| 24 Jun 2026
Range-resolved retrieval of aerosol particle size distribution during Saharan dust intrusions over the Bavarian Alps using multiwavelength lidar observations
Abstract. We present ground-based remote sensing observations of Saharan dust over the Bavarian Alps during three events on 29 February, 29 March, and 29 April 2024, with emphasis on aerosol particle size distribution (APSD) retrieval from multiwavelength lidar measurements. Raman–depolarization lidar observations from Garmisch-Partenkirchen were combined with sun-photometer data from the summit of Zugspitze. Long-range transport from North Africa was confirmed using back-trajectory analysis and aerosol forecasts. Elevated dust layers extending up to 6–7 km a.s.l. were observed, frequently structured into multiple layers.
APSDs were retrieved by applying an inversion method that directly substitutes predefined (bi-modal log-normal) size distributions into the lidar equations, reducing the inversion to the estimation of a limited set of microphysical parameters. This approach enables range-resolved retrievals of effective particle radius, yielding values up to 3–4 µm within dust layers and decreasing with altitude. Comparison with column-integrated sun-photometer retrievals shows consistent coarse-mode effective radii (1.2–1.4 µm). The results highlight both the strengths and limitations of APSD retrieval from lidar, particularly regarding assumptions on particle shape and refractive index.
The manuscript addresses an important topic: retrieving range-resolved aerosol particle size distributions during Saharan dust outbreaks using multiwavelength lidar observations. The combination of lidar, AERONET observations, trajectory analysis, and operational models provides a valuable observational framework. The paper demonstrates that the APSD inversion software can be successfully applied to real dust cases over Central Europe and presents interesting first results. I really like that the authors openly acknowledge the limitations associated with spherical particle assumptions and fixed refractive indices.
However, some improvements of the current manuscript are still needed, and should focus on strengthening the discussion of retrieval uncertainties, clarifying methodological assumptions, and refining the presentation. There is no need in additional measurements or a different analysis. Specifically:
1. The manuscript presents retrieved effective radii but provides almost no quantitative uncertainty estimates. Since aerosol microphysical retrievals are highly sensitive to measurement errors and model assumptions, uncertainty bars or sensitivity analyses can be very helpful.
2. For mineral dust, Mie theory is a considerable simplification. Although the authors acknowledge this issue, the manuscript would benefit from a more quantitative discussion of how particle nonsphericity could bias the retrieved effective radius.
3. The inversion is based on a predefined bimodal lognormal distribution with several fixed parameters. Such constrained inversions may produce stable solutions but not necessarily unique ones. The paper should discuss more explicitly how strongly the retrieved APSDs depend on these assumptions.
4. Limited validation: The retrieved effective radius is compared only with AERONET coarse-mode values. Additional validation against established lidar inversion products, GRASP retrievals, or previous dust studies would strengthen confidence in the method.
5. The conclusions are based on three individual dust events. The presented cases demonstrate the applicability of the APSD retrieval method to Saharan dust observations. However, since only three events are analyzed, I recommend emphasizing that the reported effective-radius values and vertical structures are representative of the investigated cases rather than of Saharan dust transport in general. The broader applicability of the method could be highlighted as a perspective for future studies involving a larger statistical dataset. For example, in the Conclusions the manuscript currently states: "These observations provide compelling evidence for the frequent and significant impact of Saharan dust on atmospheric composition over Central Europe...", I would recommend something like: "The analyzed events illustrate the significant impact that Saharan dust intrusions can have on atmospheric composition over Central Europe." Similarly, instead of: "Effective radius profiles consistently showed large particle sizes..." I would suggest: "In the analyzed events, the retrieved effective-radius profiles showed large particle sizes within the identified dust layers..." Authors shall consider what is better and decide revision of the text.
6. Broader implications should be emphasized. maybe even in a separate section. The conclusions mainly summarize the observations. The manuscript would have greater impact if it discussed how this retrieval approach could contribute to future aerosol monitoring within ACTRIS, satellite validation, aerosol data assimilation, or climate model evaluation.
7. The retrieved effective radii should be discussed more extensively alongside previous lidar studies of Saharan dust, particularly those from SAMUM, EARLINET, and long-range transport experiments, to better position the present results within existing knowledge. I was missing these additional studies and literature background.
8. Finally, I also find that the manuscript doesn't explains much why particle size changes with altitude or transport history. I would suggest providing a bit deeper physical interpretation when possible.