From the journal Environmental Science: Atmospheres Peer review history

01-Dec-2022

Dear Dr Zhao:

Manuscript ID: EA-ART-11-2022-000151
TITLE: Aqueous-phase Photochemical Oxidation of Water-Soluble Brown Carbon Aerosols Arising from Solid Biomass Fuel Burning

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Reviewer 1

The manuscript prepared by Choudhary et al. titled “Aqueous-phase Photochemical Oxidation of Water-Soluble Brown Carbon Aerosols Arising from Solid Biomass Fuel Burning” investigated the changing chemical composition and light properties of BrC aerosol from burning different solid biomass fuels (IND dung, CAD dung, and pine wood) during photochemical oxidation in the aqueous phase. Photolysis and OH oxidation were studied as the two dominant photochemical oxidation pathways. Exposure to short-wavelength UV light was observed to be resposible for the photo-enhancement of BrC from solid fuel burning. Second order reaction rate constants for BrC aqueous phase OH oxidation were measured for different types of solid fuels. The authors also reported the independence of aqueous phase evolution on the solid fuel types. This is the first study studying the auqueous phase chemistry of BrC from solid fuel bunring, which has implications for reducing uncertainties in modelling prediction of the BrC climate impacts in the developing word, such as South Asia. It is recommended for publication on Environmental Science: Atmosphere once the following points were addressed.
Major comments:
1. The photo-enhancement of BrC from burning different types of solid fuel was attributed to the oligomer formation. However, the link between the light absorption properties and chemical composition is relatively weak in the current manuscript. In line 342, the authors actually mentioned both decomposition and formation of oligomers can take place upon photolysis. The authors used non-targeted analysis using high-resolution LC-MS for chemical characterization. Pimelic acid (m/z = 159.15) was used as an internal standard during the flow-injection to account for ionization variability among different sample runs. However, oligomers are highly possible to be multifunctional compounds, for which using only one diacid to tack their ionization variability seems insufficient. Also, uncertainties from sample preparation and injection were not discussed. Based on this, detailed molecular information on oligomer formation and their relationships with BrC photo-enhancement observed in this work should be proposed and provided.
2. Why was 120 min chosen for photolysis study and 60 min chosen for OH oxidation study? It was claimed that the temperature of the experimental solution can increase up to 40 ℃ after 120-min of direct UV exposure. Temperature can make a difference to the chemical mechanisms and kinetics. Can the authors at least add some uncertainty analysis/control experiments for their results in terms of the potential impacts from temperature?
3. The authors mentioned that substantial amount of H2O2 remained in the experimental solution after OH oxidation, which is much higher than what has been reported for ambient condition. It is well known that carbonyls can react with H2O2. Have the authors evaluated potential impacts from this pathway and its products on the chemical composition as well as light properties reported in this paper?



Minor comments:
The use of notations and acronyms (e.g., WS-BrC) could be more clear and should be properly defined at first mention.
Line 16：To be more accurate, this is not the first study investigating the photochemical change of BrC aerosol from dung burning (check Cappa et al., 2020). Please add “aqueous phase” in the description.
Line 54: particulate matter .
Line 68, 78 and other places: Start a new sentence using “However”.
Line 100: Burning conditions matters in the chemical composition and light properties of organic aerosol. As the authors mentioned in Line 480, please provide the reason of choosing “pyrolysis” as your burning conditions for generating BrC in this study.
Line 110: provide the size and pore size of the filter .
Line 111: how long were all the filterds stored before chemical analysis?
Line 118: be more specific on “CAD”.
Line 117-118: Why CAD was extracted using a different amount of solvent compare to other types of BrC?
Line 206 and 223: provide the version of the software.
Fig.3g: Please provide error bars for all points on this plot unless it was only one MS run.
Line 361-363: rephrase.
Line 442-443: rephrase.
Line 447: please add “aqueous phase”.
Line 487: for BrC from burning dung cakes and wood.

Reviewer 2

This study reported the evolutions of absorption and chemical compositions during photolysis and OH oxidation of BrC from the burning of different types of biomasses. In particular, the results from dung cake burning, a new type of biomass burning, were provides. The work provided new results that would bridge the knowledge gap in the related field. However, the authors are encourged to give more detailed descriptions about the new part, specifically dung cake burning in this study, to signify the new findings (refer to the recommendation file).

Point by Point Response to Reviewer(s)’s Comments
Manuscript ID: EA-ART-11-2022-000151

REVIEWER REPORT(S):
Referee: 1
Comments to the Author
The manuscript prepared by Choudhary et al. titled “Aqueous-phase Photochemical Oxidation of Water-Soluble Brown Carbon Aerosols Arising from Solid Biomass Fuel Burning” investigated the changing chemical composition and light properties of BrC aerosol from burning different solid biomass fuels (IND dung, CAD dung, and pine wood) during photochemical oxidation in the aqueous phase. Photolysis and OH oxidation were studied as the two dominant photochemical oxidation pathways. Exposure to short-wavelength UV light was observed to be resposible for the photo-enhancement of BrC from solid fuel burning. Second order reaction rate constants for BrC aqueous phase OH oxidation were measured for different types of solid fuels. The authors also reported the independence of aqueous phase evolution on the solid fuel types. This is the first study studying the auqueous phase chemistry of BrC from solid fuel bunring, which has implications for reducing uncertainties in modelling prediction of the BrC climate impacts in the developing word, such as South Asia. It is recommended for publication on Environmental Science: Atmosphere once the following points were addressed.
Response: Firstly, we would like to thank the reviewer for comprehensive review and helping us in improving the manuscript. We have now addressed all the comments suggested by the reviewer and incorporated them in the revised submission. All the changes are kept in track change mode. The point-by-point response for all the comments are given below:

Major comments:
1. The photo-enhancement of BrC from burning different types of solid fuel was attributed to the oligomer formation. However, the link between the light absorption properties and chemical composition is relatively weak in the current manuscript. In line 342, the authors actually mentioned both decomposition and formation of oligomers can take place upon photolysis. The authors used non-targeted analysis using high-resolution LC-MS for chemical characterization. Pimelic acid (m/z = 159.15) was used as an internal standard during the flow-injection to account for ionization variability among different sample runs. However, oligomers are highly possible to be multifunctional compounds, for which using only one diacid to track their ionization variability seems insufficient. Also, uncertainties from sample preparation and injection were not discussed. Based on this, detailed molecular information on oligomer formation and their relationships with BrC photo-enhancement observed in this work should be proposed and provided.
Response: We have now incorporated more details about oligomer formation and photo-enhancement, ionization variability during flow injection mass spectrometry and associated uncertainties in revised version of the manuscript as given below:
Oligomer formation and photo-enhancement
We agree with the reviewer’s statement that ‘the link between the light absorption properties and chemical composition is relatively weak’. We were unable to extract more detailed chemical information from the oligomer region. This is because the MS signals were very small, which reflects the fact that the enhancement is likely due to the formation of highly-light absorbing compounds that are present in a very small concentration. This is why we have taken the current approach of lumping all the peaks in the dimer region. The fact that we see a difference between UVB and UVA supports our discussion, and with error bars newly added to Figure 3(g) below, we are more confident about this observation. Following the reviewer’s comment, we have made the following changes to Section 3.4. 1) We have reviewed the current state of understanding of oligomer formation and added corresponding citations, 2) We have described the challenges associated with providing more detailed chemical information related to oligomers and 3) We suggested future studies should employ advanced analytical techniques better suited for characterizing these oligomer species.
“3.4. Potential Formation of Oligomers
Biomass burning emissions are highly enriched in aromatic compounds, such as phenols, nitrophenols, carbonyls, and organics acids, etc.1–3 These conjugated compounds, especially monoaromatics, form oligomers through radical coupling reactions involving either oxygen atom (of hydroxyl functional group) or carbon atom (of aromatic ring) during exposure to aqueous-phase photochemical oxidation processes.4,5 The oligomerization of conjugated compound/s causes it to become larger in size (increase in molecular weight) resulting the energy required for π-π* transition to become narrower and wavelength of light absorbed by the oligomer correspondingly becomes longer.6
In this study, WS-BrC aerosols from different solid biomass fuel burning types (mainly pine wood burning) underwent varying degree of photo-enhancement (particularly in the visible region) during aqueous-phase direct photolysis as discussed in previous sections. The corroboration and extent of oligomer formation during photolysis of dung cakes and pine wood burning WS-BrC may potentially substantiate the relationship between oligomer formation and photo-enhancement.
To ascertain the potential formation of oligomers in this study, several offline chemical analyses (e.g., high-resolution ESI- LC-MS, WSOC and direct MS flow-injection) were performed on samples collected during aqueous-phase UVB(Q) and UVA(Q) direct photolysis. The most dominant peaks in ESI- base peak chromatograms (BPC) of these solid biomass fuel burning water-extracts eluted between 2 to 14 min, and almost all of them were associated with C6 to C10 monoaromatic compounds, as listed in Table S3. During direct photolysis, these BPC peaks dissipated much faster upon exposure to UVB(Q)-light (Figure S7a) rather than UVA(Q)-light (Figure S7b). As a matter of fact, the total intensity of monoaromatic compounds reduced by ~80 to 100% after 120 min of UVB(Q) photolysis. However, monoaromatics loss was within ~20% during UVA(Q) photolysis (Figure S7c). Meanwhile, the observed loses in WSOC concentration during both UVB(Q) and UVA(Q) direct photolysis were similar and within ~15% (Figure S7c; S8). The comparatively faster disappearance (~4 to 5 times) of monoaromatic compounds during UVB(Q) direct photolysis compared to UVA(Q), but similar changes in WSOC concentrations, indicates the possibility of monoaromatic compounds reacting with each other through radical coupling reactions4,5 and forming dimeric compounds (oligomer structures) upon exposure to UVB-light, which we investigate further in the following section.

Figure 3. Temporal variation in direct flow-injection MS spectra at 0, 60 and 120 min for pine wood burning during UVB(Q) (a, b, c) and UVA(Q) (d, e, f) direct photolysis experiments normalized using internal standard. Also shown is % change in ratio of total direct MS intensity of higher (m/z = 215-350) and lower (m/z = 100-215) molecular-weight compounds (g). The black bar is the normalized signal of the internal standard (i.e., pimelic acid, m/z = 159.15).
To further corroborate the formation of oligomers, we utilized direct flow-injection MS on photolyzed aliquots of pine wood burning (Figures 3; S9) and IND dung burning (Figure S10) water-extract samples from UVB(Q) and UVA(Q) experiments. The signals of all detected peaks were normalized against the signal of the internal standard (i.e., pimelic acid, m/z = 159.15). For pine wood burning, we observed a decreasing trend in direct flow-injection MS signal intensity during both UVB(Q) (Figure 3a-c) and UVA(Q) (Figure 3d-f) photolysis. During UVB(Q) photolysis, the decrease in the total MS signal intensity in the monomeric region (m/z = 100 to 215) was much faster than in the dimeric region (m/z = 215 to 350). However, the decrease in the monomeric and dimeric regions appeared to be nearly similar during UVA(Q) photolysis. In this regard, previous studies have also shown that biomass burning organic aerosols undergo both formation7–9 of high molecular-weight compounds (oligomers) and loss7,8,10 of a few existing ones simultaneously during aqueous-phase photochemical oxidation processes. We have attempted to identify individual dimer peaks formed during the direct photolysis under UVB(Q), however, it was proven challenging due to the small signal intensities. Instead, we took an alternative approach and obtained the total signal over the dimeric and monoaromatic regions. Further, we assessed and compared the temporal variations in % change in the ratio of the total MS signal intensity over the dimeric and monoaromatic regions (Figure 3g; Table S4). We observed that the ratio consistently increased (~35% after 120 min) upon exposure to UVB(Q) photolysis. However, it decreased (~10 to 15% after 120 min) during UVA(Q) photolysis (Figure 3g) signifying that potential formation of oligomers may be higher during UVB(Q) photolysis than UVA(Q). The standard deviation of % change in the ratio was within ± 10% indicating efficacy of the method in identifying oligomer formation qualitatively. For IND dung burning, we hardly observed any significant MS signal in the dimeric region (Figure S10) indicating oligomeric compounds were forming to a lesser extent compared to pine wood burning. The consistency observed between the potential oligomer formation and the absorbance behavior of pine wood and IND dung burning WS-BrC during direct photolysis in this study substantiate their relationship.
In summary, these observations signify two major findings: firstly, oligomer formation would be more likely during exposure to shorter wavelength UV-light. While previous studies used either UVB or UVA as light sources, our study represents the first direct comparison of the two. Secondly, the chemical structure of monomers and their abundance would be the driving factors for oligomerization during aqueous-phase direct photolysis. For example, previous studies have reported that monoaromatic compounds, such as guaiacol9, vanillin,4 syringol,5 and coniferyl aldehyde4 can react with each other and form light-absorbing oligomers during aqueous-phase UVB(Q) direct photolysis. These monoaromatic compounds (except syringol) were observed in emissions of all three solid biomass fuel burning types in this study (Table S5). It is noted that we did not observe syringol in our study, however, it has been reported to be a good tracer for wood burning.11 Additionally, the emissions of these monomers from wood burning were ~2 to 4 times higher than IND dung burning in our study (Table S5). A recent study3 has also reported that wood burning emissions are highly rich in lignin-degradation products (monomers) compared to animal dung cakes. The enrichment of such monoaromatic compounds is likely responsible for the more profound photo-enhancement in pine wood burning WS-BrC than dung burning observed in this study. Unfortunately, the method we employed did not allow molecular-level identification of oligomers that are responsible for the observed photo-enhancement. In future studies, instruments and method better suited for oligomer detection – e.g., SEC - should be employed to confirm our observation.”

Ionization variability during flow injection mass spectrometry
We agree with the reviewers and have reviewed the literature related to ion suppression. We found that suppression of signals for single- and multi-functional internal standards can differ up to 15%. This piece of information is now added to the manuscript. We have also cited a number of studies employing a single internal standard, with a hope to justify that this is a common practice used in analyses.
“Size-exclusion chromatography (SEC) is a more common technique to investigate formation of high molecular-weight compounds (oligomer).7,12,13 However, in this study, direct flow-injection MS (ThermoFisher Scientific model LTQ-XL) was used to identify oligomer formation during aqueous-phase direct photolysis. The MS sensitivity in this technique is susceptible to changes due to suppression of ionization signal from matrix interferences, such as structural complexity, loading/concentration of analyte solution, etc. The use of an internal standard during the flow-injection can account for this ionization suppression. However, oligomers (dimers) are likely to be multifunctional compounds, and therefore using only a single-functional compound (pimelic acid) to account for their ionization suppression may not be enough. In this context, a previous study14 compared ionization suppression effect of both single-functional (e.g., aniline, lauric acid, 2-methylquinoline, etc.) and multifunctional internal standards (e.g., gemfibrozil) in complex sample matrices (e.g., waste water) and found bias to be within ± 15%. Additionally, several studies15–17 have used single-functional compounds as an internal standard to identify a range of compounds in complex sample matrices during direct flow-injection MS analysis.
In this work, pimelic acid (m/z = 159.15) was used as an internal standard during the flow-injection MS analysis. Two samples each for pine wood and IND dung burning were analyzed. The total runtime for the experiment was 10 min and the flow-injection flow rate was 0.005 mL/min, achieved with a syringe pump. The signal intensity for all the compounds in monomer region (m/z < 210)9 and dimer region (m/z = 250 to 350)5,9 were normalized to that of pimelic acid (10 µM), an internal standard used to account for ionization suppression. The % change in the ratio was found to be within ± 10% (discussed in section 3.4) signifying efficacy of pimelic acid as internal standard in this study.”
2. Why was 120 min chosen for photolysis study and 60 min chosen for OH oxidation study? It was claimed that the temperature of the experimental solution can increase up to 40 ℃ after 120-min of direct UV exposure. Temperature can make a difference to the chemical mechanisms and kinetics. Can the authors at least add some uncertainty analysis/control experiments for their results in terms of the potential impacts from temperature?
Response: The original experimental length was 120 min for both direct photolysis and OH oxidation experiments. Yet, a majority fraction (~70-100%) of brown carbon (BrC) photobleached/disappeared only within 60 min of OH oxidation for all three solid biomass fuel burning types, whereas, no loss in WS-BrC absorbance (instead photo-enhancement was observed; except for IND dung) was observed during direct photolysis (Figure 1, 4). This resulted in distinct experimental durations for direct photolysis (120 min) and OH oxidation (60 min) experiments in this study.
Further, we agree with the reviewer that change in temperature of the experimental solution during photochemical processes can make a difference to the chemical mechanisms and kinetics. In this context, previous studies18–20 have shown that a change in temperature in order of 10 to 20°C from room temperature will not affect composition and kinetics of organic compounds, including BrC chromophores, substantially. As a matter of fact, these studies have found change in rate constant to be within 10% due to increase in room temperature by 20°C, way less than precision/uncertainty of ~20 to 55% (for different solid biomass fuel burnings) in 2nd order rate constant of WS-BrC (Table 1) reported in this study. We have now incorporated all these details in the revised submission of the manuscript.
“A fan was used to cool the experimental solution. Still, the temperature of the aqueous solution rose up to ~40 °C after 120 min of irradiation. Previous studies21,22 have shown that a change in temperature during photochemical oxidation processes have potential to impact chemical mechanisms and kinetics of organic molecules. However, raising the temperature change from room temperature by 10 to 20°C would not affect composition and kinetics of organic compounds, including BrC chromophores, substantially.18–20 As a matter of fact, variability in kinetics parameters would likely to be within 10% due to a temperature change of 20°C, a very small change compared to the variability observed in this study.”

3. The authors mentioned that substantial amount of H2O2 remained in the experimental solution after OH oxidation, which is much higher than what has been reported for ambient condition. It is well known that carbonyls can react with H2O2. Have the authors evaluated potential impacts from this pathway and its products on the chemical composition as well as light properties reported in this paper?
Response: Dear reviewer, we apologize for the confusion, but we have not quantified the amount of H2O2 remained in the solution after completion of OH oxidation experiments. Further, the initial concentration of H2O2 added in the experimental solution may look higher. However, it should be noted that it is [OH]ss concentration which governs the changes in composition and WS-BrC absorbance during OH oxidation, not the initial concentration of H2O2 added in the experimental solution. We have now modified the relevant text in the revised submission to communicate this content and enhance the clarity.
Furthermore, we agree with the reviewer that carbonyls can react with H2O2. The reaction proceeds via a nucleophilic addition of H2O2 to a carbonyl compound, yet carbonyl carbon must be an efficient electrophile for this reaction to proceed.23–25 Aromatic compounds with functional groups such as phenols, carbonyls, aldehydes, etc., have been identified as predominant constituents of biomass burning emissions.1–3 However, due to the electron donating nature of aromatic rings, the aromatic carbonyls are less reactive towards nucleophilic addition reactions. In fact, a recent study26 has found the nucleophilic additions of water to aromatic carbonyls negligible. These findings combined with our observations showing no change in WS-BrC absorbance during the dark control experiments (Figure 4) signifies non-substantial relevance of this chemistry to our study. We have now incorporated these details briefly in the revised manuscript.
“The aqueous-phase OH oxidation experiments were performed with the same experimental apparatus utilized in the UVB(Q) direct photolysis experiment. The experiments were conducted at room temperature for 1 h with aliquots being taken at 0, 5, 10, 15, 20, 30, 45 and 60 min for absorbance measurements. Hydrogen peroxide (H2O2) 30% w/w (Sigma Aldrich) was added to WS-BrC aqueous solution to act as in-situ source of OH radicals upon exposure to UVB radiation in quartz vessel during OH oxidation experiments. The added concentration of H2O2 in the aqueous solution varied from 7.5 mM to 125 mM, depends on the measured TOC concentration of each WS-BrC solution, to maintain a relatively consistent steady-state OH concentration ([OH]ss) similar to that in ambient cloud water (~10-13 M). The added H2O2 concentrations may look very high compared to the ambient quantity, yet it is the [OH]ss that governs the photochemistry and crucial to determine the WS-BrC lifecycle parameters such as photochemical decay, lifetime, etc. in aqueous medium.”
“However, there might be a potential bias in aqueous-phase OH oxidation of WS-BrC due to the reaction between carbonyl compounds and H2O2. The carbonyl compounds can react with H2O2 via a nucleophilic addition of H2O2 to a carbonyl compound, but carbonyl carbon must be an efficient electrophile for this reaction to proceed.23–25 Aromatic compounds with functional groups such as phenols, carbonyls, aldehydes, etc., have been identified as predominant constituents of biomass burning emissions.1–3 However, due to the electron donating nature of aromatic rings, the aromatic carbonyls are less reactive towards nucleophilic addition reactions. In fact, a recent study26 has found the nucleophilic additions of water to aromatic carbonyls negligible. A few dark control experiments were carried out in this study to confirm that reactions between H2O2 and WS-BrC constituents (especially with carbonyl functional group) under dark conditions (i.e., no UVB illumination), if any, do not alter WS-BrC optical properties.”

Minor comments:
The use of notations and acronyms (e.g., WS-BrC) could be more clear and should be properly defined at first mention.
Response: We have now defined all the notations and acronyms at their first mention in the revised version of the manuscript.
“This is the first study to explore aqueous-phase photochemical aging of laboratory-generated water-soluble brown carbon (WS-BrC) aerosols arising from cow dung cakes burning, with a focus on cloud-water photochemistry.
.
…throughout the manuscript.”

Line 16：To be more accurate, this is not the first study investigating the photochemical change of BrC aerosol from dung burning (check Cappa et al., 2020). Please add “aqueous phase” in the description.
Response: We have now incorporated this comment in the revised manuscript.
“This is the first study to explore aqueous-phase photochemical aging of laboratory-generated water-soluble brown carbon (WS-BrC) aerosols arising from cow dung cakes burning, with a focus on cloud-water photochemistry.”

Line 54: particulate matter .
Response: We have now incorporated this comment in the revised manuscript.
“Previous studies (carried out between 2004 to 2015) have pegged the annual contribution of solid biomass fuel burning emissions to PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 µm) concentration from ~20 to 40% in India28–30 and ~40% in Africa.30”

Line 68, 78 and other places: Start a new sentence using “However”.
Response: We have now incorporated this comment throughout the manuscript in the revised submission.
“Previous laboratory studies on aqueous-phase photochemical oxidation of secondary BrC showed rapid loss of chromophores also referred to as photobleaching.8,31 However, photo-enhancement preceded photobleaching for biomass burning primary BrC aerosols.7–9
A few recent studies have assessed the effects of UVB7,9 and UVA8 direct photolysis in the aqueous medium on BrC absorbance separately. However, a comparative analysis in conjunction with the effects of different wavelengths of UV-light on the evolution of BrC has never been done before.

…. throughout the manuscript”

Line 100: Burning conditions matters in the chemical composition and light properties of organic aerosol. As the authors mentioned in Line 480, please provide the reason of choosing “pyrolysis” as your burning conditions for generating BrC in this study.
Response: Biomass burning have been identified as a dominant source of particulate matter, especially brown carbon (BrC) aerosols, in the ambient atmosphere.30,32 A major fraction of biomass burning emissions comprises organic molecules such as phenols, methoxyphenols, nitrophenols, and organic acids.1–3,9,33 These molecules are products from pyrolysis of cellulose and lignin, key elements of biomass tissues.3,33 The greatest advantage of our pyrolysis setup is the achievement of highly reproducible air flow and temperature heating profiles, which is difficult for open burning and smouldering setups. Given that one of our objectives was the comparison of the behaviour of BrC from different fuels, generating BrC under a reproducible combustion/pyrolysis condition was both beneficial and necessary. For similar reasons, the advantage of such pyrolysis setup has been recognized by a number of laboratory studies for BrC aerosols generation.3,7–9,34 We have now incorporated these details in the revised version of the manuscript.
“Cellulose and lignin are key components of biomass tissues. Pyrolysis of these components produce BrC chromophores such as phenols, nitrophenols, and organics acids etc. that are highly prevalent in the ambient atmosphere.1–3,9,33 Therefore, BrC aerosol particles were generated by pyrolysis of above-mentioned solid biomass fuels in a tube furnace, using the method described in Loebel Roson et al,3 in an aerobic atmosphere.”
“The setup is highly efficient in reproducing air flow and temperature heating profiles, which is difficult for open burning and smouldering setups. We note that our setup does not fully reproduce the combustion environment of actual solid biomass fuel burning. However, the BrC produced from pyrolysis should be good representative of those emitted from real combustion. Similar tube furnace setups have been used in a number of recent investigations for biomass burning.26,34–37”

Line 110: provide the size and pore size of the filter .
Response: We have now provided these details in revised version of the manuscript.
“The aerosol particles emitted during the pyrolysis of different solid biomass fuels were collected onto 0.22 µm prebaked quartz filters (WhatmanTM; 47 mm diameter).”

Line 111: how long were all the filterds stored before chemical analysis?
Response: We have now added the relevant details in the revised version of the manuscript.
“The collected filters were stored in a freezer (-19°C), usually for a week, until chemical analysis.”

Line 118: be more specific on “CAD”.
Response: We have now incorporated this comment in the revised submission of the manuscript.
“The IND and wood burning samples were extracted in 30 mL DI water, whereas CAD burning filters were extracted in 25 mL DI water due to their lower particulate loading compared to other two solid biomass fuel burning types. The blank filters were extracted in 25 mL DI water.”

Line 117-118: Why CAD was extracted using a different amount of solvent compare to other types of BrC?
Response: Spectrophotometric technique was used to measure WS-BrC absorbance in this study. As we all know that this technique is based on Beer-Lambert law signifying that magnitude of WS-BrC absorbance signal would be proportional to its concentration in the experimental solution. It is noted that particulate loading onto CAD filters was comparatively lower than other solid biomass types. Therefore, they were extracted in smaller amount of solvent (i.e., water) than IND and wood samples. We have incorporated this detail in the revised version of the manuscript.
“The IND and wood burning samples were extracted in 30 mL DI water, whereas CAD burning filters were extracted in 25 mL DI water due to their lower particulate loading compared to other two solid biomass fuel burning types. The blank filters were extracted in 25 mL DI water.”
“Measurements of WS-BrC absorbance in the aqueous extracts were based on Beer-Lambert law. A multi-channel UV-vis spectrophotometer (Genesys; Model: 10S Vis spectrophotometer) with path-length of 1 cm was used for the measurements.”

Line 206 and 223: provide the version of the software.
Response: We have now added these details in the revised version of the manuscript.
“The analysis and molecular formula assignment of high-resolution LC-MS data was performed using MassHunter software (v. B.07.00).3
The data from Thermo Scientific LC-MS were analyzed using FreeStyleTM 1.8 software.”

Fig.3g: Please provide error bars for all points on this plot unless it was only one MS run.
Response: We have now provided the error bars, using datasets obtained from multiple MS runs (previously only one MS run was used), in the Figure 3g in the revised submission of the manuscript.

Figure 3. Temporal variation in direct flow-injection MS spectra at 0, 60 and 120 min for pine wood burning during UVB(Q) (a, b, c) and UVA(Q) (d, e, f) direct photolysis experiments normalized using internal standard. Also shown is % change in ratio of total direct MS intensity of higher (m/z = 215-350) and lower (m/z = 100-215) molecular-weight compounds (g). The black bar is the normalized signal of the internal standard (i.e., pimelic acid, m/z = 159.15).

Line 361-363: rephrase.
Response: We have now modified this sentence in the revised version of the manuscript.
“These monoaromatic compounds (except syringol) were observed in emissions of all three solid biomass fuel burning types in this study (Table S5). It is noted that we did not observe syringol in our study, however, it has been reported to be a good tracer for wood burning.11”

Line 442-443: rephrase.
Response: We have now modified this sentence in the revised version of the manuscript.
“Atmospheric BrC aerosols are short-lived climate pollutants contributing to both air pollution and global warming. The atmospheric warming effect of BrC aerosols has become emergent in recent years, constituting ~7 to 48% of total anthropogenic aerosol warming globally.37,38”

Line 447: please add “aqueous phase”.
Response: We have corrected this sentence in the revised submission of the manuscript.
“However, due to limited investigations on biomass fuel types that are relevant to these areas, the exact climatic impacts of aqueous-phase processing of BrC over these regions is poorly understood.”

Line 487: for BrC from burning dung cakes and wood.
Response: We have now corrected this sentence in the revised submission of the manuscript.
“Therefore, we propose that future studies should be centered on understanding the evolution and fate of BrC emitted from burning of different biomass types (e.g., dung cakes, agriculture-residues, wood, etc.) at different burning conditions under atmospherically relevant oxidation processes.”





 
Referee: 2
Comments to the Author
This study reported the evolutions of absorption and chemical compositions during photolysis and OH oxidation of BrC from the burning of different types of biomasses. In particular, the results from dung cake burning, a new type of biomass burning, were provides. The work provided new results that would bridge the knowledge gap in the related field. However, the authors are encourged to give more detailed descriptions about the new part, specifically dung cake burning in this study, to signify the new findings (refer to the recommendation file).
This study reported the evolutions of absorption and chemical compositions during photolysis and OH oxidation of BrC from the burning of different types of biomasses. In particular, the results from dung cake burning, a new type of biomass burning, were provides. I recommend accepting the paper after the following issues are well addressed.
Response: We are very thankful to the reviewer for comprehensive review of the manuscript and helping us in improving the manuscript. We have now added more discussion about dung cake burning and addressed all the comments suggested by the reviewer in the revised submission. All the changes are kept in track change mode in the revised manuscript. The point-by-point response for all the comments are given below:

Line 15&16: add “burning” after wood and animal dung to make the description clearer. throughout the paper, please check the work is discussing biomass burning emissions rather than biomass emissions.
Response: We have now incorporated this comment throughout the manuscript in the revised submission.
“A large fraction of the population in developing countries is still dependent on wood and animal dung cake burning for household cooking and heating. While wood emissions are well studied, a lack of understanding about animal dung burning emissions still persists.”
This is the first study to explore aqueous-phase photochemical aging of laboratory-generated water-soluble brown carbon (WS-BrC) aerosols arising from cow dung cake burning, with a focus on cloud-water photochemistry.
.
.
…..throughout the manuscript”

Line 21-23: the two sentences “The photo-enhancement was induced more effectively upon exposure to UV-light 22 with shorter wavelengths during direct photolysis. The photo-enhancement in WS-BrC 23 absorbance coincided with oligomer formation” can be combined.
Response: We have now incorporated this comment in the revised submission of the manuscript.
“The photo-enhancement in WS-BrC absorbance was induced more effectively upon exposure to UV-light with shorter wavelengths and coincided with oligomer formation during direct photolysis.”

Line 23: suggest deleting “Further”.
Response: We have now modified the sentence in the revised submission of the manuscript.
“During OH oxidation, WS-BrC emitted from burning of all three solid biomass fuels followed similar evolution pathways, with their second-order rate constant values being (25.4 ± 3.9) × 108, (19.2 ± 13.1) × 108 and (23.3 ± 6.5) × 108 M-1s-1 for WS-BrC arising from burning Indian cow dung, Canadian cow dung and pine wood, respectively.”


Line 50-53: the proportion of dung cakes in all utilized solid biomass, if available, is better to be presented here.
Response: We have now added more details about uses of dung cakes for household cooking and heating in India in revised version of the manuscript.
“For example, more than one billion of the global population, mainly from developing countries, is still using solid biomass fuel burning (e.g., dung cakes, wood etc.) as an affordable source of energy for cooking and heating.39 As a matter of fact, ~60% of Indian and ~86% of African households were using solid biomass fuels for cooking and heating in 2016.39 Moreover, atleast 10-15% of Indian household (~150 million people) rely on dung cakes for cooking and heating implying their potentially substantial role in very poor air quality in India.40 Burning such solid biomass fuels is often practiced with old and poorly designed burning devices; as such the emissions arising from such fuels may be both more abundant and toxic than the fraction of household can indicate.40,41”

Line 81: “oxidation” might be better replaced by “processes” as the work include direct photolysis and OH oxidation.
Response: We have now modified this sentence in the revised submission of the manuscript as given below.
“The overall objective of this study was to comprehensively examine the evolution of BrC emitted from burning of solid biomass fuels during aqueous-phase photochemical oxidation processes.”
Line 137: the intensity peak for UVB(G) should be provided here to make the reading clear.
Response: We have now added the information about the intensity peak for UVB(G) light in the revised submission of the manuscript.
“The flux spectra appeared polychromatic, with intensities peaking at 313 and 366 nm for UVB(Q) and UVA(Q) light, respectively (Figure S2). Overall, the predominant fraction of photon flux for these UV-lights fell within 280 to 400 nm wavelengths (Figure S2). The flux intensity of UVB(G) light also peaked at 313 nm, however, the glass vessel (compared to quartz) blocked shorter wavelength UVB flux (≤ 320 nm) quite efficiently (~50 to 90%) and loss of flux at longer wavelengths (> 320 nm) was very small.”

Line 154: what is “final concentration of H2O2”? I guess here it is added concentration? final
concentration seems to refer to H2O2 concentration after the 1 hour reaction.
Response: Dear reviewer, we apologize for the confusion. We have not quantified the amount of H2O2 remained in the experimental solution after completion of OH oxidation experiments. We only mentioned the concentration of H2O2 added in the experimental solution before start of OH oxidation experiment in our study. We have now modified the relevant text to communicate this content in the revised version of the manuscript.
“The aqueous-phase OH oxidation experiments were performed with the same experimental apparatus utilized in the UVB(Q) direct photolysis experiment. The experiments were conducted at room temperature for 1 h with aliquots being taken at 0, 5, 10, 15, 20, 30, 45 and 60 min for absorbance measurements. Hydrogen peroxide (H2O2) 30% w/w (Sigma Aldrich) was added to WS-BrC aqueous solution to act as in-situ source of OH radicals upon exposure to UVB radiation in quartz vessel during OH oxidation experiments. The added concentration of H2O2 in the aqueous solution varied from 7.5 mM to 125 mM, depends on the measured TOC concentration of each WS-BrC solution, to maintain a relatively consistent steady-state OH concentration ([OH]ss) similar to that in ambient cloud water (~10-13 M). The added H2O2 concentrations may look very high compared to the ambient quantity, yet it is the [OH]ss that governs the photochemistry and crucial to determine the WS-BrC lifecycle parameters such as photochemical decay, lifetime, etc. in aqueous medium.”

Line 233: I feel water-soluble organic nitrogen (WSON) over WSOC, i.e., WSON/WSOC ratio, is a better proxy of chemical characteristics of BrC than TN/WSOC. Please provide the information if available.
Response: Dear reviewer, the inorganic nitrogen constituents (e.g., 〖NO〗_2^-,〖NO〗_3^-) in water-extracts of solid-biomass fuel burning samples were negligible compared to TN in this study (Table S2). Therefore, TN/WSOC ratio would likely to reflect more closely with water-soluble organic nitrogen fraction (i.e., WSON/WSOC) than inorganic fraction. We have now added theses details and modified the discussion about WSON/WSOC ratio, including its role in governing BrC absorption characteristics, accordingly in main manuscript text and supplementary information (Table S2) of revised manuscript.
“MAEWS-BrC, also referred to as absorptivity, indicates the mass normalized absorption cross-section of WS-BrC chromophores. The absorptivity values of WS-BrC emitted from dung cakes (both IND and CAD) and pine wood burning, calculated by utilizing WS-BrC absorbance and WSOC concentrations (Table S2) in Eq. 1, were determined at 365 nm and 400 nm. We observed that the absorptivity values of WS-BrC aerosols from dung cakes burning (averaged at ~0.75 and 0.30 m2/gC at 365 nm and 400 nm, respectively) were ~2 to 3 times higher than pine wood burning (Figure S5). In this context, previous studies3,9,42–44 have reported that the nitrogen containing organic compounds govern optical properties (e.g., absorptivity) of biomass burning BrC. In this study, the higher water-soluble organic nitrogen (WSON) over WSOC (WSON/WSOC) ratio was observed for dung cakes burning than pine wood (Table S2). A recent mass-spectrometry based study3 also observed higher prevalence of nitrogen containing OA in dung cakes burning than wood burning. This probably explains the higher absorptivity values for dung cake burning WS-BrC than pine wood.
Table S2. WSOC and TN concentrations for different types of solid biomass fuel burning.
Fuel type Parameters*
WSOC (mg-C/L) TN (mg-N/L) TIN$ (µg-N/L WSON/WSOC&
Blanks a, c 3.8 ± 0.1 0.37 ± 0.07 < LOD -
IND dung #, b, d 139.7 ± 30.2 10.65 ± 2.05 < LOD 0.077 ± 0.004
CAD dung #, a, e 34.2 ± 4.5 3.90 ± 1.55 - 0.115 ± 0.047
Pine Wood #, b, e 403.6 ± 300.4 5.85 ± 4.44 <LOD 0.015 ± 0.006
* Mean ± standard deviation; # all values are blank corrected; WSOC = water-soluble organic carbon; TN = total nitrogen (water-soluble); TIN = total inorganic nitrogen (water-soluble); WSON = water-soluble organic nitrogen
$ TIN = 〖(NO〗_2^--N+〖NO〗_3^--N). Measured using colorimetric autoanalyzer (ThermoFisher Gallery Beermaster Plus). Limit of detection (LOD) = 4.3 µg/L
& Since, inorganic nitrogen constituents (e.g., 〖NO〗_2^-,〖NO〗_3^-) are negligible compared to TN. Therefore, TN/WSOC ratio (i.e., organic + inorganic fraction) would likely to reflect more closely with water-soluble organic nitrogen fraction (i.e., WSON/WSOC) than inorganic fraction.
a extracted in 25 mL; b extracted in 30 mL (diluted by 8 times during WSOC, TN and TIN measurements)
c n = 2; d n = 3; e n = 4
”

Line 234-236: “the absorptivity values observed in this study were comparable to previous studies…” the description is ambiguous. As mentioned above, absorptivity of WS-BrC aerosols from dung cake burning is significantly different from that from wood burning. Thus, please give explicit descriptions about the comparisons between this study and previous studies (with similar/different types of biomass burning). In addition, the author highlights this work is the first effort of measuring the absorptivity of WS-BrC from dung cake burning, so it is better to provide more detailed comparisons between the results from dung cake burning and other biomass burning.
Response: We have now significantly improved the discussion on absorptivity values of WS-BrC from different solid-fuel burning types and their comparison with previous studies in the revised version of the manuscript.
Further, we want to clarify that this is first study to assess aqueous-phase photochemical processing of WS-BrC from dung cakes burning. There are a few studies3,45,46 that reported absorptivity of freshly emitted dung cakes burning BrC. We have added a brief discussion about them in the revised submission.
“Further, previous studies3,7–9,44–48 observed a large variability in the absorptivity values of biomass burning WS-BrC. For example, the absorptivity values at 365 nm varied from 0.7 to 1.5 m2/gC for dung cakes burning,3,9,45,46,48 0.1 to 2.0 m2/gC for wood burning,3,7–9,44 and 1.2 to 3.0 m2/gC for agriculture-residue burning.47,48 The absorptivity values observed in our study fell in lower side of the ranges reported by these studies and it could be the result of different type of burning set-up (discussed in section 2.1) utilized for burning in this study.”
“This is the first study to explore aqueous-phase photochemical aging of laboratory-generated water-soluble brown carbon (WS-BrC) aerosols arising from cow dung cakes burning, with a focus on cloud-water photochemistry.
It should be noted that this is first study to assess aqueous-phase photochemical oxidation of WS-BrC from dung cakes burning.”

Line 239: in the caption of Figure 1, please point out that the MAE changes at wavelength of 365 and 400 nm act as examples of the changes in near UV-region and visible region, respectively, which corresponds to the following discussion.
Response: We have now incorporated this comment in the revised version of the manuscript.
“Figure 1. Spectral change in absorbance from 330 to 700 nm of WS-BrC emitted from pyrolysis of IND dung (a, d), CAD dung (b, e) and pine wood (c, f) during aqueous-phase direct photolysis using UVB(Q) (panel A) and UVA(Q) (panel B) light. Where, (Q) denotes an experiment performed with a quartz vessel. The color code of absorption spectra indicates the length of irradiation. The inset plots indicate changes in mass absorption efficiency (MAEWS-BrC) at 365 and 400 nm for the corresponding fuels. The MAEWS-BrC changes at 365 and 400 nm act as case studies of the changes in near UV-region and visible region, respectively.”

Line 258: I guess it is “at shorter reaction time” rather than “at shorter wavelength”. Similarly for the “at longer wavelengths”.
Response: No. We don’t agree. During UVA(Q) photolysis, WS-BrC from all three solid biomass fuel burning types underwent photobleaching in the near UV region. However, in visible region, WS-BrC from dung cakes burning underwent photobleaching at shorter wavelengths (400 to 425 nm) and photo-enhancement at longer wavelengths (425 to 550 nm) of visible spectrum. (Figure 1B). We have now modified this portion a little to communicate above content in the revised submission.
“In contrast, during UVA(Q) photolysis, all three solid biomass fuel burning types underwent varying scale of photobleaching in the near UV region; whereas, in the visible region, WS-BrC from both dung cakes burning underwent a combination of photobleaching (at shorter wavelengths, 400 to 425 nm) and photo-enhancement (at longer wavelengths, 425 to 550 nm), but only photo-enhancement (scale was smaller compared to UVB(Q)) was observed for pine wood burning (Figure 1B).”
Line 304: for the “Potential Formation of Oligomers” section, the authors are strongly encouraged to compare in detail the differences/similarities of changes of molecules during photolysis of BrC from dung cake burning and wood (pine) burning. One thing is that the variations in absorption discussed in the above sections are actually attributed to the changes of light absorption molecules. A detailed description of molecule change during photolysis explains the variations in absorption, which makes readers easier to get connections between absorptions of chemical property of BrC. The authors need to give more related discussion (absorption ~ chemical property); another thing is that the authors highlight this is the first effort presenting photochemical processes of BrC from dung cake burning, while there is only very limited content describing the chemical evolution of dung cake burning derived BrC during photolysis and limited comparison with that from wood burning.
Response: We have added the mechanism about role of oligomer formation in the photo-enhancement of WS-BrC absorbance. The discussion on dung burning has also been improved. We now modified section 3.4 substantially by incorporating all the suggestion of the reviewer as given below.
“3.4. Potential Formation of Oligomers
Biomass burning emissions are highly enriched in aromatic compounds, such as phenols, nitrophenols, carbonyls, and organics acids, etc.1–3 These conjugated compounds, especially monoaromatics, form oligomers through radical coupling reactions involving either oxygen atom (of hydroxyl functional group) or carbon atom (of aromatic ring) during exposure to aqueous-phase photochemical oxidation processes.4,5 The oligomerization of conjugated compound/s causes it to become larger in size (increase in molecular weight) resulting the energy required for π-π* transition to become narrower and wavelength of light absorbed by the oligomer correspondingly becomes longer.6
In this study, WS-BrC aerosols from different solid biomass fuel burning types (mainly pine wood burning) underwent varying degree of photo-enhancement (particularly in the visible region) during aqueous-phase direct photolysis as discussed in previous sections. The corroboration and extent of oligomer formation during photolysis of dung cakes and pine wood burning WS-BrC may potentially substantiate the relationship between oligomer formation and photo-enhancement.
To ascertain the potential formation of oligomers in this study, several offline chemical analyses (e.g., high-resolution ESI- LC-MS, WSOC and direct MS flow-injection) were performed on samples collected during aqueous-phase UVB(Q) and UVA(Q) direct photolysis. The most dominant peaks in ESI- base peak chromatograms (BPC) of these solid biomass fuel burning water-extracts eluted between 2 to 14 min, and almost all of them were associated with C6 to C10 monoaromatic compounds, as listed in Table S3. During direct photolysis, these BPC peaks dissipated much faster upon exposure to UVB(Q)-light (Figure S7a) rather than UVA(Q)-light (Figure S7b). As a matter of fact, the total intensity of monoaromatic compounds reduced by ~80 to 100% after 120 min of UVB(Q) photolysis. However, monoaromatics loss was within ~20% during UVA(Q) photolysis (Figure S7c). Meanwhile, the observed loses in WSOC concentration during both UVB(Q) and UVA(Q) direct photolysis were similar and within ~15% (Figure S7c; S8). The comparatively faster disappearance (~4 to 5 times) of monoaromatic compounds during UVB(Q) direct photolysis compared to UVA(Q), but similar changes in WSOC concentrations, indicates the possibility of monoaromatic compounds reacting with each other through radical coupling reactions4,5 and forming dimeric compounds (oligomer structures) upon exposure to UVB-light, which we investigate further in the following section.

Figure 3. Temporal variation in direct flow-injection MS spectra at 0, 60 and 120 min for pine wood burning during UVB(Q) (a, b, c) and UVA(Q) (d, e, f) direct photolysis experiments normalized using internal standard. Also shown is % change in ratio of total direct MS intensity of higher (m/z = 215-350) and lower (m/z = 100-215) molecular-weight compounds (g). The black bar is the normalized signal of the internal standard (i.e., pimelic acid, m/z = 159.15).
To further corroborate the formation of oligomers, we utilized direct flow-injection MS on photolyzed aliquots of pine wood burning (Figures 3; S9) and IND dung burning (Figure S10) water-extract samples from UVB(Q) and UVA(Q) experiments. The signals of all detected peaks were normalized against the signal of the internal standard (i.e., pimelic acid, m/z = 159.15). For pine wood burning, we observed a decreasing trend in direct flow-injection MS signal intensity during both UVB(Q) (Figure 3a-c) and UVA(Q) (Figure 3d-f) photolysis. During UVB(Q) photolysis, the decrease in the total MS signal intensity in the monomeric region (m/z = 100 to 215) was much faster than in the dimeric region (m/z = 215 to 350). However, the decrease in the monomeric and dimeric regions appeared to be nearly similar during UVA(Q) photolysis. In this regard, previous studies have also shown that biomass burning organic aerosols undergo both formation7–9 of high molecular-weight compounds (oligomers) and loss7,8,10 of a few existing ones simultaneously during aqueous-phase photochemical oxidation processes. We have attempted to identify individual dimer peaks formed during the direct photolysis under UVB(Q), however, it was proven challenging due to the small signal intensities. Instead, we took an alternative approach and obtained the total signal over the dimeric and monoaromatic regions. Further, we assessed and compared the temporal variations in % change in the ratio of the total MS signal intensity over the dimeric and monoaromatic regions (Figure 3g; Table S4). We observed that the ratio consistently increased (~35% after 120 min) upon exposure to UVB(Q) photolysis. However, it decreased (~10 to 15% after 120 min) during UVA(Q) photolysis (Figure 3g) signifying that potential formation of oligomers may be higher during UVB(Q) photolysis than UVA(Q). The standard deviation of % change in the ratio was within ± 10% indicating efficacy of the method in identifying oligomer formation qualitatively. For IND dung burning, we hardly observed any significant MS signal in the dimeric region (Figure S10) indicating oligomeric compounds were forming to a lesser extent compared to pine wood burning. The consistency observed between the potential oligomer formation and the absorbance behavior of pine wood and IND dung burning WS-BrC during direct photolysis in this study substantiate their relationship.
In summary, these observations signify two major findings: firstly, oligomer formation would be more likely during exposure to shorter wavelength UV-light. While previous studies used either UVB or UVA as light sources, our study represents the first direct comparison of the two. Secondly, the chemical structure of monomers and their abundance would be the driving factors for oligomerization during aqueous-phase direct photolysis. For example, previous studies have reported that monoaromatic compounds, such as guaiacol9, vanillin,4 syringol,5 and coniferyl aldehyde4 can react with each other and form light-absorbing oligomers during aqueous-phase UVB(Q) direct photolysis. These monoaromatic compounds (except syringol) were observed in emissions of all three solid biomass fuel burning types in this study (Table S5). It is noted that we did not observe syringol in our study, however, it has been reported to be a good tracer for wood burning.11 Additionally, the emissions of these monomers from wood burning were ~2 to 4 times higher than IND dung burning in our study (Table S5). A recent study3 has also reported that wood burning emissions are highly rich in lignin-degradation products (monomers) compared to animal dung cakes. The enrichment of such monoaromatic compounds is likely responsible for the more profound photo-enhancement in pine wood burning WS-BrC than dung burning observed in this study. Unfortunately, the method we employed did not allow molecular-level identification of oligomers that are responsible for the observed photo-enhancement. In future studies, instruments and method better suited for oligomer detection – e.g., SEC - should be employed to confirm our observation.”

Line 328: for Figure 3, the changes of MS intensity are not visibly significant for both UVB(Q) and UVA(Q) results. The authors are encouraged to present the experimental results in another way.
Response: Yes, we agree that our evidence of oligomer formation in Figure 3 is very subtle. We tried different ways to shows the results, but all our efforts (except the one shown in Figure 3) were futile. However, we have carried out some modification in Figure 3 in response to previous comments from the reviewers to make the oligomer formation evidence more noticeable in the revised manuscript as given below.
 
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21-Feb-2023

Dear Dr Zhao:

Manuscript ID: EA-ART-11-2022-000151.R1
TITLE: Aqueous-phase Photochemical Oxidation of Water-Soluble Brown Carbon Aerosols Arising from Solid Biomass Fuel Burning

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below. One review had a minor suggestion on the discussion of future directions. If possible, please consider briefly addressing that suggestion in your final uploaded version. I will not be sending out your uploaded version for further review.

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Reviewer 2

The authors have well adressed the issues raised by reviewers. I do not have further questions and comments. The work bridged the knowledge gap in the aqueous-phase chemical evolution of water-soluble light-absorptive species from burnings of dung cakes. The revised manuscript is well-written. I think this work is within the scope of Environmental Science: Atmospheres and recommend to accept the paper.

Reviewer 1

The manuscript revised by Choudhary et al. (ID: EA-ART-11-2022-000151) has addressed most of my comments, which can be accepted for publication on Environmental Science: Atmospheres after making minor changes.
For one of my previous major concerns, the authors have intensified the links between chemical composition (oligomer formation) and light absorption properties of BrC by adding more thoroughly reviewed literature and a more detailed discussion of the experimental observation. Besides SEC, I suggest the authors adding more future directions for audience who would like to perform aerosol oligomer research in from line 454 to 456. For instance, comparing MS signals among MSs coupled with different ionization sources may be helpful.
Line 395: should be “the observed loss”