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
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-424/egusphere-2026-424-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-424-RC1 -
RC2: 'Reply on RC1', Sujai Banerji, 06 Apr 2026
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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
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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
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