the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Feb 2026
Characterization of the complex refractive index of a polluted dust storm over the Western Himalayas
Abstract. A polluted dust episode over the western Himalaya was characterized using in situ optical measurements, size-resolved aerosol sampling, single-particle analysis, and spectroscopic techniques. During the event, coarse-mode mass increased substantially, accompanied by elevated fine-mode particle number and black carbon concentrations. In situ observations revealed a threefold enhancement in the scattering coefficient and a 2–2.5-fold increase in the absorption coefficient relative to background conditions. Beyond these extrinsic property changes, intrinsic optical properties shifted markedly. Single scattering albedo and scattering Ångström exponent increased, and absorption Ångström exponent decreased, indicating compositional changes toward dust dominance with embedded anthropogenic absorbing aerosols. Single-particle analysis confirmed internal mixing of Fe-bearing mineral dust with carbonaceous material and secondary inorganic species. These compositional and morphological changes were reflected in the refractive index, which increased by approximately 4–6 % in the real part and 30–35 % in the imaginary part across visible wavelengths. The results demonstrate that polluted dust differs fundamentally from pure mineral dust and exhibits enhanced optical efficiency per unit mass and altered aerosol radiation interactions due to chemical transformation during transport. Accurate representation of such region-specific intrinsic properties is essential for quantifying aerosol radiative effects and boundary-layer interactions over the Himalayas.
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RC1: 'Comment on egusphere-2026-424', Sujai Banerji, 05 Apr 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-424/egusphere-2026-424-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-424-RC1 -
RC2: 'Reply on RC1', Sujai Banerji, 06 Apr 2026
Womack et al. (2021) retrieved complex refractive indices from multiple size-selected aerosol extinction measurements, together with measured size distributions and a least-squares retrieval algorithm under explicit model assumptions.
Citation: https://doi.org/10.5194/egusphere-2026-424-RC2
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RC2: 'Reply on RC1', Sujai Banerji, 06 Apr 2026
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RC3: 'Comment on egusphere-2026-424', Anonymous Referee #2, 20 May 2026
The authors present an analysis of particle optical property measurements made over India, focusing on measurements made during an approximately two-week period during the overall study. They use their measurements to characterize the air masses encountered during the two-week period into a “background” period and an “event” period, where the “event” period is referred to as a “polluted dust” period. They use their measurements to derive changes in the complex refractive index of the particle ensemble from the background to the event period, concluding that the “results demonstrate that polluted dust differs fundamentally from pure mineral dust and exhibits enhanced optical efficiency per unit mass and altered aerosol radiation interactions due to chemical transformation during transport.”
I find that the overall measurements seem of good quality and the general framework for the analysis is fine. However, I have serious concerns about the authors interpretation and presentation of their work. As noted above, they conclude that “that polluted dust differs fundamentally from pure mineral dust.” They have in no way demonstrated this. The fundamental problem with their interpretation is that they have not separated absorption by dust from absorption by black carbon. The authors determine complex refractive index values for the entire particle ensemble and make no effort to distinguish between black carbon and dust (and give no consideration to potential contributions from brown carbon). They refer to a particle classification scheme to argue that they are primarily looking at dust. However, they insufficiently recognize that the classification scheme is inherently imprecise, and mixtures of different particles types can drive the results. Based on the current analysis, the authors can at best conclude that the ensemble average imaginary refractive index increased during the event period over the background period. But they have not done an analysis that allows them to identify the reasons for this change and certainly have not demonstrated that the change is a result of chemical transformation of the dust during transport.
I think this manuscript could be acceptable for publication if the authors either fundamentally change their interpretation and discussion (including rewriting the abstract almost entirely, and certainly the second half) or fundamentally changing their analysis to try and distinguish between contributions from black carbon, brown carbon, and dust, and then pinpointing the dust properties. I suspect this latter approach will be very difficult and will likely end up with statistically insignificant results.
Now, if the authors simply mean by “polluted dust” that it is “dust that is internally mixed with BC” then they should say this clearly. But, they should also make clear that the change is driven by the introduction of BC into the system, and not by “chemical changes” to the dust itself.” They should also reframe their work, as this is not an especially novel result. If they mean instead to argue that it is chemical changes to the dust itself (rather than dust-containing particles) that drive the apparent optical changes, then I do not find that the analysis or data support the conclusions.
I will also note that the authors provide no statistical analysis to justify their conclusion that the properties during the event period differ from the background period. They show smoothed (very smoothed) distributions, but a statistical analysis comparing the background and event period is necessary if the authors wish to contend that there were real changes.
Citation: https://doi.org/10.5194/egusphere-2026-424-RC3 -
RC4: 'Comment on egusphere-2026-424', Anonymous Referee #3, 09 Jun 2026
Chandel et al. present observations of aerosol optical properties over the western Himalaya, with a particular focus on a selected dust-influenced period. The study combines in situ optical measurements, aerosol size distributions, black carbon measurements, and single-particle chemical analysis to characterize changes in aerosol properties during this period. The topic is potentially important because observations of aerosol optical properties in high-altitude Himalayan regions remain limited, and such measurements may be useful for evaluating the regional climatic effects of transported dust and pollution.
However, I have substantial concerns about the current manuscript. In my view, the manuscript is difficult to follow, the interpretation is often stronger than what the data can support, and the logic is not sufficiently consistent throughout the paper. The measurements themselves may be valuable, but the present analysis and discussion do not yet provide a robust basis for the major conclusions. I therefore do not think the manuscript is suitable for publication in ACP in its current form.
My first major concern is that the authors appear to overstate the distinction between the background and dust-event periods. The event is defined using a relatively modest PMcoarse threshold, but the evidence for a strong dust event at the sampling location is not fully convincing. From the temporal evolution of PMcoarse, a more pronounced coarse-mode enhancement appears later, around 26 May–3 June, yet this period is not selected or discussed in sufficient detail. The authors should clearly justify why 18–24 May was chosen as the primary dust-event period instead of the later period with apparently higher PM10 or PMcoarse levels. The MERRA-2 dust flux and CALIPSO information are used to support the selected period, but the connection between these regional-scale products and the actual surface measurements at the site needs to be made more quantitatively and cautiously. At present, the event selection appears somewhat arbitrary, and several of the subsequent conclusions depend strongly on this period definition.
Second, I am concerned that the interpretation of the optical measurements is not sufficiently rigorous. The authors report increases in scattering, absorption, SSA, SAE, MEE, and changes in AAE during the selected event period. These changes indicate that the aerosol ensemble changed during the event, but they do not by themselves demonstrate that the intrinsic optical properties of dust were fundamentally modified. In particular, absorption increased during the event together with BC concentration, and AAE decreased toward values more characteristic of BC-like absorption. This suggests that enhanced BC or other anthropogenic absorbing components may explain much of the absorption increase. The manuscript does not adequately separate the contributions of mineral dust, BC, and possible BrC to the retrieved absorption and imaginary refractive index. Therefore, the authors should avoid implying that the absorption enhancement reflects chemical transformation of dust itself unless they can explicitly demonstrate this separation.
Relatedly, the current wording throughout the manuscript gives the impression that the authors have characterized the refractive index of “polluted dust,” but the retrieval actually appears to represent an ensemble-average aerosol refractive index. This distinction is critical. If “polluted dust” simply means an aerosol population containing dust mixed with BC, sulfate, and other anthropogenic components, then the authors should state this clearly and interpret the results as ensemble-average optical properties of a mixed aerosol population. However, if the authors intend to claim that chemical aging of mineral dust itself caused the enhanced absorption or modified refractive index, the current data and analysis do not support that conclusion. The abstract, discussion, and conclusion should be substantially rewritten to avoid overclaiming.
Third, the discussion structure is difficult to follow. The manuscript often presents results in a report-like sequence rather than developing a coherent scientific argument. For example, the aerosol-type classification based on SAE–AAE appears suddenly in the optical-properties section, but it is not well integrated with the later chemical-composition analysis. The classification scheme is also inherently qualitative and should not be used as strong evidence that the observed optical changes are specifically caused by polluted dust. The chemical data should be used more directly and carefully to support, constrain, or qualify the optical interpretation. As written, the optical, chemical, and refractive-index sections are not sufficiently connected.
Fourth, the diurnal section does not yet provide a clear scientific contribution. It is not obvious why the diurnal analysis is central to the manuscript, and the current interpretation is not fully convincing. The authors argue that the diurnal behavior reflects boundary-layer growth and valley-scale transport, but the figures suggest that the differences between background and event periods are persistent across many hours rather than being limited to specific transport windows. If local or valley-scale processes enhance absorption around 08:00–09:00, the authors should explain why a similar feature is not more clearly observed during the background period. Likewise, if boundary-layer evolution controls the transport of polluted aerosol to the site, this mechanism should produce a more clearly defined enhancement during the event period. The discussion should also be revised because some interpretations do not appear consistent with the plotted data; for example, absorption appears to be strongly enhanced in the afternoon as well, not only in the morning. Unless the authors can provide a more quantitative and physically consistent interpretation, this section should be shortened or moved to supporting information.
Fifth, the chemical-composition analysis is not well aligned with the main background-versus-event framework. The main discussion up to Section 3.3 is based on a comparison between background and dust-event periods. However, Section 3.4 shifts to a comparison between two selected days within the dust-event period. The authors should explain why 20 May and 22 May were selected, whether they are representative of the event, and why comparable chemical measurements from the background period are not available. Without background chemical composition, it is difficult to use the SEM-EDX analysis to support claims about changes from background to polluted dust conditions. The chemical data may show that dust and carbonaceous particles coexisted during the event, but they do not clearly establish the magnitude or mechanism of the transition from background to event conditions.
Finally, the ACRI interpretation needs major revision. The authors state that both RRI and IRI show marked enhancement during the dust event, but this conclusion is not fully convincing from the current presentation. The reported increase in RRI appears relatively modest, and the IRI behavior is more difficult to interpret because it is strongly affected by absorbing components such as BC. The authors should provide formal statistical tests or uncertainty-aware comparisons between background and event periods before claiming marked enhancement. More importantly, the retrieved RRI and IRI should be described as properties of the aerosol ensemble rather than dust-specific values. The manuscript should not compare these ensemble-average values directly with literature values for relatively pure mineral dust without carefully acknowledging the role of BC, BrC, and other internally or externally mixed absorbing particles.
Overall, I find that the manuscript addresses a potentially interesting and under-observed region, and the measurement dataset may be valuable. However, the current manuscript substantially over-interprets the observations. The authors need to either fundamentally reframe the manuscript as an ensemble characterization of a mixed dust–pollution aerosol episode or conduct additional analysis that separates the optical contributions of dust, BC, BrC, and other aerosol components. Without such revision, the conclusion that polluted dust differs fundamentally from pure mineral dust due to chemical transformation during transport is not adequately supported.
Citation: https://doi.org/10.5194/egusphere-2026-424-RC4
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