Review Reports - Characterization of Fresh and Aged Smoke Particles Simultaneously Observed with an ACTRIS Multi-Wavelength Raman Lidar in Potenza, Italy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General comments

This study compares optical and microphysical properties of local and transported Canadian wildfire smoke observed in Italy on 16 July 2024. The local smoke (below 3 km) showed hygroscopic growth and cloud formation, while the aged Canadian smoke (6-8 km) exhibited higher absorption and distinct lidar ratios, highlighting altitude-dependent differences in aerosol properties. These findings emphasize the need to account for such variability in climate models, particularly given increasing wildfire frequency. While this study offers significant reference value, the background, methodologies, and data sections require more supporting literature. Furthermore, the analysis of local smoke (below 3 km) should consider boundary layer diurnal variations. Detailed recommendations are provided below:

(1) The logic and written expression of the manuscript

Lines36-38 “The particle linear depolarization ratio (PLDR) at 532 nm was 0.067 ± 0.002, while backscatter related Ångström exponents (AEβ) values were 1.21 ± 0.03 and 1.21 ±0.03 in the spectral ranges 355-532 nm and 532-1064 nm, respectively. ”, but there is no corresponding line in the full text. Only in Table 1 are there “1.21 ± 0.03 and 1.23 ±0.03” and only in Table 1 are there “1.21 ± 0.03 and 1.21 ±0.04”. Please check carefully.

The statement in lines 75-77, "In large wildfires, the high temperatures result in a more efficient combustion, leading to higher BC concentrations.", needs references to facilitate further reading and to highlight the significance of the article.

The statement from lines 117 to 119: "As a consequence, their contribution to the atmospheric radiative budget is frequently overlooked or underestimated, emphasizing the necessity of ground-based observations to accurately assess their true impact." needs to include references to support this view.

Overall, the references for intruction are relatively weak, and many statements need to add relevant references to support this point of view. Not only the two I pointed out above, but also the others. Please check carefully.

Lines 173 to 176, "This study showcases the full potential of the ACTRIS Potenza site by synergistically using the different ACTRIS components. It further exemplifies that multidisciplinary observation strategies are not only effective but necessary for advancement." are suitable for the end of the article because the specific comparison content of the article has not been given at this point.

6.Lines 209 to 211 mention "The system is equipped with a classical Cassegrain telescope, with a primary mirror diameter of 400 mm utilized for far field measurements, and a Dall Kirkham telescope". The purpose is to let readers better understand the details of Lidar. It is recommended to further add relevant references to help readers further understand the differences between different telescopes, why two telescopes are used and their principles.

Line214 "25 km.Cross" needs a space in the middle

Lines 180-235 of the article are a bit too long, and it is recommended to divide them into paragraphs or abbreviate them. In addition, references 25, 26 and the linked SOP are far from enough, and new references need to be added to help readers further understand some details and expand their reading.

Article summary 257-267 introduces ceilometer, and relevant references need to be added

Lines 381: The date format of 15/07/2024 should be consistent with Line 367's 16 June 2024

(2) Figures and tables

The date "16/07/2024" in the figure caption of Fig3. was changed to be consistent with the others: July 7 2024.
It is recommended to add vertical lines indicating the sunset time and the time when the fire started in Figures 3 and 5 for the convenience of readers’ reference.
It is recommended that AE and PLDR in Figure 6 be swapped because PLDR shares the legend with the first sub-image.

(3) Scientific of the manuscript

Lines 144-145 of the article mention that "the study provides novel insights, essential for refining climate models and improving aerosol radiative forcing estimates." However, the author should further discuss how to quantitatively refine climate models to advance the novelty and importance of the article.
Figure 1 clearly shows that the fresh smoke diffuses and layers after 21:00. The article compares aged smoke at 6-7 km with fresh smoke within the boundary layer and evaluates various parameters. However, the boundary layer height exhibits diurnal variations, which are not addressed in this study. The boundary layer height can influence the concentrations of various atmospheric components, and differences in solar radiation can affect atmospheric photochemical reactions. How do the authors account for these factors? It is recommended that the authors at least provide a comparison or present data on the diurnal variations in boundary layer height and the corresponding changes in atmospheric components on previous day without fresh smoke.
Lines 449-451 mention "As already mentioned previously, CL51 ceilometer measurements (figure 5a) show the presence of an aerosol layer progressively descending from an altitude of 1.5-3.5 km at 17:00 reaching the PBL after 20:00 UTC." What does "PBL" mean? PBLH or PBL top? The definition given in line 225 is "planetary boundary layer (PBL)", which seems to be an inaccurate description of reaching the PBL, because the uniformity is within the PBL.
Figure 5c and 5d clearly show that AAE and BB decrease monotonically from 14:00 to 18:00 UTC, corresponding to the increase in the height of the boundary layer. Using astronomical calculation tools (such as SunCalc), the sunset time is about 20:30 (CEST, UTC+2), so the UTC time is about 18:30 UTC. It corresponds to the lowest position around 18:30 in Figure 5. Therefore, it is still necessary to consider the diurnal changes of the boundary layer or the diurnal changes of the previous day to make readers more convinced.
Table 1 gives the comparison of different parameters for "3-3.5 km for the 16 July 2024 from 16:50-17:29 UTC" and "2.6-2.9 km for the 16 July 2024 from 21:19-21:34 UTC". However, it is a comparison of parameters at different times and altitudes. It is very necessary to explain why different altitudes are chosen, so this article must be supplemented with relevant knowledge on the diurnal evolution of the boundary layer. It is best to give the daily variation of the boundary layer height to make readers more convinced.
Why didn't the author compare different altitudes at the same time in Tables 2 and 3? Are they both 20:19 to 21:19 or 22:19 to 22:50?

Author Response

General comments

Comments 1: The logic and written expression of the manuscript

Lines36-38 “The particle linear depolarization ratio (PLDR) at 532 nm was 0.067 ± 0.002, while backscatter related Ångström exponents (AEβ) values were 1.21 ± 0.03 and 1.21 ±0.03 in the spectral ranges 355-532 nm and 532-1064 nm, respectively. ”, but there is no corresponding line in the full text. Only in Table 1 are there “1.21 ± 0.03 and 1.23 ±0.03” and only in Table 1 are there “1.21 ± 0.03 and 1.21 ±0.04”. Please check carefully.

Response 1: Thank you very much for pointing out this inconsistency.

We have now integrated the correct values into the main text to ensure consistency with Table 1 (now is table 2). The sentence at lines 36–38 (file word) has been revised as follows:

“The particle linear depolarization ratio (PLDR) at 532 nm was 0.067 ± 0.002, while backscatter-related Ångström exponents (AEβ) values were 1.21 ± 0.03, 1.23 ± 0.03, and 1.22 ± 0.04 in the spectral ranges 355–532 nm, 355–1064 nm, and 532–1064 nm, respectively.”

In addition, further clarification was added at lines 597–599:

“ Specifically, the extinction Ångström exponents (AEα) at 355-532 is 1.93± 0.05, the AEβ 355-532 is 1.21±0.03, AEβ 355-1064 is 1.23±0.03 and the AEβ 532-1064 is 1.22±0.04.”

Comments 2: The statement in lines 75-77, "In large wildfires, the high temperatures result in a more efficient combustion, leading to higher BC concentrations.", needs references to facilitate further reading and to highlight the significance of the article.

Response 2: We added the following references

[7] ]Kuhlbusch, T.A.J.; Andreae, M.O. Black carbon formation during savanna fires: Seasonal variation of emission factors. Sustainability 2010, 2, 294–320. https://doi.org/10.3390/su2010294

[8] Popovicheva, O.B.; Persiantseva, N.M.; Kireeva, E.D.; Timofeev, M.A.; Shonija, N.K. Microstructure and physicochemical properties of black carbon particles from wildfire emissions. Atmosphere 2022, 13, 115. https://doi.org/10.3390/atmos13010115

Comments 3: The statement from lines 117 to 119: "As a consequence, their contribution to the atmospheric radiative budget is frequently overlooked or underestimated, emphasizing the necessity of ground-based observations to accurately assess their true impact." needs to include references to support this view.

Response3: We added the following references

22 Chen, W.; Yan, L.; Ding, N.; Xie, M.; Lu, M.; Zhang, F.; Duan, Y.; Zong, S. Analysis of Aerosol Radiative Forcing over Beijing under Different Air Quality Conditions Using Ground-Based Sun-Photometers between 2013 and 2015. Remote Sens. 2016, 8, 510. https://doi.org/10.3390/rs8060510

23 Liang, Y.; Che, H.; Wang, H.; Zhang, W.; Li, L.; Zheng, Y.; Gui, K.; Zhang, P.; Zhang, X. Aerosols Direct Radiative Effects Combined Ground-Based Lidar and Sun-Photometer Observations: Cases Comparison between Haze and Dust Events in Beijing. Remote Sens. 2022, 14, 266. https://doi.org/10.3390/rs14020266

Comments 4: Overall, the references for intruction are relatively weak, and many statements need to add relevant references to support this point of view. Not only the two I pointed out above, but also the others. Please check carefully.

Response 4: Thank you for your comment. We have carefully revised and improved the Introduction section and added the following references to support the key statements:

1 Johnston, F. H., Henderson, S. B., Chen, Y., Randerson, J. T., Marlier, M., DeFries, R. S., ... & Brauer, M. (2012). Estimated global mortality attributable to smoke from landscape fires. Environmental Health Perspectives, 120(5), 695-701.

3 Tian, L.; Wu, X.; Tao, Y.; Li, M.; Qian, C.; Liao, L.; Fu, W. Review of Remote Sensing-Based Methods for Forest Aboveground Biomass Estimation: Progress, Challenges, and Prospects. Forests 2023, 14, 1086. https://doi.org/10.3390/f14061086

6 Turpin, B.J.; Lim, H.J. Species Contributions to PM2.5 Mass Concentrations: Revisiting Common Assumptions for Estimating Organic Mass. Aerosol Sci. Technol. 2001, 35, 602–610. [Google Scholar] [CrossRef]

7 Kuhlbusch, T.A.J.; Andreae, M.O. Black carbon formation during savanna fires: Seasonal variation of emission factors. Sustainability 2010, 2, 294–320. https://doi.org/10.3390/su2010294

8 Popovicheva, O.B.; Persiantseva, N.M.; Kireeva, E.D.; Timofeev, M.A.; Shonija, N.K. Microstructure and physicochemical properties of black carbon particles from wildfire emissions. Atmosphere 2022, 13, 115. https://doi.org/10.3390/atmos13010115

9 Liu, D.; Allan, J.D.; Whitehead, J.D.; Young, D.E.; Flynn, M.J.; Coe, H.; Beddows, D.C.S.; Taylor, J.W.; McFiggans, G.; Fleming, Z.L.; et al. Black-Carbon Absorption Enhancement in the Atmosphere Determined by Particle Mixing State. Nat. Geosci. 2017, 10, 184–188. https://doi.org/10.1038/ngeo2901.

12 Romshoo, B.; Müller, T.; Pfeifer, S.; Saturno, J.; Nowak, A.; Ciupek, K.; Quincey, P.; Wiedensohler, A. Optical Properties of Coated Black Carbon Aggregates: Numerical Simulations, Radiative Forcing Estimates, and Size-Resolved Parameterization Scheme. Atmosphere 2021, 12, 12989. https://doi.org/10.5194/acp-21-12989-2021.

13 Li, Y.; He, C.; Liu, X.; Kang, H.; Zhang, Y.; Liu, J.; Yang, M.; Gong, X.; Zhu, T.; Martin, S.T. Liquid–Liquid Phase Separation Reduces Radiative Absorption by Aged Black Carbon Aerosols. Commun. Earth Environ. 2023, 4, 123. https://doi.org/10.1038/s43247-023-00761-2.

Comments 5: Lines 173 to 176, "This study showcases the full potential of the ACTRIS Potenza site by synergistically using the different ACTRIS components. It further exemplifies that multidisciplinary observation strategies are not only effective but necessary for advancement." are suitable for the end of the article because the specific comparison content of the article has not been given at this point.

Response 5: Thank you for your helpful suggestion.

We agree that the sentence is more appropriate for Conclusions rather than Section 2. As recommended, we moved the following sentence to Conclusions (now lines 825–827):

“This study showcases the full potential of the ACTRIS Potenza site by synergistically using the different ACTRIS components. It further exemplifies that multidisciplinary observation strategies are not only effective but necessary for advancement. Indeed …”

Comments 6: Lines 209 to 211 mention "The system is equipped with a classical Cassegrain telescope, with a primary mirror diameter of 400 mm utilized for far field measurements, and a Dall Kirkham telescope". The purpose is to let readers better understand the details of Lidar. It is recommended to further add relevant references to help readers further understand the differences between different telescopes, why two telescopes are used and their principles.

Response 6: We thank the reviewer for this question. The topic is very technical, and we think that providing further details on the telescopes of our lidar is out of the scope of this study which is mainly focused on the characterization of smoke layers using lidar measurements. Moreover, in general, a lidar is characterized by other equally important aspects such as the field of view, the F number, the iris diameter, the collimation lenses, the eyepieces before the detectors which need to be provided in parallel with the telescope details. Such a description would require a dedicated section which is not related to the main topic of the study we propose. For this reason, we prefer not to add further technical details in the paper. Nonetheless, we provide here some more clarification for the reviewer. The choice of a telescope depends on the particular application one is interested in. In our case this choice, together with other characteristics of the lidar optical system, has been made on the basis of geometrical optics principles and come from a customized optical design study carried out using ray tracing analysis techniques. Therefore, no specific references can be reported.

Based on the abovementioned analysis, the system is equipped with two telescopes, one provides measurements in the far range and the other optimized for the near range detection. The gluing of the two signals measured by near and far range telescopes allows to highly extend dynamic range of lidar signals keeping its linearity. In this way we span from about 200 m (full overlap altitude of the near range telescope) up to about 25 km (considering the far range telescope) with a good signal to noise ratio. For far range measurements, we use a classical Cassegrain telescope with a primary mirror diameter of 400 mm as our simulations show this is a good set-up to get backscattered radiation with good signal to noise ratio from far range atmosphere. For near range measurements, we use a Dall-Kirkham telescope, with a primary mirror diameter of 200 mm. Also in this case the selection has been made by considering ray-tracing simulations showing that the Dall-Kirkham telescope (which is a particular type of Cassegrain telescope) is a good option , because it is compact, stable, less expensive and suitable for near range observations, providing a full overlap height of about 200 m.

The following paragraph is inserted in the submitting paragraph lines (209-215):

"From a ray-tracing analysis, the system is equipped with two telescopes, one for measurements in the far range (classical Cassegrain telescope with a primary mirror diameter of 400mm), and the other for measurements in the near range (Dall-Kirkham telescope with a primary mirror diameter of 200 mm). The gluing of the signals measured by two telescopes allows us to highly extend the detected dynamic range, spanning from about 200 m (full overlap height of the near range telescope) up to about 25 km (considering the far range telescope) with a good linearity and signal to noise ratio."

Comments 7: Line214 "25 km.Cross" needs a space in the middle

Response 7: Corrected, thank you.

of the article are a bit too long, and it is recommended to divide them into paragraphs or abbreviate them. In addition, references 25, 26 and the linked SOP are far from enough, and new references need to be added to help readers further understand some details and expand their reading.

Response 8: Thank you for the helpful comment.

We reorganized lines 180-235. Lines 180-185 consist now a separate paragraph.

Furthermore, we are now providing the definition of SOP by adding the following sentences:

Specifically, SOP refers to the full set of quality assurance (QA) tests that are submitted every two months to ensure that the system complies with the high standards required by ACTRIS. These tests typically include: telecover tests, Rayleigh fit analysis, polarization calibration, zero bin determination, and dark signal measurement.

For further reading, we added the following references in the submitting manuscript:

35 Freudenthaler, V. (2016). About the effects of polarising optics on lidar signals and the Δ90-calibration. Atmospheric Measurement Techniques, 9(9), 4181–4255. https://doi.org/10.5194/amt-9-4181-2016

36 Freudenthaler, V. Optimized Background Suppression in Near Field Lidar Telescopes. In Proceedings of the 6th International Symposium on Tropospheric Profiling (ISTP), Leipzig, Germany, 14–20 September 2003.

37 Freudenthaler, V. Effects of Spatially Inhomogeneous Photomultiplier Sensitivity on Lidar Signals and Remedies. In Proceedings of the 22nd International Laser Radar Conference (ILRC 2004), Matera, Italy, 12–16 July 2004.

Comments 9: Article summary 257-267 introduces ceilometer, and relevant references need to be added

Response 9: We added the following references in the submitting manuscript:

41 Granados-Muñoz, M.J.; García, S.; Mora, J.M.; Cuevas, E.; Alonso-Pérez, S.; Querol, X. Analysis of Four Years of Ceilometer-Derived Aerosol Backscatter Profiles in a Coastal Site of the Western Mediterranean. Remote Sens. 2021, 13, 2576. https://doi.org/10.3390/rs13142576.

42 Chen, J.; Zeng, X.; Li, S.; Song, G.; Li, S. Water Vapor Correction in Measurements of Aerosol Backscatter Coefficients Using a 910 nm Vaisala CL51 Ceilometer. Remote Sens. 2025, 17, 2013. https://doi.org/10.3390/rs17122013

Comments 10: Lines 381: The date format of 15/07/2024 should be consistent with Line 367's 16 June 2024

Response 10: Corrected, thank you.

(2) Figures and tables

Comments 11: The date "16/07/2024" in the figure caption of Fig3. was changed to be consistent with the others: July 7 2024.

Response 11: Corrected. Thank you.

Comments 12: It is recommended to add vertical lines indicating the sunset time and the time when the fire started in Figures 3 and 5 for the convenience of readers’ reference.

Response 12: Figures are modified accordingly and are now part of the submitting manuscript.

Comments 13: It is recommended that AE and PLDR in Figure 6 be swapped because PLDR shares the legend with the first sub-image.

Response 13: Figure is modified accordingly and is now part of the submitting manuscript

(3) Scientific of the manuscript

Comments 14: Lines 144-145 of the article mention that "the study provides novel insights, essential for refining climate models and improving aerosol radiative forcing estimates." However, the author should further discuss how to quantitatively refine climate models to advance the novelty and importance of the article.

Response 14: Thank you for your insightful comment. We agree that a more detailed discussion on how the study’s findings can quantitatively refine climate models would enhance the novelty and impact of the manuscript. In response, we have expanded the relevant section to include specific mechanisms and parameters through which our results can contribute to improving aerosol representation and reducing uncertainties in aerosol radiative forcing in climate models.

Lines 135-143

These findings offer quantitative constraints on key aerosol properties, such as size distribution, chemical composition, and hygroscopic growth factors, which can be directly integrated into global climate model parameterizations. Specifically, the observed variability in aerosol optical properties across different atmospheric regimes provides critical input for refining aerosol–radiation and aerosol–cloud interaction schemes. Implementing these empirically derived parameters in Earth system models can reduce the uncertainty in aerosol radiative forcing estimates, especially for short-lived climate forcers. Future modeling efforts should aim to assimilate these observational constraints to evaluate their influence on climate sensitivity and long-term projections.

Comments 15: Figure 1 clearly shows that the fresh smoke diffuses and layers after 21:00. The article compares aged smoke at 6-7 km with fresh smoke within the boundary layer and evaluates various parameters. However, the boundary layer height exhibits diurnal variations, which are not addressed in this study. The boundary layer height can influence the concentrations of various atmospheric components, and differences in solar radiation can affect atmospheric photochemical reactions. How do the authors account for these factors? It is recommended that the authors at least provide a comparison or present data on the diurnal variations in boundary layer height and the corresponding changes in atmospheric components on previous day without fresh smoke.

Response 15: Thank you for the comment as it allows us to better clarify the point raised. Our statement suggesting that the fresh biomass burning layer remains confined within the planetary boundary layer (PBL) is not fully correct. From the nighttime measurements shown in Figure 1, it is clear that the fresh smoke layer is not observed only in the nocturnal PBL but also in the residual layer above the PBL up to about 3 km. However, due to not complete overlap, the lidar retrieved optical properties we provide refer mostly to the particles located in the residual layer. As the residual layer is decoupled from PBL the impact of boundary layer height on the particle concentration we observe does not apply in this case. For the same reason we think that a correlation study considering the diurnal PBL variability would have a negligible impact on the results calculated out of the measurement shown in Figure 1. .

Additionally, our measurement site is located in a remote mountainous region in Southern Italy located at 760 m a.s.l. with low local activity . Despite the latitude, nighttime surface cooling is pronounced, often resulting in very shallow boundary layer — typically below 100 m. The radiosonde systems operative at our observatory confirm that even during summer months, the nocturnal PBL top often drops below the lidar overlap range (around 200 m agl), while PBL top height can go over 2 km.

In this specific case, the observed biomass burning aerosol layer persisted above the nocturnal PBL as seen in Figure 1, and it reached the surface due to gravitational settling rather than boundary layer mixing. While the PBL often plays a key role in modulating aerosol dynamics, especially for dust intrusions, our data suggest it was not the dominant driver in this event.

Following the reviewer’s suggestion, we included in situ data from the previous day (without fresh smoke influence) to highlight the differences. We added here ACSM data for the previous day (15), clearly showing the impact the smoke layers had after 20:00 UTC on 16 July 2024. See the pdf file. (Figure 1 in the word file attacched).

All references to the PBL influence on this specific event have been revised accordingly to avoid misinterpretation.

Comments 16: Lines 449-451 mention "As already mentioned previously, CL51 ceilometer measurements (figure 5a) show the presence of an aerosol layer progressively descending from an altitude of 1.5-3.5 km at 17:00 reaching the PBL after 20:00 UTC." What does "PBL" mean? PBLH or PBL top? The definition given in line 225 is "planetary boundary layer (PBL)", which seems to be an inaccurate description of reaching the PBL, because the uniformity is within the PBL.

Response 16: We thank the reviewer for pointing out the confusing statement in lines 449–451. We agree that the phrasing was imprecise. In this revised version of the manuscript, we removed the phrase suggesting that the aerosol layer reached or is confined within the planetary boundary layer (PBL), as this does not accurately reflect the observed dynamics.

At this time, some aerosols are reaching the surface.

As clarified previously, the aerosol layer remains centered around 2.5–3 km even during nighttime hours, well above the nocturnal PBL height, which is typically below 100 m at our site. The observed descent of the aerosol layer is gradual and primarily driven by gravitational settling rather than entrainment into or interaction with the boundary layer.

To avoid misinterpretation, the text has been revised to emphasize the downward movement of the aerosol layer due to gravitational processes, without implying interaction with or intrusion into the PBL. We have also ensured that the term "PBL" is used consistently throughout the manuscript and does not refer ambiguously to its height (PBLH) or top.

We write in the test at line 510:

Furthermore, a thin layer reaches the surface after 20:00 UTC due to gravitational settling.

Comments 17Figure 5c and 5d clearly show that AAE and BB decrease monotonically from 14:00 to 18:00 UTC, corresponding to the increase in the height of the boundary layer. Using astronomical calculation tools (such as SunCalc), the sunset time is about 20:30 (CEST, UTC+2), so the UTC time is about 18:30 UTC. It corresponds to the lowest position around 18:30 in Figure 5. Therefore, it is still necessary to consider the diurnal changes of the boundary layer or the diurnal changes of the previous day to make readers more convinced.

Response 17: We thank the reviewer for this valuable observation. We agree that the monotonic decrease in AAE and BC between 14:00 and 18:00 UTC could plausibly be linked to processes within the planetary boundary layer (PBL). However, we do not agree that the decrease is related to an increase in the PBL height, which reached its maximum height earlier that day as shown in the figure below. On the contrary, this monotonic decrease might be the result of less traffic emissions. As shown in Figure 2 of the submitted paper, CIAO is a background station with low activity (the area consists of few companies for processing and distribution of goods) therefore this decrease most likely coincides with the end of shift or the end of the day for those activities. Furthermore, as presented in Response 15, we observe that the evolution of the aerosol concentration between 16:00-19:00 UTC coincides with that of the previous day (15 July 2024). Furthermore, based on the vertical distribution of the aerosol layer and supporting measurements (Figures 4 and 6), we believe that the aerosol present near the surface during that period is not associated with the biomass burning aerosols but is rather influenced by local sources, most likely vehicular traffic. For this reason, this portion of the data was not included in the characterization of biomass burning aerosol in our main analysis.

As far as the diurnal evolution of the PBL, the convection that develops due to surface heating is characterized by convective motions such as thermals and updrafts. MIRA35, a vertically pointing Ka-band radar, provides high-resolution measurements of the vertical Doppler velocity that can be used to approximate the convective PBL. Specifically, the cloud radar measures the vertical component of air motion through the Doppler shift of radar signals backscattered by particles in the atmosphere. In the convective PBL, the updrafts and downdrafts lead to variability in the measured Doppler velocity. These vertical motions are generally confined below the convective PBL top, which appear as a transition to more laminar flow or reduced vertical velocity variance. Therefore, we can infer the evolution and height of the convective PBL by looking at the time-range evolution of the cloud radar measurements of the vertical Doppler velocity. As can be observed in the figure below, the updraft and downdraft motions in the convective PBL can be identified by the highest values of the vertical Doppler velocity. The highest values of Doppler velocity (in absolute values) occur within these areas, where the updrafts are represented in red (positive velocity, away from the radar) and the downdrafts in blue (negative velocity, towards the radar). By looking at the daily time-height evolution of the vertical Doppler velocity for the 16 of July 2024, it can be observed that the convective PBL starts to develop slightly before 8:00 UTC, keeps increasing in height throughout the morning and then becomes rather stable. After 14:00 UTC, the vertical reach of the convective movements starts to decrease, with decreasing convection between 16:00 and 17:00 UTC.

(See figure 2 in attached file.)

Figure 2: Time series of the Doppler Velocity retrieved from MIRA35 on 16 July 2024 (https://cloudnet.fmi.fi/).

Also, a similar evolution is observed on the previous day: https://cloudnet.fmi.fi/search/visualizations?site=potenza&dateFrom=2024-07-15&dateTo=2024-07-15

Coming back to the Figure 5b, 5c, and 5d, the spikes observed occur under the nocturnal boundary layer regime. Considering that no activities can induce them that otherwise would be present during the day (e.g., traffic), it is safe to say that smoke particles entrained the shallow nocturnal boundary layer and deposited on the surface.

Nonetheless, supporting statements are included in the submitting manuscript. Furthermore, we added ACSM and ceilometer data from the previous day, which show a similar diurnal trend in surface aerosol properties and reinforce the idea that local sources can dominate near-surface composition during daytime PBL expansion.

Comments 18: Table 1 gives the comparison of different parameters for "3-3.5 km for the 16 July 2024 from 16:50-17:29 UTC" and "2.6-2.9 km for the 16 July 2024 from 21:19-21:34 UTC". However, it is a comparison of parameters at different times and altitudes. It is very necessary to explain why different altitudes are chosen, so this article must be supplemented with relevant knowledge on the diurnal evolution of the boundary layer. It is best to give the daily variation of the boundary layer height to make readers more convinced.

Response 18:We thank the reviewer for this very pertinent comment. As correctly pointed out, the two altitude ranges considered (3–3.5 km at 16:50–17:29 UTC and 2.6–2.9 km at 21:19–21:34 UTC) are different, and the rationale behind their selection has now been explicitly clarified in the text.

Both layers are located above the boundary layer, both in the afternoon and in the evening. They have been selected based on the homogeneity of intensive parameters (like lidar ratio, particle depolarization ratio, Angstrom exponents) and the intensity of extensive properties (like backscatter and extinction coefficients) . This is essential to retrieve accurate microphysical properties.

Comments 19: Why didn't the author compare different altitudes at the same time in Tables 2 and 3? Are they both 20:19 to 21:19 or 22:19 to 22:50?

Response 19: We used the same criteria described in the previous point.

For the lower layer, the integration interval from 20:19 to 21:19 UTC was selected as it provided the best compromise between signal stability and extinction intensity. The same criterion was applied to the upper layer, and the time window was selected accordingly.

As shown in Figure 1, the aerosol layer around 6 km becomes more pronounced towards the end of the observation period, which justifies the choice of a different time interval for the upper layer. Therefore, the time ranges were selected independently for each layer, based on when the extinction signal was strong and stable enough to allow a robust retrieval.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

General Commnents : In this manuscript, the authors reported measurements of particles from two wildfires: one local wildfire (fresh) and one wide wildfire from Canada (aged), using multiple sensors, including Ceilometer, radar, microwave radiometer, Raman lidar, etc. They found the aged and fresh particles from these two wildfires have distinct optical and microphysical properties, suggesting distinct physical and chemical mechanisms behind these short- and long-term transports. This study is interesting and can provide significant references for climate, wildfire, and atmospheric particles related studies. The writing of this manuscript tends to be lengthy with structural issues. Some evidences are needed to prove that the observed signals are related to fresh and aged wild fires, for example, local and large-scale wind fields. More direct comparisons between the fresh local fire and the long-transport aged fire should be provided, for now, they are mostly separated.

Response : We thank the reviewer for the insightful and constructive comment.

We have carefully revised the manuscript to address both the structural issues and the clarity of the comparisons between the two wildfire cases.

Regarding the evidence supporting the identification of the observed layers as originating from fresh and aged wildfire smoke, we would like to highlight that the The drivers for this study are the optical and microphysical properties for the two discrete aerosol layers. The summarizing tables in the submitted manuscript clearly illustrate these differences. When it comes to the aged smoke aerosol layer in the mid-troposphere, Figure 3 (in the new version) of the submitted manuscript nicely pinpoints the source of biomass burning aerosol layer in boreal forests of North America. The travelled path is common during the burning season and our measurements have contributed to the monitoring of pyroCb-related smoke particles as shown in Baars et al. (2019). During the afternoon hours of 16 July 2024, a visually thick plume of smoke from a local and small-scale forest fire was the trigger for POLPO measurements, that were not scheduled (see Figure 4b of the submitted manuscript). An almost identical situation was observed in De Rosa et al. 2022. For this reason and the close proximity of the source to CIAO, we did not provide dispersion modeling output.

In the submitting manuscript, we are including a wind rose. The figure indicates prevailing winds from southeast and speed 4-6 m/s. This information coupled with Figure 2 (submitting manuscript) demonstrate that air masses advected over CIAO from the location of burnt forest.

We acknowledge that in the original manuscript the analyses of the two wildfire events appeared somewhat separated. To address this, we have extensively revised the text. The description of the Canadian wildfire now directly follows that of the local wildfire. Subsequently, the discussion transitions seamlessly to the comparison of their respective optical and microphysical properties. This restructuring improves the clarity and coherence of the manuscript, allowing for a more integrated and direct comparison between the fresh local fire and the aged long-range transported smoke.

We have clarified these aspects more explicitly in the revised manuscript to make our interpretation more transparent and better supported.

Commnents 1: Please rephrase the sentence in line 28-29.

Response 1: We thank the reviewer for the suggestion. The sentence has been revised accordingly. We included the following text in the submitting manuscript:

“The fresh smoke was originated by a local wildfire about 2 km far away from the measurement site and observed about one hour later its ignition.”

Commnents 2:Line 31, “a result”

Response 2:Corrected.

Comments 3: The structure of the abstract needs further refinement.

Response 3: We have carefully revised and refined the structure of the abstract to improve clarity and coherence.

Comments 4: The structure of the introduction section is a chaos now. Please refine this section with

clearer logic flow and focus on your study.

Response 4: We have redefined the structure of the introduction to ensure a clearer logical flow and precise presentation of the study objectives. We believe that the revised version now provides a more coherent and effective context.

Comments 5: Line 159, missing “.”.

Response 5: Corrected

Comments 6: Line 180-188 could be moved to section 2, before section 2.1.

Response 6:Thanks. Done.

Comments 7. Line 238, “equipped”.

Response 7: Corrected.

Comments 8: A table containing all the instruments and their observing targets is recommended.

Response 8: Done.

Comments 9. A wind rose or wind fields should be added to Fig. 2.

Response 9 Done. A wind rose is added.

Comments 10: Reference 38&39 are wrongly cited in line 393. Please provide solid evidence regarding the well mixed smoke that is close to the source.

Responce 10: We acknowledge that references [38, 39] do not directly support the specific statement about the well-mixed nature of smoke near the source in the case of small fires. The sentence has been revised accordingly in the manuscript.

To strengthen the argument, we have now added a more appropriate references :

20 De Rosa, B.; Amato, F.; Amodeo, A.; D’Amico, G.; Dema, C.; Falconieri, A.; Giunta, A.; Gumà-Claramunt, P.; Kampouri, A.; Solomos, S.; et al. Characterization of Extremely Fresh Biomass Burning Aerosol by Means of Lidar Observations. Remote Sens. 2022, 14, 4984. https://doi.org/10.3390/rs14194984

56 Heilman, W.E. Atmospheric Turbulence and Wildland Fires: A Review. Int. J. Wildland Fire 2023, 32, 476–495. https://doi.org/10.1071/WF22053

Comments 11. Line 401, “nigh”

Response 11: Corrected.

Comments 12: From Fig. 3c, it seems clouds may form at about 14:00 UTC at higher altitudes (~5 km). RH increased below 4 km starting at 16:00 UTC. The connections and discussions between cloud formation and the local fire are weak. I would rather believe that this cloud formation is due to water vapor transport from a local lake.

Response 12: We agree that the interpretation of cloud formation processes should be further supported. Nonetheless, we would like to clarify that the cloud identified by Cloudnet (see Figure 5 of the submitted manuscript) occured below 4 km asl and, precisely, on top of the local wildfire smoke plume. In addition, the lidar attenuated backscatter coefficient, Figure 4b, confirms the forming cloud marked by the “shadow” effect (blue arrows in the figure below), which a typical demonstration of cloud in lidar signals. As far as the RH (Figure 4c), the dark blue areas indicate high relative humidity, but high RH alone cannot lead to cloud formation. Our hypothesis, which is supported by different sensors’ observations, is that the fresh biomass burning aerosols act as CCN (cloud condensation nuclei) upon which water vapor condenses. The boxed area of Figure 4c indicates that within that area there are sufficient levels of RH which when coupled with the presence of aerosols could trigger cloud formation. The lake as seen in Figure 2 can, of course, modulates the relative humidity patterns around CIAO, however, aerosols are needed to form clouds.

Comments 14: These two wild fires should be introduced before the analysis of all data.

Response 14: We thank the reviewer for the useful suggestion. Following this advice, we have moved the description of both wildfires to an earlier section of the manuscript, before the analysis of the measurements. This reordering provides clearer context for the reader and improves the logical flow of the results and discussion sections.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author's response effectively addressed all my concerns. I agree with the author's statement that the nocturnal boundary layer height is very low, even below 100 meters. While a systematic study of the diurnal variation of boundary layer height is beyond the scope of this paper, the definition of boundary layer height in the article remains insufficiently clear. It is recommended that the author adopt the approach of defining PBLH as the sum of the mixed layer and the residual layer to improve clarity in this paper. The description of Figure 10 in the reference DOI: https://doi.org/10.1364/OE.451728 can be consulted for guidance. Subsequently, the content of the article should be revised accordingly. The research presented in this paper is highly significant; however, the definition of boundary layer height is critical and has guiding implications for further studies. Therefore, the paper can only be accepted once the boundary layer description is accurately revised.

Author Response

Comments 1: The author's response effectively addressed all my concerns. I agree with the author's statement that the nocturnal boundary layer height is very low, even below 100 meters. While a systematic study of the diurnal variation of boundary layer height is beyond the scope of this paper, the definition of boundary layer height in the article remains insufficiently clear. It is recommended that the author adopt the approach of defining PBLH as the sum of the mixed layer and the residual layer to improve clarity in this paper. The description of Figure 10 in the reference DOI: https://doi.org/10.1364/OE.451728 can be consulted for guidance. Subsequently, the content of the article should be revised accordingly. The research presented in this paper is highly significant; however, the definition of boundary layer height is critical and has guiding implications for further studies. Therefore, the paper can only be accepted once the boundary layer description is accurately revised.

Response 1: The authors are pleased to know that the revised version of the manuscript addressed all the reviewer’s concerns.

Concerning the specific request to adopt specific definition of the PBLH, the authors would like to recall that the PBL is not the main topic of this study. The measurement case we present refers to the characterization of the optical and microphysical properties of two smoke layers (one aged and the other fresh) observed at same time. According to that, it is not fully clear how the adoption of the suggested PBLH definition would improve the clarity of the paper.

Moreover, the authors fully agree with the reviewer that the definition of the PLBH is in general a key aspect for many reasons and it needs to be addressed carefully but this work is not intended to provide guidelines for further study on the PBL but for the smoke detection by using lidar observations.

With these points in mind, we tried anyway to address the reviewer’s comment. Specifically, in discussing the fresh smoke layer we provide more information connected to the PBL dynamics. We thank the reviewer for this suggestion, which has improved the description of the event. We now explain that during the day, convective activity transports the smoke plume up to about 3 km, whereas at night, the same smoke resides in the residual layer above a shallow nocturnal boundary layer. With our lidar measurements (started at about 20:30 UTC) we characterize the smoke in the residual layer (as the lidar overlap prevents us to go lower). However, in-situ observations at the same time show increasing BC on the surface. As our site is characterized as background, we can reasonably exclude other sources of BC. Consequently, our observations suggest that smoke particles from the residual layer start to penetrate the underlying nocturnal layer and deposit on the ground. This is also confirmed by the ceilometer observations (virtually no overlap region) revealing that starting from about 20:30 UTC residual and underlying nocturnal layers seem to merge (or better, the temperature inversion defining the top of the nocturnal layer is located at quite low height). Additionally, following the reviewer’s suggestion we have added references related to PBL supporting our interpretation.

We believe this clarification improves the physical interpretation of the observations and is in line with the reviewer’s recommendation (lines 454-471 in the file Word).

At this time, the solar activity is still strong enough to initiate and sustain the development of a convective mixing layer (ML) driven by buoyant forces [57]. This turbulent motion is highly effective in vertically redistributing local air masses and aerosol particles, allowing the smoke emitted by the local fire to be transported upward to about 3 km. This mechanism facilitates the formation of elevated aerosol layers that can remain aloft for several hours. Additionally, approximately at 17:00 UTC, the smoke particles act as cloud condensation nuclei and a thin cloud forms at 3 km (dashed rectangle in Fig 4a). As solar activity starts to decrease, the turbulent processes within the mixing layer start to decay. Under these conditions, the mixing layer transits into a quite shallow nocturnal boundary layer (not anymore characterized by turbulence) which is typically located below a residual layer characterized by a stratified and stable regime [58]. As a consequence, the elevated aerosol layer observed at approximately 3 km during night is no longer sustained by active convection but instead resides within the residual layer, where it remains decoupled from the surface. However, ceilometer observations indicate that after 20:00 UTC the bottom of the residual layer starts to penetrate the underlying nocturnal layer following approximately the dashed white line reported in Fig.4a. This hypothesis is subsequently confirmed by the in-situ measurements shown later in the paper.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have improved the manuscript significantly and I am happy to recommand an accept of this manuscript.

Author Response

Comments 1: The authors have improved the manuscript significantly and I am happy to recommand an accept of this manuscript.

Response 1: We sincerely thank the reviewer for the constructive comments and suggestions that helped us improve the manuscript. We also appreciate the time and effort dedicated to reviewing our work.