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| 28 Jul 2026
Analysis of the Composition and Direct Radiative Effects of Polluted Dust Using Satellite Observations and Reanalysis Data
Abstract. Mineral dust particles influence the global energy budget through interactions with both solar and terrestrial radiation. During long-range transport, dust can be not only externally but also internally mixed with local pollutant aerosols such as sulfate, nitrate, organic carbon, and black carbon. These physical and chemical interactions alter the optical properties of the resulting polluted dust particles, which remain a major source of uncertainty in computing the dust direct radiative effect (DRE). This study aims to quantify how pollution influences the shortwave and longwave optical properties and the direct radiative effects of dust aerosols using observational and reanalysis data. Following previous studies, a concentric spherical core-shell model is adopted to simulate the mixing state of dust-sulfate mixtures. In the urban environment, the pollutants such as sulfate and nitrate mainly originate from industrial activities. In the shortwave region, internal mixing of dust and pollution reduces the atmospheric reflectance, increases radiative absorption, resulting in a weaker enhancement of optical depth than that of external mixing. In the longwave region, internal mixing absorbs more radiation than external mixing, resulting in a stronger positive DRE. The study highlights the need for improved representation of dust–pollution mixing in climate models.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2026-4228', Anonymous Referee #1, 11 Aug 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-4228/egusphere-2026-4228-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-4228-RC1 -
RC2: 'Comment on egusphere-2026-4228', Anonymous Referee #2, 05 Sep 2026
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This preprint (egusphere‑2026‑4228) combines CALIOP satellite lidar observations, MERRA‑2 reanalysis datasets, and core‑shell Mie scattering radiative transfer simulations to identify global climatological hotspots of polluted dust and characterize aerosol compositional features across these hotspots. It further investigates how external versus internal mixing modulates short‑wave (SW) and long‑wave (LW) optical properties and direct radiative effects (DRE) of polluted dust and reveals a novel SW‑LW asymmetric response of dust radiative effects driven by internal mixing. The topic is scientifically valuable. The work helps reduce uncertainties in simulated dust radiative forcing and provides useful insights for improving polluted‑dust parameterizations in climate models. Major weaknesses stem from idealized model assumptions, biases in MERRA‑2 reanalysis outputs, radiative simulations limited exclusively to China, together with incomplete methodological descriptions and ambiguous scientific analyses in several sections. Substantial revisions addressing these major concerns will significantly strengthen the manuscript and bring it up to journal publication standards.
Major comments
- The text briefly describes polluted‑dust identification using CALIOP Level‑2 data and the use of ERA5 reanalysis, yet critical information on how each aerosol parameter is derived remains missing. Examples include how MERRA‑2 outputs are temporally and spatially matched against CALIOP observations; which input datasets are used when recalculating AOD differences for different mixing states (Figures 7 and 8); how extinction efficiency and single‑scattering albedo (SSA) are computed; and the full procedure for DRE calculation. Many methodological steps are insufficiently documented. It is recommended to expand a dedicated methodology section to explicitly describe the derivation of each aerosol‑related variable, detail satellite‑reanalysis spatiotemporal matching procedures, and include essential formulas or appropriate algorithm citations.
- Three polluted‑dust hotspots are selected for December‑January‑February (DJF), but the explicit justification for choosing DJF is absent. Moreover, Figure 1 suggests that the Middle‑East region (10‑90° E, 0‑30° N) also represents a prominent polluted‑dust hotspot, which is not addressed in the current analysis. The authors should explain why DJF is chosen as the target season and discuss polluted‑dust features over the Middle‑East domain.
- The manuscript acknowledges that MERRA‑2 systematically underestimates dust AOD while reproducing seasonal phases consistent with CALIOP; for this reason MERRA‑2 is used only for relative sensitivity tests. However, the negative bias in MERRA‑2 dust AOD may not only alter absolute radiative‑effect magnitudes but also modify dust‑sulfate mass ratios, which in turn changes core‑shell coating thickness and could perturb the relative differences between internal‑ and external‑mixing scenarios. The current comparison is based purely on column‑integrated AOD. Vertical structures strongly influence radiative calculations; comparisons of extinction‑coefficient profiles between CALIOP and MERRA‑2 should be added.The method for retrieving dust AOD from CALIOP is not described and needs to be documented. The discussion section must elaborate on how such biases may impact the key finding of SW‑LW asymmetry, rather than simply stating that relative comparisons remain unaffected.
- Figure 3 shows vertical mass‑concentration profiles for different aerosol components, whereas extinction‑coefficient profiles (analogous to Figure 9) would be more relevant for this radiative‑focused study. Figure 3 should be replaced with extinction‑coefficient profiles. Additionally, only DJF profiles are displayed, while subsequent DRE simulations are not restricted to DJF. This inconsistency requires clarification.
- All internal‑mixing simulations adopt idealized concentric spherical core‑shell geometries with three sulfate‑coating allocation schemes. Real‑atmosphere polluted dust exhibits complex microphysics including non‑sphericity, hygroscopic growth, partial and heterogeneous coatings, as well as dust‑carbonaceous aggregates. All three implemented schemes assume complete spherical coatings and lack scenarios for partial coating.
- Expanded discussion is required on biases induced by perfect core‑shell assumptions, clarifying the validity domain of the presented results.
- Consistent with the viewpoint of Reviewer 1, this study focuses on sulfate‑dominated coatings, while nitrate is also an important coating material for atmospheric polluted dust. The potential impacts of nitrate coatings should be discussed, and it should be explicitly stated that the findings cannot be directly generalized to nitrate‑dominated polluted‑dust regimes.
- CALIOP can classify aerosol layers as polluted dust but cannot distinguish internal from external mixing. The study identifies hotspots from CALIOP observations and then conducts idealized internal‑/external‑mixing simulations without observational constraints on mixing‑state fractions. So discuss to what extent existing satellite and in‑situ measurements support the assumption of fully sulfate‑coated internally mixed particles within these hotspots. Summarize field‑based evidence supporting or contradicting core‑shell particle assumptions across the three hotspots. Since in‑situ measurements indicate that internal‑mixing fractions vary continuously in nature, robustness of conclusions across variable internal‑mixing fractions should be further explored.
- As reviewer 1 mentioned radiative‑sensitivity experiments are only performed for eastern China. West‑Central Africa is characterized by dust mixed predominantly with organic carbon, and aerosol compositions vary with altitude over the Indian Subcontinent. Radiative impacts for these two hotspots remain untested. Therefore, it is unclear whether the SW‑LW asymmetry conclusion can be generalized to these other regions. At minimum, qualitative discussion is needed on how organic‑carbon coatings and vertically variable aerosol composition may alter optical properties and radiative responses. Idealized supplementary simulations for West‑Central Africa or the Indian Subcontinent are encouraged if computational resources permit.
- Comparisons of complex refractive indices among pure dust, pure sulfate and core‑shell particles should be provided to explain the fundamental physical reasons driving opposite responses between short‑wave and long‑wave bands, which will improve readability for readers regarding this key novel finding.
Minor comments
- The unit for attenuated backscatter should be Mm⁻¹ sr⁻¹ or km⁻¹ sr⁻¹. Extinction‑coefficient values in Figure 9 should be reported in Mm⁻¹. The fact that derived extinction coefficients at 550 nm and 10 μm only differ by a factor of ~5 appears suspicious; please double‑check calculations and unit conversions.
- The legend of Figure 11 contains overlapping curves for land‑ and ocean‑mean DRE as a function of internal‑sulfate fraction, reducing readability. Optimize the legend or split sub‑panels. Several key results are placed only in supplementary materials; move critical figures into the main text.
- Section 4.2 reports numerous quantitative DRE values for land, ocean and the Chinese Capital Region. A summary table compiling key DRE metrics (pure dust, fully external mixing, three internal‑mixing schemes) for TOA and surface, for SW, LW and net radiative effects is recommended for convenient reference.
- Add recent relevant publications addressing dust non‑sphericity effects.
- Explicitly state the shell‑volume proportionality relationships for the three schemes within the caption of Figure 6.
Citation: https://doi.org/10.5194/egusphere-2026-4228-RC2 -
RC3: 'Comment on egusphere-2026-4228', Anonymous Referee #3, 06 Sep 2026
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Review of "Analysis of the Composition and Direct Radiative Effects of Polluted Dust Using Satellite Observations and Reanalysis Data" by Ronghao Wang et al.
Recommendation: Reject
The manuscript "Analysis of the Composition and Direct Radiative Effects of Polluted Dust Using Satellite Observations and Reanalysis Data" utilizes CALIPSO and MERRA-2 data to identify high-occurrence regions of polluted dust and discusses the impacts of the sulfate-dust mixing state on shortwave and longwave radiation. However, the first issue addressed in this manuscript lacks novelty, as the initial few figures primarily present topics frequently covered in previous literature. The main valuable discussion appears in the second part, which is indeed a crucial topic in aerosol radiation research; nevertheless, a number of issues remain unresolved or unclearly articulated. My comments are listed as follows:Major Comments:
1.The topics addressed in the first five figures are well-worn issues that have been extensively covered in previous literature, offering few particularly new findings. Furthermore, these figures occupy a relatively large portion of the manuscript, failing to highlight the unique value and novelty of this work.
2.The main optical and radiative conclusions regarding internal versus external dust–sulfate mixing appear highly similar to those already reported in the previous theoretical study (Zhang et al. (2024)), particularly the weaker SW cooling and stronger LW warming associated with internally mixed/coated dust. The authors should clearly distinguish which results are genuinely new and which represent regional applications or quantitative extensions of established mechanisms.
3.CALIOP polluted-dust classification and MERRA-2 aerosol composition can demonstrate the coexistence of dust and pollution, but they cannot determine whether sulfate is externally mixed, partially coated, or forms a core–shell structure around dust particles. Therefore, the observational analysis does not directly support the specific internal-mixing configurations assumed in the radiative calculations.
4.The observational and modeling components are only indirectly connected. The manuscript effectively assumes the pathway “polluted dust + sulfate coexistence → sulfate-coated dust,” but this transition is not observationally demonstrated.
5.The large DRE differences between fully external and fully internal mixing represent differences between two idealized end-member assumptions, rather than estimates of the actual atmospheric bias. Without observational constraints on the fraction of internally mixed particles, these values should be interpreted as sensitivity bounds rather than realistic corrections to atmospheric DRE.
6.The manuscript does not describe the methodology in sufficient detail, nor does it contain a dedicated section to thoroughly discuss the models, equations, and formulations used in this study.
Specific Comments:
1. The background introduction in the abstract accounts for too large a proportion, and the summary of the main conclusions is inadequate, lacking any quantitative description.
2. Page 4, line 123: In Figure 1, it should refer to "white boxes" rather than "black boxes".
3. In Figure 2, please adjust the position and size of the legend, as it currently obscures other details of the figure.
4. It is recommended to keep the legend labels consistent between Figure 2 and Figure 3.
5. In Figure 4, the subplots in the second row should have their own panel labels.
6. The authors' justification for the plausibility of the concentric core-shell model appears somewhat overstated. While Unga et al. (2018) indeed provided observational evidence for coated or core-shell particles, this does not prove that such morphology broadly represents polluted dust, nor does it demonstrate that this model is sufficient for accurately estimating dust DRE.
7. Regarding the statement: "This SW-LW asymmetry in terms of the impacts of internal mixing on dust DRE is, to our knowledge, a new result of the present study.", I believe this finding/novelty has already been reported in previous literature.
Citation: https://doi.org/10.5194/egusphere-2026-4228-RC3 -
RC4: 'Comment on egusphere-2026-4228', Anonymous Referee #4, 08 Sep 2026
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Please find the review comments in the attached file.
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