the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2025
Understanding divergent Brown Carbon Photobleaching Rates from Molecular Perspective
Abstract. The global radiative effect of brown carbon (BrC) remains highly uncertain. BrC’s photobleaching, which significantly alerts its radiative effect, has been still poorly constrained. This study investigates photobleaching rates of laboratory-synthesized secondary BrC (aq-BrC), biomass burning-derived BrC (b-BrC), and ambient PM2.5-derived BrC (p-BrC). Our results reveal a source dependence in BrC photobleaching rates. The highest photobleaching rate constant (kBrC) is observed for aq-BrC (1.13 ± 0.08 h⁻¹), followed by p-BrC (0.12 ± 0.02 h⁻¹) and b-BrC (0.05 ± 0.01 h⁻¹), indicating the stable light-absorption capacity of b-BrC in the atmosphere. The OH oxidation of imidazole-2-carboxaldehyde (2-IC) and methylglyoxal oligomers, nitrophenols (including phenols), and lignin derivatives governs the photobleaching of aq-BrC, p-BrC, and b-BrC, respectively. The high kBrC of aq-BrC is attributed to the high reactivity of the chain structures in 2-IC and methylglyoxal oligomers. In contrast, the highly conjugated structures of lignin derivatives in b-BrC impart stability against OH oxidation, resulting in a low kBrC. Our findings reveal the significant differences in the photobleaching behavior of BrC originated from different sources, underscoring the crucial need to account for source differences in assessments of BrC’s global radiative forcing effect.
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Status: open (until 05 Jan 2026)
- RC1: 'Comment on egusphere-2025-5502', Anonymous Referee #1, 22 Dec 2025 reply
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- 1
In this study, the authors report on the aqueous OH-initiated oxidation of secondary, primary, and ambient BrC. In the experiments, the authors monitored changes in absorption, using UV-vis spectroscopy, and composition, using high-resolution mass spectrometry, and the experiments appear to have been done carefully. Representatives of primary and secondary BrC were chosen based on the objective of synthesizing the differing rates of photobleaching, known from the literature. The structure of the study and the presentation of the results in figures are generally clear. On the other hand, some of the rationale (e.g., for the choice of secondary BrC) and word choice are not clear. With revisions, the manuscript may be suitable for publication in ACP.
1 - The title is suitable and describes an important problem of broad interest to the readers of ACP - note that divergent should be capitalized.
19 - Here, alerts should be affects - throughout, there are odd typos and word choices, which I will not list out in detail. A revision for clarity of language is needed.
20 - I appreciate the list of the three BrC types here, but a brief clause about the motivation (e.g., to resolve differences in rates of photobleaching in the literature) would be helpful, as well.
32 - I think the abstract should include something about the effort to synthesize the divergent rates here and in the literature (i.e., represented in Fig. 1b).
70 - Does the prefix p- stand for primary here? If so, some rationale regarding the absence of secondary BrC in the ambient samples should be provided.
71 - This is not a particularly compelling way to frame the results - it is well known already that rates for these different types of BrC are different, from the literature.
78 - The products of methylglyoxal and ammonium sulfate represent just one sub-type of secondary BrC - another type, e.g., is from the aqueous processing of phenolic precursors. What was the rationale for this choice? I am skeptical that it is a representative choice partly because it is known to photobleach so much faster than ambient secondary BrC.
89 - This is where I was expecting some rationale for labelling ambient samples with p- for primary.
103 - A brief description of how this scaling factor was determined would be helpful.
135 - It would be helpful to specify that the decay discussed here is the overall decay, rather than the route-specific decays, which are discussed later.
144 - It is not clear to me why the left- and right-hand tick marks are labelled with different numerical values.
166 - My perception is that much of the novelty of the study lies in this rough clustering of rates with BrC type.
195 - A table of values of k_BrC; k_BrC,OH; k_BrC,pho; and k_BrC,ctrl would be helpful for clarity. Additional comparisons to route-specific values from the literature are warranted.
198 - The predominant role of OH oxidation is presented here as a central conclusion of this study - some discussion of past studies, which have also shown this role extensively, should be included.
234 - Phenols could also be representative of secondary BrC, if it came from phenolic precursors - an acknowledgement of the variable composition of secondary BrC should be included.
248 - Include here why exact mass concentration cannot be derived (i.e., unknown ionization efficiency and lack of standards).
265 - Have these species been identified in biomass burning BrC previously? What other studies have investigated BrC from corn straw?
271 - This conclusion is well-known - indeed, it was presented as the motivation for undertaking this study in the first place. I recommend reframing the Conclusion to focus on new insights.
286 - I recommend acknowledging that even perfect knowledge about the sources of BrC is not enough to accurately predict its climate effects - e.g., impacts of environmental conditions (temperature and relative humidity) are also important.