the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jan 2024
Comparison of scanning aerosol LIDAR and in-situ measurements of aerosol physical properties and boundary layer heights
Abstract. The spatial-temporal distribution of aerosol particles in the atmosphere has a great impact on radiative transfer, clouds, and air quality. Modern remote sensing methods as well as airborne in-situ measurements by unmanned aerial vehicles (UAV) or balloons are suitable tools to improve our understanding of the role of aerosol particles in the atmosphere. To validate the measurement capabilities of three relatively new measurement systems and to bridge the gaps that are often encountered between remote sensing and in-situ observation as well as to investigate aerosol particles in and above the boundary layer, we conducted two measurement campaigns and collected a comprehensive dataset employing a scanning aerosol LIDAR, a balloon-borne radiosonde with the Compact Optical Backscatter Aerosol Detector (COBALD), an optical particle counter (OPC) on a UAV, as well as a comprehensive set of ground-based instruments. The extinction coefficients calculated from near-ground-level aerosol size distributions measured in-situ are well correlated with those retrieved from LIDAR measurements with a slope of 1.037 ± 0.015 and a Pearson correlation coefficient of 0.878, respectively. Vertical profiles measured by an OPC-N3 on a UAV show similar vertical particle distributions and boundary layer heights as LIDAR measurements. However, the sensor, OPC-N3, shows a larger variability in aerosol backscatter coefficient measurements with a Pearson correlation coefficient of only 0.241. In contrast, the COBALD data from a balloon flight are well correlated with LIDAR-derived backscatter data from the near ground level up to the stratosphere with a slope of 1.063 ± 0.016 and a Pearson correlation coefficient of 0.925, respectively. This consistency between LIDAR and COBALD data reflects a good data quality of both methods and proves that LIDAR can provide reliable and spatial distributions of aerosol particles with high spatial and temporal resolutions. This study shows that the scanning LIDAR has the capability to retrieve backscatter coefficients near ground level (from 25 m to 50 m above ground level) when it conducts horizontal measurement which isn't possible for vertically pointing LIDAR. These near-ground-level retrievals compare well with ground-level in-situ measurements. In addition, in-situ measurements on the balloon and UAV validated scanning LIDAR retrievals within and above the boundary layer. The scanning aerosol LIDAR allows us to measure aerosol particle distributions and profiles from the ground level to the stratosphere with an accuracy equal or better than in-situ measurements and with a similar spatial resolution.
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Status: final response (author comments only)
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RC1: 'Comment on ar-2024-3', Anonymous Referee #1, 15 Feb 2024
The focus of the manuscript is on the comparison of (a) almost horizontal aerosol lidar observations with respective in situ aerosol observations at ground, of (b) lidar-derived optical properties with respective UAV OPC measurements, and (c) of lidar profiles of aerosol backscatter up to the tropopause with respective balloonborne COBALD backscatter sonde observations.
The experimental effort is large and that aspect deserves publication. However, the results are not new. Lidar comparisons with COBALD observations and with in situ aerosol observations have already been performed and published. The use of a scanning lidar in these comparisons may be a new aspect.
The manuscript provides the impression that the goal is to validate lidar observations. However, it is known at least since the 1980s that lidar observations are trustworthy. To my opinion, the lidar data (presented here) confirm that in situ aerosol observations can be used (after proper corrections of instrumental effects and other short comings) to compute optical properties. Furthermore, the lidar observations show that the COBALD sonde allows proper aerosol measurements. But the high quality of COBALD observations is also known since many years.
Detailed comments:
p2, l25: you mean …climate change simulations…
p2, l49: CALIPSO is a simple backscatter lidar. Advanced techniques are the HSRL and Raman lidar techniques. That should be mentioned.
p2, l41: Kotthaus et al. does not focus on aerosol optical properties, only on PBL heights.
p2, l45: MPLNET consists of simple backscatter lidars…
Nothing is mentioned regarding state-of-the-art HSRL and Raman lidars and their potential (Burton et al., 2012, 2015, Gross et al., 2011, 2015, Baars et al. 2016, Hu et al. 2022) and many other papers, e.g., from Veselovskii and others…
p3, l62: Ceolato and Berg, 2012 review article: This is quite a poor review article, mainly covering just the simplest of all aerosol lidar techniques.
p3, l63: 2 times Duesing et al., 2018, is mentioned.
All in all: The introduction does not reflect the latest status of lidar efforts, studies, and publications in the field of aerosol remote sensing.
p5, l131: The boundary layer detection method (Haar wavelet transform method) has often been presented. Please omit, just provide a reference… maybe Baars 2008.
p7, l176: You are able to measure ozone as well… should be mentioned.
p7, l183: The COBALD sonde is used to validate lidar observations. To my opinion, the opposite is always the case. And comparisons of lidar with COLBAD observations remain difficult in the case of large, non-spherical particles such as cirrus ad PSC particles and even coarse dust particles because the FOV of the COBALD system is 6°, the lidar FOV is 0.1°. The lidar measures exactly backscatter (or 180° scattering properties), the COLBALD sonde is not able to do that. The scattering phase function can change a lot over 3 degress around 180° in the case of dust and cirrus particles (non spherical particles).
Figure 2: The legend in Figure 2 is confusing. Please state clearly what is shown in Figure 2.
p9, l201: …Fidas200 underestimates…. and at the end … you believe that these in situ observation validate the lidar measurements?
Figure 3: the use of container humidity and ambient humidity in the correction of in situ aerosol properties is rather confusing… J have the feeling the in situ observations are far away from being trustworthy.
Figure 4: In these modern times with lidars producing data with 7.5 m vertical resolution and 10 s temporal resolution, the color plot is rather poor.
p11, l250 – p12, l264: The paragraph is trivial. I would skip it. What is new? … should be always the driving question. PBL determination with lidar and comparison with radiosondes and other approaches was often presented during the past 30 years.
Figure 5: Again, a poor color plot because of the rather poor resolution!
Figure 6: The agreement is reasonable. In situ observation with UAVs are just snaphots. Lidar observations seem to be more representative. What is the signal averaging period? Please state!
Figure 7: The correlation is rather poor. The UAV observations from 9 July are much too high, and the ones measured on 12 July are much too low. The quality of the in situ observations is therefore rather low. What do we learn from such a poor result?
p16, l304: I asked myself, why do the authors want to validate lidar observations of the backscatter coefficient? Ground based lidars (and airborne lidars) are used since the 1970s and space lidars since the 1990s. Do we still need to validate lidars? That makes sence. But why do we need to validate ground-based lidars? Please explain, what is so critical with lidar backscatter observations?
Figure 8: Again, the color plot is very poor because the vertical and temporal resolution is so poor. What does the color show in (b), probably distance, is not given in the figure.
p18, l341: Again, I think the opposite is true. The lidar does not need validation after 40 years of consistent, high quality aerosol observations. Moreover, the lidar shows (in Figure 10) that even a balloonborne backscatter sonde (one snapshot-like measurement per height) can be used to characterize the height profile of aerosols. The advantage of lidar, on the other hand, is the potential to monitor aerosol developments of the vertical aerosol distribution over long time periods, continuously.
Citation: https://doi.org/10.5194/ar-2024-3-RC1 -
AC1: 'Reply on RC1', Hengheng Zhang, 26 Apr 2024
We would like to thank reviewer for taking the time to review this manuscript and for providing valuable, constructive feedback and corresponding suggestions that helped us to further improve the manuscript. Our detailed replies to each of the raised concerns are in the supplementary material. Our point-to-point replies are marked by “R” and are in blue,Changes to the manuscript text are in green.
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AC1: 'Reply on RC1', Hengheng Zhang, 26 Apr 2024
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RC2: 'Comment on ar-2024-3', Anonymous Referee #2, 20 Feb 2024
General Comments:
The manuscript compares the aerosol optical properties and boundary layer height retrieved from a scanning lidar with various in-situ measurements, including ground-based, UAV, and balloon measurements. This comparison to some extent validates the feasibility of scanning lidars and new vertical in-situ measurement techniques, and also identifies their calibration methods.The research content aligns with the publication scope of the Atmospheric Measurement Techniques journal, and publication is recommended after revisions.
Specific Comments:
1) The English grammar and structure of the manuscript need further improvement.
2) It is difficult to discern the research purpose the authors intend to convey from the manuscript. If the aim of the study is to validate and analyze the errors of the lidar retrieval using in-situ measurements, it would be more appropriate to calibrate and correct the in-situ measurements first. Then, the corrected results should be compared with the lidar retrieval results for a more reliable comparison, rather than initially comparing the lidar retrieval results with the uncorrected in-situ measurements.
3) The analysis and explanation of the comparison results in Chapter 3.1 are confusing. Lidar detects aerosols in ambient conditions, so why is indoor humidity being incorporated into the Mie calculation for comparison? It seems the authors intend to convey that the in-situ measured aerosols are not completely dry, but when incorporated into the Mie calculation, they are treated as if they are in a dry state, leading to higher results. This could serve as a potential explanation for the differences in the comparison results, but it still does not clarify why indoor humidity is being used in the calculation.
4) Figures 4 and 5 should be redrawn. It is inappropriate to directly plot potential temperature profiles while the x-axis represents time. Instead, the measured potential temperature profiles should first be used to retrieve the boundary layer height based on drone detection, and then labeled on the graph for comparison with boundary layer heights from other sources.
5) The experimental effort is large and that aspect deserves publication. However, the results are not new. Lidar comparisons with COBALD observations and with in situ aerosol observations have already been performed and published. The use of a scanning lidar in these comparisons may be a new aspect.
6) Several “scatter” in the manuscript should be “scattering”, like scattering coefficient, single scattering albedo, etc.
7) "LIDAR" is a common way to write lidar, so the author's choice to use it to refer to lidar is fine. However, when introducing terms such as CALIPSO and EARLINET, their official writing "Lidar" should be used.
Line 27: “the” should be removed.
Line 31: “and” should be “of”.
Citation: https://doi.org/10.5194/ar-2024-3-RC2 -
AC2: 'Reply on RC2', Hengheng Zhang, 26 Apr 2024
We would like to thank reviewer for taking the time to review this manuscript and for providing valuable, constructive feedback and corresponding suggestions that helped us to further improve the manuscript. Our detailed replies to each of the raised concerns are in the supplementary material. Our point-to-point replies are marked by “R” and are in blue,Changes to the manuscript text are in green.
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AC2: 'Reply on RC2', Hengheng Zhang, 26 Apr 2024
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