the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Sep 2026
Contrasting absorption and transport regimes of black and brown carbon across the western, central, and eastern Himalayas
Abstract. Carbonaceous aerosols strongly influence atmospheric radiative forcing over the Himalayas, yet their spatial variability and transport mechanisms remain poorly constrained. Here we present high-time-resolution measurements of black carbon (BC), primary brown carbon (BrCpri), and secondary brown carbon (BrCsec) at three high-altitude sites across the western (Purang), central (Yadong), and eastern (Zayü) Himalayas during the pre-monsoon period. Distinct spatial gradients in light absorption and biomass burning (BB) tracers (levoglucosan and K⁺) reveal pronounced regional heterogeneity. Yadong exhibits the strongest absorption (Babs: 53.1 ± 18.5 Mm−1 at 370 nm; 14.2 ± 5.5 Mm−1 at 880 nm) and the highest BB influence, indicating a central Himalayan hotspot of polluted aerosol accumulation. BC dominates light absorption across all sites, with its contribution increasing eastward and reaching ~95 % at 660 nm in Zayü. In contrast, BrC contributions exhibit strong regional contrasts, with BrCpri dominating absorption in Yadong (8–28 %) and BrCsec enhanced in Purang (3–26 %). Trajectory analyses indicate that central Himalayan aerosol loading is strongly modulated by orographic uplift and terrain-driven circulation. CALIPSO observations further demonstrate a transition from mineral dust in the west, to pollution-modified dust in the central Himalayas, and biomass-burning aerosols in the east. These results improve our understanding of the sources, transport pathways, and atmospheric transformations of BC, BrCpri, and BrCsec in the Himalayas.
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Status: open (until 05 Nov 2026)
- RC1: 'Comment on egusphere-2026-4438', Anonymous Referee #1, 20 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-4438', Anonymous Referee #2, 26 Oct 2026
reply
General Comments
This manuscript investigates the optical properties, sources, and transport of black carbon (BC) and brown carbon (BrC) across three Himalayan regions using ground-based measurements, chemical tracers, trajectory analysis, and satellite observations. The observational dataset is potentially valuable, particularly given the scarcity of ground-based measurements in this region.
However, the manuscript has substantial weaknesses in its experimental design, methodological justification, integration of different datasets, and scientific interpretation. Although numerous analyses and figures are presented, the scientific insights remain limited, and several conclusions are not sufficiently supported by the available evidence. The overall organization and presentation also require considerable improvement.
I recognize the potential value of this study, but the manuscript is not yet sufficiently mature for publication. Substantial revision and further analysis are necessary to improve its scientific rigor and overall quality.
Major Comments
1.Experimental design and measurement quality control
The rationale for conducting measurements at three different sites in three different years (2021–2023) is insufficiently explained. It is also unclear whether the same AE33 instrument was used at all sites or whether different instruments were deployed. The authors should clarify the experimental design and provide essential information on instrument calibration, maintenance, quality control, and data completeness.
2.Comparability of observations
The three sites differ substantially in observation year, elevation, and environmental conditions. Therefore, the observed differences cannot be directly interpreted as regional spatial patterns without accounting for interannual variability and meteorological influences. The authors should justify the comparability of these datasets or appropriately qualify their spatial conclusions.
3.Methodological assumptions and data consistency
The separation of BC, BrCpri, and BrCsec relies on assumptions, including a fixed AAEBC of 1.1 and the MRS method, whose applicability and uncertainties are insufficiently evaluated. Furthermore, the reported total absorption coefficients at 370 nm appear inconsistent with the sums of the individual components in Sections 3.1–3.3. These discrepancies require careful verification, together with appropriate sensitivity and uncertainty analyses.
4.Insufficient integration of observational and modeling evidence
The AE33 measurements, chemical tracers, HYSPLIT trajectories, and CALIPSO observations are largely presented as separate analyses. Their temporal and spatial relationships, as well as their combined ability to support the proposed source and transport mechanisms, remain unclear. In particular, the limited satellite and trajectory analyses are insufficient to establish generalized regional transport regimes without further validation.
5.Limited scientific insights and insufficient support for the conclusions
Despite the numerous figures and analytical approaches, much of the Results and Discussion remains descriptive. The proposed explanations for regional differences, secondary BrC formation, orographic effects, and radiative impacts are not adequately supported by independent evidence. The simplified assumptions used in the radiation absorption estimates also require further justification. The authors should critically reassess the reliability and scientific significance of their main conclusions.
6.Weak logical structure and insufficiently focused Introduction
The Introduction lacks a clear logical progression and is not sufficiently focused on the specific scientific questions addressed in this study. Much of the background discussion is general, while the connections among previous studies, knowledge gaps, research objectives, and the adopted methods remain weak. Similarly, the Results and Discussion section lacks a coherent scientific narrative, with several analyses appearing loosely connected or repetitive. Substantial restructuring is needed to improve the focus, logical flow, and scientific depth of the manuscript.
Minor Comments
1.Reference formatting
The reference list contains numerous inconsistencies in journal names, abbreviations, capitalization, and author formatting. For example, Environmental Science & Technology is written as "Environmental science and technology" in Line 600 but as "Environ. Sci. Technol." in Line 615. The authors should carefully review and standardize the entire reference list according to the journal guidelines.
2.Inconsistent information and duplicated funding statements
The elevations of some sampling sites differ between the main text and Table S1, and the wavelength information used for AAE calculations requires verification. In addition, nearly identical funding information appears in both the Acknowledgments and Financial support sections. These inconsistencies should be corrected.
3.Figure references and presentation
Some figures and their subpanels are not adequately referenced or discussed in the main text. For example, Figure 5 contains nine subpanels (a–i), but only panels (a–c) are explicitly referenced, while the remaining panels receive limited discussion. The figure caption also incorrectly refers to panels (d–g) instead of (d–i). The authors should carefully review all figures for similar issues and make the necessary corrections to ensure consistency between the figures, captions, and corresponding discussions.
Overall Assessment
The observations presented in this study have potential scientific value. However, substantial improvements are needed in experimental documentation, methodological validation, data consistency, scientific interpretation, and manuscript organization. I therefore cannot recommend publication in its present form and encourage the authors to undertake a thorough revision before the manuscript can be reconsidered for publication.
Citation: https://doi.org/10.5194/egusphere-2026-4438-RC2
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- 1
Review comments on “Contrasting absorption and transport regimes of black and brown carbon across the western, central, and eastern Himalayas” by Ju-Lian Shi et al. submitted to Atmospheric Chemistry & Physics.
###########General Comments#########
This manuscript investigates the light-absorption characteristics and transport mechanisms of black carbon (BC) and brown carbon (BrC) across three high-altitude Himalayan sites, combining ground-based measurements, chemical tracers, trajectory analysis, and CALIPSO observations. The study addresses an important topic and provides valuable observations in a relatively understudied region. However, I have concerns regarding the methodology used to separate BC, primary BrC (BrC_pri), and secondary BrC (BrC_sec). In particular, the assumption of a fixed BC absorption Ångström exponent (AAE_BC = 1.1) may introduce systematic biases into the estimated BrC absorption, potentially affecting the central conclusions regarding regional differences and radiative contributions. Additional concerns relate to the observations of aerosol source attribution and the representativeness of satellite observations used in the anaylisi are not acquired synchronously.
The manuscript contains potentially valuable observations, but the reliability of its central findings depends critically on the validity of the BC–BrC absorption separation. Substantial methodological validation and uncertainty assessment are required before the conclusions can be considered for publication in ACP. Overall, I would recommend major revision, with particular emphasis on validating the BC–BrC separation framework and demonstrating the robustness of the main conclusions.
###########Specific Comments#########
Section 2.1: Measurements at Purang, Yadong, and Zayü were conducted in different years (2021, 2022, and 2023, respectively). Interannual variations in biomass burning and atmospheric circulation may substantially influence the observed differences. The conclusions regarding west-to-east spatial gradients should be appropriately qualified.
Method Section 2.2:
L131-137: The BC–BrC separation assumes a fixed AAE_BC of 1.1 and extrapolates BC absorption from 880 nm to shorter wavelengths. However, coated BC may exhibit non-power-law spectral behavior due to variations in core size, coating thickness, and mixing state. Simply varying the AAE value would not fully address this structural uncertainty.
Sections 3.5-3.6: CALIPSO profiles from only three selected pollution episodes are insufficient to establish representative regional differences in aerosol vertical structure. The authors should clarify the temporal and spatial collocation with ground observations, discuss CALIOP aerosol classification uncertainties, and avoid generalizing individual case studies to seasonal regional characteristics.
L187-198: The authors should clarify CALIOP aerosol subtypes are algorithm-derived classifications rather than direct measurements of aerosol chemical composition, and discuss uncertainties in distinguishing dust, polluted dust, and smoke, particularly over complex terrain. The spatial and temporal correspondence between satellite observations and ground measurements should also be provided.
L206-207 and L267：Some reported absorption coefficients appear inconsistent between Sections 3.1 and 3.3 (e.g., the values reported for Yadong at 370 nm). These discrepancies should be checked and clarified.