the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Composition and Formation Mechanism of Brown Carbon: Identification and Quantification of Phenolic Precursors
Abstract. Light-absorbing organic aerosols, collectively known as brown carbon (BrC), significantly influence climate and air quality, particularly in urban environments like Dhaka, Bangladesh. Despite their significance, the contributions and transformation pathways of phenolic compounds—major precursors of brown carbon (BrC)—are still insufficiently understood in the South Asian megacities. This study addresses this gap by investigating the surface morphology of PM2.5, quantifying seven phenolic BrC precursors, and exploring the aqueous-phase formation pathway of nitrophenols at two urban sites (Dhaka South and Dhaka North) from July 2023 to January 2024. Phenolic compounds, including phenol, methylphenols, methoxyphenol, hydroxyphenol, and nitrophenol were identified and quantified using gas chromatography–flame ionization detection (GC-FID). PM2.5 surface morphology and elemental composition were analyzed via FESEM-EDX, and functional groups were characterized using ATR-FTIR. Results revealed that PM2.5 particles were predominantly spherical or chain-like with carbonaceous elements (C, O, N, S), mineral dust, and trace metals. The dominant functional groups included aromatic conjugate double bond, carbonyl, and nitro group. Aqueous-phase nitration of 2-hydroxyphenol under acidic conditions, analyzed via UV-Vis spectroscopy, demonstrated an alternative nitrophenol formation pathway. Among the detected compounds, 2-hydroxyphenol and 4-nitrophenol showed the highest average concentrations (2.31±1.39 and 2.20±1.21 µg m-3, respectively). Seasonal variations showed elevated nitrophenol levels during winter, especially in Dhaka South (4.54±2.94 µg m-3). These findings highlight the role of aqueous-phase reactions and urban sources in BrC formation, offering critical insights for future atmospheric modeling and air quality management strategies in South Asia.
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CC1: 'Comment on egusphere-2025-1954', Hong Wang, 02 Jul 2025
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CC2: 'Reply on CC1', Hong Wang, 01 Aug 2025
The authors had revised their manuscript carefully, now it is acceptable as it is.
Citation: https://doi.org/10.5194/egusphere-2025-1954-CC2
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CC2: 'Reply on CC1', Hong Wang, 01 Aug 2025
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RC1: 'Comment on egusphere-2025-1954', Anonymous Referee #1, 22 Jul 2025
Md. Al-amin Hossen et al. report new data sets on phenolic precursors identified in brown carbon (BrC) sampled in the megacity of Dhaka. The new data found by using different state-of-the-art instruments such as scanning electron microscopy and infrared spectrometry is of great importance. In addition, they improve the understanding of BrC formation in highly populated regions. However, the novelty of the results and the applied laboratory methods such as the nitration experiment lack a scientific standard to being published in ACP as a research article. Hence, I agree with the editor’s suggestion to publish this manuscript as a ‘Measurement report’ if the addressed revisions below are taken care of:
Major comments
- I believe the definition of BrC is not correctly introduced and hence, I suggest the authors review such parts of the text. Here is one examples: Abstract and line 48: Brown carbon is not the only OA particle type absorbing light: Please carefully read the definitions e.g. here: https://pubs.acs.org/doi/pdf/10.1021/cr5006167?ref=article_openPDF and rephrase.
- Especially the Methods section needs thorough revision on English grammar and check when to use past or present tense (past for experimental steps, present tense for general valid statements). E.g.:
- Line 125: change ‘for obtaining’ to ‘to obtain’
- Line 126: change ‘freeze-drying’ to ‘freeze-dried’
- Line 152: change ‘to identification and quantification’ to ‘to identify and quantify’
- Please use a table for the chemical reagent on line 195
- Please review the caption of all Figures and mention the abbreviations again so that one can understand the Figure without reading the main text. Also, it might facilitate to follow Figure 1 if you shortly summarize the main flows in the text after line 117.
- The paragraphs starting on line 222 reads like part of the Introduction. It does not describe or explain results you obtained but introduces aerosol nucleation theory. Please carefully revise this. Similarly, the first paragraph from line 248 and the first paragraph from line 341 on describe a method and not results.
- What does Fig. 4 add to the story? It seems that Fig. 4 shows redundant data already depicted in Fig. 3. Please explain or delete one.
- I have a hard time to understand how section 3.5 fits into this manuscript. May you elaborate on this new production mechanism of nitro-aromatic compounds and put it into atmospheric context such as its relevance? It may help if you show the differences between your newly discovered pathway and the one(s) elaborated in the literature.
Minor comments
- Please check all abbreviations and use them throughout the text. I have found e.g. ‘brown carbon’ even though you have introduced its abbreviation. E.g.: line 89.
- Please use ‘secondary organic aerosol (SOA) particles’ throughout the text and not ‘secondary organic aerosols (SOAs)' (e.g. line 26).
- Please do not use abbreviations if not introduced already (e.g. line 35)
- Please add column titles for Table 1.
- Line 43: Change ‘critical’ to ‘new’.
- Line 55: Add a citation or combine both sentences.
- Line 69: the citation is misplaced. Please review accordingly.
- Line 72: change ‘a study’ to ‘studies'.
- Line 83: change ‘are comes’ to ‘come’
- Please rephrase the sentence starting on line 85.
- Line 91: add ‘we’ before ‘focused’.
- Line 93: delete ‘in the laboratory’.
- Line 110: Change ‘concurrently’ to ‘simultaneously’.
- Line 207: Please revise the sentence.
- Line 209: Which panels in Fig. 2 are you referring to?
- The sentence on line 210 sounds like a repetition of the sentence above. Please revise.
- Line 211: You cannot say PM2.5 particles. Either aerosol particles or PM2.5.
- Panels a-c in Fig. 2: The font is too small. Either you get rid of all the text, or you enlarge the images. The size bar must be clearly readable. In addition, in panel e) one cannot see the description of the peaks in the spectrum and thus, the Fig. seems to be useless. Panel f) is a Table and not a Figure. Please revise.
- Line 237: I can only follow this from Table 2f.
- Line 241: What have the sulphur emissions with nitrogen in common? Otherwise, make two sentences, please.
- Line 277: Which phenolic compound? You did not elaborate this until now.
- Line 279: Again here, I am unsure why you add this here as you should talk about the results and not the introduction of and why you were looking for nitrophenols as SOA proxies.
- Line 286: Please already explain this in the first sentence of this paragraph and add it to the caption of Fig. 3. Please also explain the meaning of the grey error bar and show how you derived it in the main text.
- Line 300: Please use the same unit for readability throughout the text (ug instead of ng).
- Panel b) of Fig. 6 is a Table and not a Figure.
- Line 398: change ‘has been’ to ‘have been’.
- Conclusions: Please do not distinguish between ‘Summary’ and ‘Conclusions’, but incorporate it smoothly in the main text of the ‘Conclusions’.
Citation: https://doi.org/10.5194/egusphere-2025-1954-RC1 -
RC2: 'Comment on egusphere-2025-1954', Anonymous Referee #2, 31 Jul 2025
The manuscript discusses the composition of brown carbon in PM2.5 collected from Dhaka, Bangladesh. The manuscript focuses on the composition of phenolic precursors of brown carbon in PM2.5. The surface morphology and elemental composition were also analysed in this study. Overall the study is interesting and worth publication since it is important to have information on brown carbon in the South Asian region.
Detail comment
1. The introduction of the manuscript requires improvement to ensure a better flow of information and proper paragraphing. The first paragraph is too long and should be separated into different paragraphs.
2. There is no information on surface morphology and its relationship with brown carbon and phenolic compounds presented in the introduction.
3. The objective of the manuscript presented in the last part of the introduction is considered weak and needs further clarification and improvement.
4. What is the connection between surface morphology and brown carbon presented in the results of this study? I can't see any clear discussion on the relationship between them in the Results and Discussion.
5. The authors should include the descriptive data of the phenolic compounds as supplementary material, providing additional important information beyond just the figures in the manuscript.
6. Section 3.5: Should the methodology of this section and the formula of the interaction be included in the Methodology Section?
Citation: https://doi.org/10.5194/egusphere-2025-1954-RC2 -
AC1: 'Comment on egusphere-2025-1954', Abdus Salam, 08 Sep 2025
Dear Editor,
We are happy to inform you that we were able to respond all the comments of two anonymous referees (RC1 and RC2) and one community comments (CC), and submitting the responses as the supplementary file. We have also revised the manuscript according to their suggestions - if you confirm us then we can submit the revised version of the manuscript.
Please have a look and also please inform us if you need anything.
Kind regards
Abdus Salam, PhD
On behalf of all coauthors.
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This research focuses on brown carbon (BrC) phenolic precursors in the South Asian megacity of Dhaka, filling a gap in the region's understanding of PM2.5 in the quantification of phenolic compounds and the study of formation mechanisms, which is of great value in understanding atmospheric pollution and climate change in tropical cities. Combining various techniques such as GC-FID, FESEM-EDX, and ATR-FTIR, we systematically analyzed the chemical composition, surface morphology, and functional groups of PM2. The chemical composition, surface morphology and functional groups of PM5 were systematically analyzed, and the aqueous-phase nitrification experiments also provided evidence for a new mechanism of BrC formation. The research design is reasonable, the data are detailed, the logic is clear, and the conclusions are reliable and of publication value. I suggest a slight revision for publication in this journal.