the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
15 Feb 2021
15 Feb 2021
Polarization lidar for detecting dust orientation: System design and calibration
- 1Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Athens, Greece
- 2Raymetrics S.A., Athens, Greece
- 3Fakultät für Physik, Meteorologisches Institut, Ludwig-Maximilians-Universität, Munich, Germany
- 4University of Vienna, Faculty of Physics, Vienna, Austria
- 5National Institute of R&D for Optoelectronics, Magurele, Romania
- 1Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Athens, Greece
- 2Raymetrics S.A., Athens, Greece
- 3Fakultät für Physik, Meteorologisches Institut, Ludwig-Maximilians-Universität, Munich, Germany
- 4University of Vienna, Faculty of Physics, Vienna, Austria
- 5National Institute of R&D for Optoelectronics, Magurele, Romania
Abstract. Dust orientation is an ongoing investigation in recent years. Its potential proof will be a paradigm shift for dust remote sensing, invalidating the currently used simplifications of randomly-oriented particles. Vertically-resolved measurements of dust orientation can be acquired with a polarization lidar designed to target the off-diagonal elements of the backscatter matrix which are non-zero only when the particles are oriented. Building on previous studies, we constructed a lidar system emitting linearly- and elliptically-polarized light at 1064 nm and detecting the linear and circular polarization of the backscattered light. Its measurements provide direct flags of dust orientation, as well as more detailed information of the particle microphysics. The system also employs the capability to acquire measurements at varying viewing angles. Moreover, in order to achieve good signal-to-noise-ratio in short measurement times the system is equipped with two laser sources emitting in interleaved fashion, and two telescopes for detecting the backscattered light from both lasers. Herein we provide a description of the optical and mechanical parts of this new lidar system, the scientific and technical objectives of its design, and the calibration methodologies tailored for the measurements of oriented dust particles. We also provide the first measurements of the system.
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Alexandra Tsekeri et al.
Status: final response (author comments only)
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RC1: 'Needs more data', Anonymous Referee #1, 02 Mar 2021
The authors describe the design of a new lidar system designed to measure dust orientation through polarimetry. The sensor concept is built on established theory that strictly backward (single) scattering polarization matrices only have non-zero off diagonal elements if the particles have a preferred orientation. Otherwise, averaging over all orientations causes those off diagonal terms to reduce to zero.
I found the work to be reasonably thorough. Overall, I found the instrument description to be mostly complete. There were a few areas where some information on tolerance could be added. There are several areas where additional information should be provided (detailed below), which should not be difficult to add as minor revisions.
I found the math difficult to follow and generally lacking elegance. There are so many subscripts that I can’t keep track of what everything means. At one point there is an unexplained change from scalar to vector representation (Eq 8 and 9) for several terms. Overall, the presentation of the math is not a strict barrier to publication and this is obviously just the opinion of this reviewer, but some definitions and explanations need to be filled in (details below).
My biggest concern with this work is that it seems incomplete. After a description of the instrument, the data example is lacking. There is hardly any data, and all of that data is shown as time integrated plots. The integration times are very long (10-15 min -- well in excess of variability in atmospheric structure) and separated considerably in time. This needs some explanation. Also the authors should really be showing time resolved plots which tend to be more revealing, allow the reader to see both the vertical and temporal structure (see, for example, the exceptional plots in figure 8 of Kokhanenko 2020, 10.5194/amt-13-1113-2020).
I don’t understand why there is so little data. What is the barrier to running this instrument continuously (a very important question for an instrument paper)? The only thing we are shown is a dust layer with no observable orientation, so we have no assurance that the instrument can observe off diagonal elements. The authors could operate the instrument to observe rain, which has very strong orientation signatures (see Hayman 2014 10.1364/OE.22.016976). That would at least provide some coverage of the measurement space. Demonstration would not be fully complete given the intended application, but it may be asking too much to demand the authors to show polarization properties of oriented dust.
To be clear, I think this work needs two things to be publishable:
- Observations of oriented scatterers (not necessarily dust)
- Observations shown as time resolved plots
Some more general comments:
Given that this is an instrument paper, I would think operability is part of the design and performance criteria. Is this somehow connected to the very small set of observation examples?
I am somewhat concerned that the authors seem to have decided that oriented dust is a foregone conclusion. The published work on this phenomena appears to be circumstantial (see specific comment about Line 13), so I would recommend the authors adopt a more cautious tone on the subject.
I assume the authors are aware, though they do not explicitly state in the manuscript, that there is no requirement that oriented particles produce non-zero off diagonal elements. Off-diagonal elements (within the constraints of single backscattering) indicate oriented particles, but the absence of off-diagonal terms does not rule out the presence of oriented particles. This is one of the weaknesses of using polarization for this type of measurement.
It is notable that there is no discussion of uncertainty in this work. This seems like a pretty important aspect of the instrument design.
Specific comments:
Line 13: “Dust particles have non-spherical irregular shapes and they have been reported to present preferential orientation (Ulanowski et al., 2007).”
It’s worth noting that the analysis presented by Ulanowski is circumstantial. Dichroism from starlight was observed and those authors, lacking another explanation, assert that it must be caused by vertically oriented dust. This is not scientifically rigorous proof of oriented dust. The limits of imagination do not constitute scientific proof. (Remember when, lacking any other explanation, a neutrino traveled faster than the speed of light at CERN?).
The correct assertion is that dichroism has been observed in starlight when Saharan dust was present and that has led to the hypothesis that Saharan dust could have a preferential orientation. If the conclusions from Ulanowski 2007 et al are already deemed sufficient and correct, why build a lidar to look at this? Clearly there needs to be more, different observations.
Line 45: The authors note that they are using high power lasers. What is the eye safety classification of the lidar system and does that affect how and when often the instrument can be run?
Line 82: The description of the telescope system does not mention a field stop. What is the angular acceptance of the receiver?
Line 86: “The signals are recorded by two cooled Avalanche PhotoDiodes (APDs) at each detection unit,”
What mode are the APDs operating in? Analog or geiger? How are signals acquired and stored? Are photon counts converted to a histogram, and if so at what time and range resolution? Are analog signals digitized with A/Ds and at what sample rate and what is the analog bandwidth of the digitizer?
Line 124: “Moreover, most of the previous works utilize visible light measurements whereas we use near infrared light measurements at 1064 nm, to better probe the larger dust particles (a more detailed discussion is provided in Tsekeri et al. (2021)”
This statement references work that is neither published nor submitted for publication. Please provide some high level explanation for why IR is better for probing dust.
Line 141: Eq. 1 is in a strange part of the text. The text immediately above is discussing laser polarization, not the scattering matrix. As a reader, I was confused when I saw the equation.
Figure 7: Can you double check the subscripts labeling the components? At this point I feel pretty confused by all the subscripts, but I’m not sure if those in the figure are correct.
Line 162: The transmission term is treated as a scalar (having no polarization effect) in this work, but Ulanowski et al., 2007 specifically measured dichroism in dust extinction. Please state the justification treating the transmission of dust as a scalar.
Line 182: Please explain why the elements of f_{ij} and I, Q, U, V are now being treated as vector quantities.
Line 184 (just below Eq. 8): I think the definition for g_{ij} has the wrong denominator. Shouldn’t it be G_{11}?
Line 224: “The optical elements are considered to be perfectly aligned with each other in the detection units after telescopes A and B”
Since there is no such thing as perfectly, this raises the question: What are the angle tolerances on the manufacturing and alignment?
Figure 13: In (c) orientation flag, there appears to be some bias above 1.0 both above and below the dust layer. I would have thought that above the layer, since the orientation is nonlinear, noise could be causing the bias, but below, the noise is quite low. Why is the orientation flag not equal to 1 when there is plenty of signal? Please provide more discussion on this new retrieved quantity and the observations.
I have attached additional comments on the structure of the equations.
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RC2: 'Promising instrument, deserves a better presentation', Anonymous Referee #2, 08 Mar 2021
The article by Tsekeri et al describes an innovative lidar system (WALL-E), able to probe dust particle orientation by measuring the off-diagonal elements of the backscatter matrix. The lidar comprises two lasers and two telescopes, each featuring 2 channels. The full overlap between the emitted beam and the received field of view is achieved at a range of 400-600 m. One laser is linearly polarised whereas the second one is elliptically polarised, and both emit at the same wavelength (1064 nm). One telescope is equipped with a beam splitter permitting to measure two linearly polarised channels, whereas the other one permits measuring two circular polarisations. The article describes the system layout and provides photographs of its main components: the optical head, the positioner, and the electronic compartment. Moreover, the measurement strategy is illustrated and the equations governing the system are provided. Finally first measurements are shown, for the case of a dust event in Athens, where particle orientation was not observed.
I feel that the new system can become a game changer in the field of dust remote sensing, able to provide us with very interesting insights in the coming years, around the debated hypothesis of dust particle orientation, and for this reason the article merits publication. On the other hand, I believe that it needs major improvements, in terms of a presentation more oriented towards atmospheric scientists which are not necessarily experts in optical systems.
The main issue for me is that the largest part occupied by the development of the mathematical formulas. Whereas these equations are very important, in my opinion the article that explains them should mainly speak the language of the atmospheric sciences. I would suggest rewriting the text in such a way that the main principles governing the instrument are explained in words to the reader, with the equations relegated to one of the sections not taking up more than 20-30% of the paper. A scientist wishing to skip this section for brevity, should still be able to understand the article. I would also suggest: on one hand, to simplify the math where possible, and on the other hand to expand on the non-mathematical parts.
Note that due to the complexity of the math on one hand, and to the ambiguity of some of the quantities used (see e.g. below about angles) I am unable to check these mathematical equations. Moreover, as I have meainly dealt with simpler optical systems in the past, I suggest to the editor to find an expert on light polarisation to check them.
Papers by Daskalopulou (2020) and Tsekeri (2021) is mentioned, however they have not been submitted yet. I suggest that the main learnings from these papers should be summarised here in the mean time, and/or that a preview should be provided for the reviewers and the colleagues taking part in the interactive discussion.
The introduction should place the research into context more. At present, the general presentation of the atmospheric science problem on dust orientation is discussed in the first 10 lines, and I believe that the topic deserves more, together with previous observations and to hypotheses on why it is believed to happen (e.g. dust electrification). See e.g. Nicoll et al (Env. Res. Lett 2010), Merrison et al (Plan. Sp. Sci. 2012), van der Does (Sci. Adv. 2018), Toth III (Atmos. Chem. Phys. 2020), Mallios et al (J. Aer. Sci. 2020, 2021). The topic of mineral dust in general could also be introduced before discussing the specific topic, citing a number of articles (easy to locate as there is plenty of literature), and mentioning the main points that need investigation (composition, particle size and shape, transport mechanisms, gaps in the observations, radiative effects, etc.) and the main methods used (in situ, remote sensing, modelling, etc.). The main applications of this research could also be mentioned.
There are some points which are unclear as well, and I suggest could be more explicitly be clarified, e.g. is the lidar a scanning one? It sounds like yes at the beginning, but later on there is a sentence about not using any moving parts. What is the preferred viewing geometry and why? Is the orientation controlled through a stepped motor, or is it manual?
Angles are expressed with respect to the horizon, but to the reader it is not fully clear what this means: it seems to make sense perhaps for a horizontal observation but not e.g. for a zenith geometry. I admit that I got lost with the different angles expressed in the article and that it should be made clearer every time what are the two planes between which an angle is measured.
The hardware set-up of the receiver could probably be better illustrated with a drawing than with Figure 3.
The function of some units of the hardware (LPC, precipitation sensor, UPS) is leaved implicit. I believe it should be explicit (e.g. “detection of precipitation causes shutdown of the lidar”, “UPS can keep the system running for X hours in case of power failure”, “the purpose of LPC is XXX”, etc.). You also mention shutting down the lasers in case of emergency: what type of emergency and how is it detected?
In general the reasons behind the design choices could be given: why two telescopes and not e.g. a single telescope with a more complex optical system behind, allowing the same states of polarisation to be measured? Why does the second laser emit elliptically polarised light and not circular polarised, and how is the optimal polarisation ellipse chosen?
First measurements are shown very briefly and they show that the system works, but the case study chosen does not allow to highlight particle orientation (the main goal of this new instrument). I would support Anonymous Referee #1’s suggestion that it would be useful to show an example where particle orientation is observed (not necessarily dust if an example has not yet been identified).
Finally, the 1-page long overview and future perspectives section is merely a summary of the article followed by a brief description of future plans. I believe that it would be useful to tie the research more widely to the wider field of research, going back to the main questions raised in the introduction and explaining how you are contributing to answer some of them. This section could be completely rewritten.
All in all, I feel that this paper is still immature for publication in the way that it is presently written, and that it is not yet ready for a detailed revision. I suggest a major rewriting along the above suggestions before I can seriously review it. As the new lidar system presented is very innovative and will certainly yield innovative results, I suggest that it is worth doing an additional effort into presenting it. I also suggest the editor to find an expert on light polarisation and Stokes vectors to review the equations.
Alexandra Tsekeri et al.
Alexandra Tsekeri et al.
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