Polar Aerosol Vertical Structures and Characteristics Observed with a High Spectral Resolution Lidar at the ARM NSA Observatory

Round 1

Reviewer 1 Report

See the attachment.

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for the recommendation of publishing our manuscript on remote sensing.

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 3 Report

First review of “Polar Aerosol Vertical Structures and Characteristics Observed with a High Spectral Resolution Lidar at the ARM NSA Observatory” by Zhang et al.  

 

September 6, 2022

 

Reviewer Recommendation:

Requires some revisions before publication

 

Summary:

The article describes and summarizes 6 years of high spectral resolution lidar data acquired in the far north at the ARM NSA site between 2014 and 2019, focusing primarily on aerosol statistics. The authors describe the motivation for measurements of aerosols in the Arctic regions and describe in somewhat sparse but probably sufficient detail the specifics of the data set that they are presenting. They present a case study of the potential of the data collected and then use a set of 6 years of operational data to annual and seasonal statistics of aerosol intensive parameters. 

 

The article as presented is well cited and well constructed. The data presented are mostly well supported and in general, the authors make convincing statements about the applicability of the acquired data. There are minor issues with some of the statements made concerning the generality of certain statements; specifically, some statements are made very specific that are more generally true while other statements are more general and should be specified. 

 

It is my opinion that the article is publishable, both from the perspective of originality and quality. I believe the content fits well within the scope of the journal. I have a few comments that should be addressed, but I would classify them as mostly minor. Beyond those comments, I would suggest this article be published.  

 

More Major Comments:

1. Some comments made within the text that are highly restrictive are actually very general. I would suggest in the following instances to generalize the comments made:

1. Page 1, Line 12-13: There is no need for the qualifier that HSRL can work in the Arctic region in this line. Suggest simply ending the sentence after “...properties.”

2. Page 2, Line 53: This sentence is equally true outside the Arctic. Suggest simply removing the word “Arctic” from this sentence. 

3. Page 2, Line 66-67: An argument can easily be made that advanced lidars are needed generally. I would suggest removing the word “polar” from this sentence. 

4. Page 2, Line 69: Again, this sentence is not restricted in validity to the polar regions. I would remove “Arctic” from this sentence. 

2. One comment made within the text that was intended to be highly general needs to be specified. I would suggest in the following instances to specify the comments made: 

1. Page 3, Line 112-113: Signals should only be dominated by background noise in high latitudes in the summer. I suspect this can further be specified (though not needed for this article) to times dominated by enhanced background levels coincident with clouds. Beyond that, I would guess your filter bandwidth and field of view should minimize issues related to daylight measurements. 

3. Page 2, Line 69-70: I have both typographic and conceptual issues with this sentence. I am not sure I would call the signal observed by a lidar “scatterings”. They are really observations of scattered light. Furthermore, the concept that the aerosol backscatter signal is “usually” smaller than the molecular scattering is troublesome to me because the environment is very important in that assertion. I would suggest altering this sentence to read: “A challenge of using traditional elastic scattering lidars to study aerosol properties is that, with the exception of thick aerosol plumes and heavily polluted environments, the total light observed that is scattered from molecules is often larger than the total light observed that is scattered from aerosol particles.”

4. Page 3, Line 104: I would suggest changing “distinct laser Doppler frequency shifts” to “distinct observed Doppler frequency shifts of scattered light”. The original wording suggests more hardware based changes to me as the reader than changes during the scattering process. 

5. Page 3, Line 109 (and other places located through the article): In general, aerosol backscatter coefficient (beta_p) is presented in this article as if it is a direct HSRL measurement. It is not. Backscatter ratio (SR) is a measurement that comes directly from observations but you require an estimate of molecular backscatter to estimate beta_p. For that, you require a temperature profile. There are several sources of this, e.g.: radiosondes, reanalysis, standard atmospheric models. This comment raises some simple-to-address but important points:

1. At a minimum, you must specify that you require an ancillary temperature profile to present these data. 

2. You should specify what ancillary temperature profile you have used. 

3. I would suggest explicitly stating the source of temperature in the figure caption of Figure 3 for completeness.  

6. Page 3, Lines 118-121: This description of depolarization measurements is incomplete. I believe the HSRL instrument at issue truthfully measures depolarization using circular polarizations. In general, the equation given is correct, but the basis vectors used for your measurements are not the same set as for this equation, i.e. the equation given doesn’t directly apply to your observations. In particular, observed circular depolarization and linear depolarization differ by at least a factor of 2 and are not converted to linear depolarization ratio via the same equation. Please clarify. 

7. Page 5, Figure 2: The sample of data shown looks to me to have distinct and persistent vertical stripes (see the high SR cloud at 2-3 km at 15 UTC for example; alternatively because of the colorbars used, they are present in beta_p profiles above 4 km). From my experiences, it looks like you are having poor spectral purity of the laser light at these points or more probably poor temperature control of the iodine cell. Either way, you probably need to address the stripes within the text and assess if they affect your long term statistics.

8. Page 7, Figure 5: You say above on line 199 that there are nearly ½ million profiles in your data set, which is impressive. I do wonder however if the uptime of the instrument is evenly distributed, i.e. are your repair and maintenance periods occurring at the same time each year and affecting anything presented. I would suggest adding uptime information to this graph or elsewhere to provide context.

9. Page 7, Line 220: Only using depolarization data above a certain backscatter ratio seems pretty odd to me. Are there any limits on the particle depolarization error bound imposed? I would suggest that, as one example, is a better metric than backscatter ratio to filter out bad data. 

10. Page 8, Figure 7 (and equally Figure 2): The use of the “jet” colormap here causes some weird pixels (especially in April and May) that stand out that are likely artifacts of the colormap. Panel (a) and (b) seem to have odd jumps that do not occur at the same intensity. I can not suggest strongly enough to simply switch to a perceptually uniform colormap. 

 

More Minor Comments:

1. Page 2, Line 68: The use of “Rayleigh” and “Mie” scattering here is somewhat ubiquitous but also lacks physical precision. Specifically, the term Mie scattering when talking about non-spherical particles later (and more generally the concept of a Mie scattering aerosol, that per the definition must be spherical) is inaccurate in almost all physically reasonable instances. I suggest simply removing these terms.

2. Page 3, Line 107: I believe this quoted filter bandwidth (1.8 GHz) is in fact the width of the iodine line. If I am not mistaken, the optical filter (etalon) bandwidth is probably closer to 6-ish GHz. Please check your instrument specifications.

3. Page 4, Lines 139-140: You use small and large here to describe the Angstrom exponents. I would suggest being more quantitative. I would simply suggest adding numerical values (something like 0 and 4) as parentheticals after these words for context. 

4. Page 4, Table 1: I would suggest changing 0.532 micrometers to 532 nm to match the units used below for the MFRSR. 

5. Page 7, Line 231: You might change the wording of “clear air” to something like “cloud free” to better reflect the physical situation. 

6. Page 8, Line 247: Suggest adding “e.g.” before the citations “[22,23]”. Without this addition, the implication given is these are the two main studies of long range transport of dust in the Arctic. This is a disservice to the vast quantity of research on the subject. 

7. Page 11, Lines 327-331: It is entirely understandable why the authors do not want to provide extinction estimates of very optically thin aerosol layers. It would however help this argument if the authors could provide a very rough estimate of the average optical thickness expected, i.e. “From our observations, extinction values are often roughly ____% of this threshold (or equally ___ orders of magnitude below this threshold)”.

8. Page 11, Line 332-333: I agree wholeheartedly with the conclusion: “single-wavelength lidars have their limits and are not able to provide aerosol microphysical property retrievals”. It does however make me question the inclusion of the 62-65 and 300-305. I would suggest removing these sentences and references as I believe they are cumbersome and don’t really make your points stronger. 

9. Page 11, Lines 333-336: I believe this statement vastly overstates the conclusions of the two papers referenced with regards to the ability of multiwavelength lidar to measure aerosol microphysical properties. Burton et al. shows that the information content of the retrievals is highly dependent on the properties of the aerosol being observed and concludes for example: “The results suggest that the 3 beta + 2 alpha measurement system is underdetermined with respect to the full suite of microphysical parameters considered in this study and that additional information is required, in the form of additional coincident measurements (e.g., sun-photometer or polarimeter) or a priori retrieval constraints.” This sentence should probably be omitted or at the very minimum qualified heavily. 

10. Page 11, Line 345: Note that discussion of preliminary results of the MOSAiC data, at least the Raman lidar, is available. You might consider reviewing and citing:

​​Engelmann, R., et al.: Wildfire smoke, Arctic haze, and aerosol effects on mixed-phase and cirrus clouds over the North Pole region during MOSAiC: an introduction, Atmos. Chem. Phys., 21, 13397–13423, https://doi.org/10.5194/acp-21-13397-2021, 2021.

11. The use of “et al.” in your references list is inconsistent. Several references have longer author lists than [13], [17], or [35] for example. For simplicity, I would suggest omitting the use of et al. in your reference list and list all author names. 

 

Typographic Comments:

1. I believe in general that “aerosol” should be used in the plural form. The spots where I noticed this are listed, but this may not be everything: 

1. Page 1, Line 25: “Aerosol plays” should be “Aerosols play”. 

2. Page 1, Line 30

2. Page 2, Line 51: “observatory facility” should be “observatory facilities” 

3. Page 2, Line 78: Suggest adding North and West latitude/longitude to match Line 86 below. 

4. Page 6, Line 199: Suggest replacing “totally” with “a total of”

5. Page 7, Line 220: Suggest adding “of” in the sentence “Approximately 28.3% ___ HSRL pixels”.

6. Page 8, Line 243: Figure 2c seems like the wrong reference here as it does not show monthly data. Do you mean 7c? 

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf