Primary sources and secondary formation of brown carbon in urban Shanghai: Insights from bihourly measurements of brown carbon and organic tracers

Wang, Qiongqiong; Huang, Zhongliang; Li, Zongjun; Zhu, Shuihui; Qiao, Liping; Zhou, Min; Huang, Dandan; Wang, Hongli; Fu, Qingyan; Liao, Kezheng; Yu, Jian Zhen

doi:10.5194/egusphere-2026-4766

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https://doi.org/10.5194/egusphere-2026-4766

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18 Aug 2026

| 18 Aug 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Primary sources and secondary formation of brown carbon in urban Shanghai: Insights from bihourly measurements of brown carbon and organic tracers

Qiongqiong Wang, Zhongliang Huang, Zongjun Li, Shuihui Zhu, Liping Qiao, Min Zhou, Dandan Huang, Hongli Wang, Qingyan Fu, Kezheng Liao, and Jian Zhen Yu

Abstract. Brown carbon (BrC) is a significant component of atmospheric carbonaceous aerosols and has important influence on climate, atmospheric chemistry, and air quality. Our knowledge about its sources and atmospheric evolution is limited due to the challenges of accurate estimation of its mass concentrations. To address this gap, we examined concurrent measurements of bihourly BrC and a suite of organic molecular tracers in urban Shanghai from March to August 2020, BrC was determined by synergistically applying the Bayesian inference (BI) model to multi-wavelength aethalometer and total carbon (TC) measurements. BrC constituted 38–40 % of TC, with pronounced seasonal differences in diurnal evolution. The source apportionment analysis resolved 15 source factors and among them 7 are secondary sources, namely, secondary nitrate and sulfate formation processes, aromatic secondary organic aerosol (SOA), α-pinene SOA, β-caryophyllene SOA, isoprene SOA, oxygenated cooking OA, and secondary biomass burning. Secondary BrC accounted for 42 % and 49 % of total BrC in spring and summer, respectively. Seasonal contrast of the BrC contributions from primary and secondary sources were noticed. The formation mechanisms of secondary BrC were investigated by isolating events with continuous increment in secondary BrC concentrations, and the analysis suggested the diversity of oxidative pathways governing secondary BrC production. This study demonstrates the potential applicability of coupling the BI framework with online measurements of organic tracers for robust, chemically resolved BrC source apportionment. Our field observations provide the first quantitative source analysis and offer new insights into the primary sources and secondary formation of BrC in the urban atmosphere.

Received: 07 Aug 2026 – Discussion started: 18 Aug 2026

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Qiongqiong Wang, Zhongliang Huang, Zongjun Li, Shuihui Zhu, Liping Qiao, Min Zhou, Dandan Huang, Hongli Wang, Qingyan Fu, Kezheng Liao, and Jian Zhen Yu

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RC1:
'Comment on egusphere-2026-4766', Anonymous Referee #1, 04 Sep 2026 reply
Wang et al. report the characteristics and sources of brown carbon (BrC) in the urban atmosphere of Shanghai based on comprehensive multi-instrument measurements combined with positive matrix factorization (PMF) analysis. The authors identify 15 potential BrC sources and investigate their seasonal variations and diurnal pattern. In particular, the study identifies several secondary BrC episodes and discusses their potential formation pathways and products. Overall, the manuscript presents a comprehensive dataset and applies an interesting and potentially valuable analytical approach. However, I have substantial concerns regarding the PMF analysis and, in particular, the interpretation and source attribution of individual PMF factors. The robustness of the PMF solutions and the evidence supporting the identification and formation pathways of several factors are not yet sufficiently established. These issues should be addressed carefully before the conclusions can be fully supported. Therefore, I recommend major revision. My specific comments are provided below.
Lines 116–117: More details regarding the sampling setup should be provided, including the particle size cut, the environmental conditions surrounding the sampling site, inlet height, and other relevant information that may affect the representativeness of the measurements.

Line 148: What does WtC represent? Please clearly define this parameter and explain whether the absorption contribution from mineral dust was considered.

Lines 149–152: These two parameters are important for the subsequent analysis. Please provide more details on how they were determined, including the analytical procedures, assumptions, and associated uncertainties.

Line 173: Since the PMF analysis includes both organic and inorganic species, the term “organic-tracer-based source apportionment” may not accurately describe the approach. Please consider revising the terminology accordingly.

Lines 178–185: One of the strengths of this study is the inclusion of a wide range of chemical species measured by different instruments in the PMF analysis. However, this also raises an important issue regarding the construction of the PMF error matrix. Since the input species were measured using different instruments and analytical methods, please explain in detail how the errors for each species were determined and how the uncertainties from different instruments were made comparable. This information should be clearly described in the Methods section. In addition, the authors should carefully evaluate whether some species may be over weighted in the PMF solution. For example, TC includes contributions from BC, BrC, and other organic carbon components and may therefore receive relatively high weight in the analysis.

Lines 237–244: The information presented here appears to describe the methodology and would be more appropriately placed in the Methods section. In addition, obtaining a 15-factor solution is relatively complex. The authors should provide stronger justification for selecting 15 factors and demonstrate that this solution is statistically and physically robust.

Lines 253–263: More evidence is needed to support the attribution of these factors to specific sources. A factor should not be assigned to a particular source based on a single characteristic or tracer alone. Multiple independent lines of evidence, such as factor profiles, temporal variations, correlations with external tracers, meteorological dependence, and/or comparison with source profiles, would provide stronger support for the source attribution. In particular, what is the likely source of biomass-burning emissions in the urban area of Shanghai? Please provide additional discussion and, if possible, supporting observations or references.

Lines 286–287: Is the reported fraction calculated separately for each season?

Lines 338–342: Does NOx exhibit a corresponding variation during this period? Additional evidence is needed to support this conclusion. Furthermore, could the observed increase in concentration be affected by variations in planetary boundary layer (PBL) height? Changes in PBL dynamics can substantially influence surface pollutant concentrations, and this possibility should be considered and discussed.

Line 352: Please check whether there is a typographical error here.

Figures 1–4: The presentation of these figures could be improved. The current horizontal layout makes the information relatively difficult to follow, particularly when comparing multiple factors. A vertical arrangement may improve readability. For Figures 3 and 4, I suggest considering a more compact presentation in which the factor spectrum and the corresponding diurnal variation are displayed side by side. This would facilitate a more direct comparison between the chemical characteristics and temporal behavior of individual factors.

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Citation: https://doi.org/10.5194/egusphere-2026-4766-RC1
RC2:
'Comment on egusphere-2026-4766', Anonymous Referee #2, 05 Sep 2026 reply
Review of “Primary sources and secondary formation of brown carbon in urban Shanghai: Insights from bihourly measurements of brown carbon and organic tracers” by Wang et al.

The authors presented very comprehensive results on the sources of brown carbon (BrC) in urban Shanghai in spring and summer, 2020. The mass concentrations of BrC were resolved from a Bayesian inference (BI) approach, and the comprehensive chemical characterization results were subjected to positive matrix factorization (PMF) analysis together with BrC results. The authors showed that among the 15 identified sources, the 7 secondary sources contributed to over 40% of BrC in these two seasons. The authors further analyzed multiple cases with incremental secondary BrC growth, and showed that there were different pathways leading to secondary BrC formation at this urban site. The study was well designed and conducted, and the manuscript clearly written. I recommend Minor Revision, with comments as below.
PMF results: 1) the individual species measured by TAG-GC-MS contain useful information on the dynamics of their emission and formation. But it seems that some tracers are grouped together to run PMF, e.g., as aromatic SOA tracers, isoprene SOA tracers etc.). Please justify on why they are not used as individual species in PMF runs. 2) The first resolved factor of secondary nitrate/sulfate formation processes contains quite a bit of metal signals. Why are they associated with secondary formation?

The discussion on SOA contribution to secondary BrC (e.g., p10) should be backed by some evidence suggesting that these SOA types are really light-absorbing. For example, pinenes such as limonene might have been shown to generate SOA that can absorb near UV light, but does isoprene SOA really can contribute light absorption significantly? Or is it related to heavy nitration during its formation?

I understand that the main focus of this study is on BrC mass concentration. But the light absorption capability of BrC is really the key to its climate effect. I suggest that some data with quantified absorption capability (e.g., b_abs, MAE, AAE etc.) should be mentioned during the discussion.

Minor editorial comments:
L158: a “.” is missing after the citation that ends the sentence.

In a number of places where alpha-pinene and beta-caryophyllene were used as the first word of the sentence (e.g., L254-255), their first English letter should be capitalized, as “alpha-Pinene” etc.

L326: why high wind speed cases should be considered as local air masses? Should it be transported instead?

L341: the phrase “which shared NOx” reads odd here. Please rephrase.

L360: does this “high RH” condition hint on possible aqueous-phase processes?

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Citation: https://doi.org/10.5194/egusphere-2026-4766-RC2

Qiongqiong Wang, Zhongliang Huang, Zongjun Li, Shuihui Zhu, Liping Qiao, Min Zhou, Dandan Huang, Hongli Wang, Qingyan Fu, Kezheng Liao, and Jian Zhen Yu

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https://doi.org/10.5194/egusphere-2026-4766-supplement

Qiongqiong Wang, Zhongliang Huang, Zongjun Li, Shuihui Zhu, Liping Qiao, Min Zhou, Dandan Huang, Hongli Wang, Qingyan Fu, Kezheng Liao, and Jian Zhen Yu

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Short summary

Brown carbon (BrC) is a poorly understood carbonaceous aerosol component that significantly affects climate and air quality. We examined the sources and secondary formation of BrC based on long-term field measurements of bihourly BrC and a suite of organic and elemental tracers in urban Shanghai. Seasonal contrast of source contributions and complexity of secondary formation and evolution was observed. This study offers a scientific basis for BrC mitigation and urban air quality improvement.


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