimised? Was it based on a mix of all 59 standards and/or spiked into the sample matrix?
L137 – How were the 59 organic molecules determined? Based on previous literature and/or commercially availability? How representative of the overall composition are the standards? Are you biasing towards this functionality?
L155/6 – Not entirely sure what this means. How was the structure annotated just based on MS1?
Section 2.5
You have run 59 surrogate standards for library building/identification. When authentic standards were identified (level 1) in your BB samples, did you use the authentic standard for quantification? If so, why not?
Could you have assessed the error of the semi-quantification technique based upon comparisons between authentic and surrogate standard quantification?
How were the 5 surrogate standards chosen?
Why does tetradodecanoic acid cover the tr range of above 35 minutes with a retention time of 26 minutes?
Why weren’t all 59 standards used for semi-quantification? Your tr range used by each standard is large, including more standards would have narrowed the windows.
Did you spike the standards into the sample matrix to determine matrix effects on concentrations given the sample complexity?Section 3- Many of the analytes will ionise in both the negative and positive mode. How were these species dealt with in terms of final concentrations and composition?
L235 – What sample did you use to determine this? I.e a mixture of standards/spiked sample
L239 The use of “identifies” here makes it seem like this is a known compound, maybe change to annotated?
Fig 6 – How representative are your standards to the overall detected composition?
Citation: https://doi.org/10.5194/egusphere-2025-1251-RC2 - AC2: 'Reply on RC2', Bartlomiej Witkowski, 22 Jul 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-1251', Anonymous Referee #1, 15 May 2025
General Comments:
This manuscript presents a thorough comparison of multiple NTA workflows for analysis of water-soluble brown carbon aerosol samples. This is a much-needed contribution to the field, and additionally, the best workflow was used to identify new potential precursors to aqueous SOA. My scientific comments are minor, and I have a few comments to address clarity and technical issues with the manuscript. Overall, this manuscript is worthy of publication in ACP.
Specific Comments:
It is not clear how model compounds were selected. Were these compounds chosen from studies focused on aqueous brown carbon, or on biomass burning in general? Or perhaps they were chosen because of commercial availability? Since workflow performance was assessed on the ability to correctly identify these compounds, it seems important to communicate how exactly these standards were chosen and cite references that informed the choice, if applicable.
I would welcome the addition of a comment from the authors about how well their combustion apparatus simulates real BB conditions, but do not feel it is absolutely necessary prior to publication. I am always a bit skeptical of these small-scale reactors in terms of their applicability to real conditions, and I am even more skeptical given this is a brand-new combustion system that has not been previously characterized. That being said, I don’t think the very nice NTA work done here is invalidated by the choice of combustor.
I found Figure 9 to be confusing – if the unlabeled areas correspond to unidentified molecules in each group, how are the authors determining that those molecules belong to that group? The information shown in Table S4 and S5 only added to my confusion. It seems like a reference to Tables S6 and S7 here would be more appropriate.
Technical Corrections:
I found Table 1 to be confusing in the sense that it implies no identifications were made at the highest level of confidence (e.g. Level 1). I suggest the authors add an example in the first row, since the SI shows that the method successfully reached Level 1 for a variety of compounds.
Several times throughout the manuscript, something to the effect of “level ≥ 4” is used. The context of these sentence conflicts with the literal meaning. In other words, what I think the authors are attempting to communicate is ‘a level of confidence higher than 4,’ which would correspond to a level with a numerical value ≤ 4. This occurs at lines 279, 344, 367, and 373 (and perhaps elsewhere). While a reader familiar with the Schymanski confidence level scheme will likely understand what is being implied, I suggest these be reworded to be more clear for those less familiar with NTA.
Line 290: missing a space between the words “abundant” and “in”
Line 293: I suggest spelling out molecular weights and organic aerosols to be more clear.
Commas in strange places in lines 299 and 301
Remove “identified” before the comma in line 303
Line 307: “characteristics” should be “characteristic”
Line 311: By the time I got here, I had totally forgotten what “STs” were. Consider not using an acronym all to refer to the surrogate standards.
Citation: https://doi.org/10.5194/egusphere-2025-1251-RC1 - AC1: 'Reply on RC1', Bartlomiej Witkowski, 22 Jul 2025
-
RC2: 'Comment on egusphere-2025-1251', Anonymous Referee #2, 23 May 2025
Overall, the manuscript is timely and provides an important comparison of multiple NTA workflows. This work will provide a benchmark for others to work towards when undertaking NTA and highlights the type of optimising and testing which should take place. The link with toxicity is a great step forward, especially highlighting the lack of correlation with general aerosol composition (O:C, H:C etc).
However, while I agree with the approach of NTA optimisation and semi-quantification, I have some major comments regarding the LC-MS and semi-quantification methods used. Overall, the manuscript should be published in ACP once comments are addressed.
Specific comments:
L125- What mobile phase (A or B) does the 5% apply to?
L135 – How were the methods optimised? Was it based on a mix of all 59 standards and/or spiked into the sample matrix?
L137 – How were the 59 organic molecules determined? Based on previous literature and/or commercially availability? How representative of the overall composition are the standards? Are you biasing towards this functionality?
L155/6 – Not entirely sure what this means. How was the structure annotated just based on MS1?
Section 2.5
You have run 59 surrogate standards for library building/identification. When authentic standards were identified (level 1) in your BB samples, did you use the authentic standard for quantification? If so, why not?
Could you have assessed the error of the semi-quantification technique based upon comparisons between authentic and surrogate standard quantification?
How were the 5 surrogate standards chosen?
Why does tetradodecanoic acid cover the tr range of above 35 minutes with a retention time of 26 minutes?
Why weren’t all 59 standards used for semi-quantification? Your tr range used by each standard is large, including more standards would have narrowed the windows.
Did you spike the standards into the sample matrix to determine matrix effects on concentrations given the sample complexity?Section 3- Many of the analytes will ionise in both the negative and positive mode. How were these species dealt with in terms of final concentrations and composition?
L235 – What sample did you use to determine this? I.e a mixture of standards/spiked sample
L239 The use of “identifies” here makes it seem like this is a known compound, maybe change to annotated?
Fig 6 – How representative are your standards to the overall detected composition?
Citation: https://doi.org/10.5194/egusphere-2025-1251-RC2 - AC2: 'Reply on RC2', Bartlomiej Witkowski, 22 Jul 2025
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