the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2026
Characterisation of hygroscopic aerosol processes using remote sensing and in situ techniques at the MeteoSwiss Payerne station
Abstract. This study presents a comprehensive assessment of aerosol hygroscopicity during the HYGRO19 field campaign conducted at the MeteoSwiss station in Payerne (Switzerland) from June to December 2019. The campaign provided a unique experimental framework by combining continuous Raman lidar observations with vertically resolved in situ measurements from balloon-borne Compact Backscatter Aerosol Detector coupled with RS41 Vaisala radiosondes (COBALD-RS41), ground-based tandem nephelometers, and complementary aerosol chemical analyses, enabling aerosol hygroscopicity to be evaluated using three independent approaches. Two contrasting aerosol regimes were observed: a summer period characterised by relatively high aerosol loading and frequent influence of long-range transported particles, and a second period dominated by low-pressure systems, persistent cloudiness, and reduced aerosol concentrations. Throughout these different atmospheric conditions, the Raman Lidar for Meteorological Observations (RALMO), which continuously provides independent profiles of aerosol backscatter, water vapour mixing ratio, and temperature during both daytime and nighttime, identified more than 200 well-mixed hygroscopic aerosol layers. Median hygroscopicity parameters were fβ (355 nm) = 2.64 ± 0.64 and γβ (355 nm) = 0.70 ± 0.18. Despite differences in measurement principle, wavelength, and sampling technique, Raman lidar retrievals showed good agreement with COBALD-RS41 and tandem nephelometer observations. Statistical analyses revealed no systematic dependence of aerosol hygroscopicity on altitude, with comparable values in the atmospheric boundary layer and free troposphere. Enhanced hygroscopic growth was generally associated with a larger contribution of inorganic aerosol species, whereas no clear dependence on particle size was identified. These results demonstrate that combining remote sensing and in situ observations provides a robust framework for characterising aerosol hygroscopicity under ambient atmospheric conditions.
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Status: open (until 22 Sep 2026)
- RC1: 'Comment on egusphere-2026-4361', Anonymous Referee #1, 07 Sep 2026 reply
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RC2: 'Comment on egusphere-2026-4361', Anonymous Referee #2, 11 Sep 2026
reply
The authors characterise the aerosol hygroscopicity using several independent methods for six months in a Swiss site. A good agreement in hygroscopicity retrieval was found between Raman lidar, radiosondes and tandem nephelometer. They claim that aerosol hygroscopic growth is primarily driven by inorganic aerosol species. While the methodology is technically sound, the main message of the manuscript is not clear. The whole manuscript reads like a lengthy descriptive report rather than a research article. Key details are missing, such as back trajectory analysis. The authors need to decide what are the most important aspects and put more emphasis on them in the main text, when moving the rest to the supporting information. Therefore, the manuscript will require substantial revision for a few rounds before being considered for publication.
Major Comment
- Lines 298-300: I do not get how the claim “in agreement with AOD-AE AERONET measurements” is made. In Fig 1 b, the time dominated by fine-mode particles is June 1st – September 7th. In Fig 2 b, the time dominated by fine-mode particles is the entire period from August to January. Whether the two instruments agree or not require a robust evaluation via statistics not by eyes.
- Lines 332-338: The definition of decoupled aerosol layers is very confused. More details and discussions are required. How about the time periods around December 1st? The β was quite strong and extended to free troposphere.
- Line 341: No significant changes in air-mass origin? Please provide the back trajectory analysis and indicate the time!
Minor Comment
- Lines 36-40: Could you specify what aerosol properties can be modified during sampling? What are the large certainties under conditions close to saturation?
- Line 50: What certain layers?
- Lines 301-304: Could you please provide the HYSPLIT or any back trajectory analysis to indicate where the emissions were from? Just citing papers without showing the detail is irresponsible!
- Lines 305-306: How do you define low and high contribution of absorbing aerosols? The range spans two orders of magnitude, from 0.06 to 26 Mm-1.
- Lines 307-309: Aerosol absorption coefficient and σ might point to the same thing, absorbing aerosols. Could you please include the range of aerosol absorption coefficient in the brackets? Otherwise, it is very confused that σ was used to indicated absorbing particles just right after aerosol absorption coefficient (in Mm-1) was mentioned.
- Line 309: What correlative analyses?
- Lines 314-315: Provide the range from the literature.
- Line 330: How was 1.5 km defined as the cut off height for the atmospheric boundary layer height?
- Line 345: Occurrence of what?
- Lines 365-367: Could you provide more context about these established statistical rules?
- Line 413: I will just use winter instead of second part.
- Lines 415-420: I disagree with not showing the figure. The figure needs to be included into the supporting information. Also include a discussion on why the size dependence is weak in light of the literature.
- Figure 7: Could you use direction instead of degree so that the information can be easily linked to the corresponding paragraph?
- Figure 8: I am not sure if any finding can be drawn from the limited RALMO data points.
- Lines 477-479: Where are the HYSPLIT analysis and map of fire hotspots?
Citation: https://doi.org/10.5194/egusphere-2026-4361-RC2
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- 1
Overall, this manuscript presents a valuable and comprehensive dataset on aerosol hygroscopicity from the HYGRO19 campaign, combining Raman lidar, COBALD-RS41 balloon measurements, tandem nephelometry, and complementary aerosol observations. The large number of hygroscopic aerosol layers retrieved from RALMO is particularly valuable and demonstrates the potential of remote-sensing observations to provide vertically resolved information on aerosol hygroscopicity under ambient atmospheric conditions. The comparison with independent in situ techniques is also an important strength of the study. The observed variability in hygroscopicity between different periods and its association with aerosol chemical composition provide useful insights into the factors controlling aerosol water uptake. However, I believe that the interpretation of the intercomparison and the term “robust characterisation” requires some further clarification. The different techniques probe distinct optical properties (backscatter versus total scattering), wavelengths, particle populations, and sampling conditions; therefore, the retrieved hygroscopicity parameters are not directly equivalent. The authors appropriately acknowledge that the comparisons are primarily qualitative, but the implications of these methodological differences and the associated uncertainties should be discussed more explicitly before drawing general conclusions regarding the capability of remote sensing to quantitatively characterise aerosol hygroscopicity. Although the paper is too lengthy for a research article, some material could be included in a supplementary session. Here are my comments.
Major comment:
Minor comments
Line 7: The manuscript refers to “three independent approaches.” Since the Raman lidar, COBALD-RS41, and tandem nephelometer measurements are based on different optical quantities and are not fully independent, the authors may consider using “three complementary approaches” instead.
Lines 464-465: “The tandem nephelometers' measurements are only representative of near-surface conditions.” Please revise to “the tandem nephelometer measurements are representative only of near-surface conditions.”
Line 639: The terminology “RALMO measurements alone enable the retrieval of aerosol hygroscopicity parameters” may be slightly too broad. Since the retrieval requires specific atmospheric conditions, particularly a well-mixed layer and suitable RH structure, I suggest changing this to “RALMO measurements can enable the retrieval…” or explicitly retaining the conditions under which the retrieval is applicable.
Lines 424-425: the statement “during summer, more wind profiles could be monitored due to lower cloud cover and bird migration”. Can you explain bird migration? Bird migration is not obviously connected to the availability of wind lidar measurements, or is that a typo?
General comment
The authors could consider moving some of the detailed case studies from Section 4.3 to the Supplementary Material. This could help shorten the manuscript and allow the main text to focus more clearly on the methodology and interpretation of aerosol hygroscopicity retrieval.