Review of the manuscript entitled:

Observations of Dust Particle Orientation with the SolPol direct sun polarimeter

Vasiliki Daskalopoulou et al. https://doi.org/10.5194/amt-2023-121

submitted to ACPD

-------------------------------------------------

Summary. This paper reports direct sun observations of the three Stokes vector components (I,Q,U) collected during one year (2020-2021) at a small island in the Mediterranean Sea (Antikythera) using a novel photometer. This instrument is a polarimeter previously used in planetary applications and adapted for daytime observations of the Earth’s atmosphere. Data shown in this report focuses on the degree of linear polarization (DLP) observed in the atmosphere during clear sky (ie. no or background aerosols) and during dusty days. Significant amount of information on the instrument and its calibration is provided in separate appendixes. The general performance of the instrument is discussed. Variability of observed DLP is analyzed and discussed in the context of particle orientation. Overall, this is a good report of a novel instrument with an exquisite degree of sensitivity and accuracy. It also provides a unique data set of observations of atmospheric linear polarization and a description of its features, all information very useful to understand the new batch of space borne polarimeters in the upcoming years. The material presented is clear as well as the figures. All this alone makes it a good report and worthy of publication.

There is, however, an important point and it is the fact this paper is titled as an investigation to suggest particle orientation whereas the evidence shown is at best consistent with the fact but by no means the only possibility. Alternate possibilities are not discussed and there are several statements in the text seem too biased to confirm that particle orientation is occurring. As it is now the text reads assuming that particles are oriented, and the discussions are biased towards this end. With this regard, I advise to change the title of the paper without mentioning particle orientation but rather emphasize the novelty of the instrument for atmospheric applications and reported observed DLP for atmospheric dust. I consider this aspect as major point as I do not consider it should be published with such title. I believe this change and some changes in the tone of the text should be relatively simple.

-------------------------------------------------------------------------

Line by line commentary (mostly cosmetic and some more nuanced)

Abstract and Lines 31-32: The concept of orientation is only mentioned in the introduction and in the last line in a suggestive sentence (as opposed to assertive sentence). Overall the tone and description of the abstract agrees with the findings of the paper but it is in remarkable contrast with the assertive tone of the title. The latter needs to be changed to be in more agreement with the findings. (Something like “Direct measurements of linear polarization in clear and dusty sky conditions with a novel sunphotometer. ”)

Line 63 : change to "The latter was addressed ... "

Lines 49 to 65: The contents and source of information of this paragraph is adequate. However, I think that an important distinction should be made at this stage. Much of the background information refers to studies focusing on intergalactically dust whereas studies focusing on atmospheric (Earth) dust in suspension are very limited with the cited Bailey et al., 2008; Ulanowski et al., 2007 being the most notable ones. This differentiation is not apparent in this discussion and I think this important to make a note since in interstellar dust , it is very reasonable to assume orientation is a result of alignment in electromagnetic fields whereas in the Earth atmosphere, this source of alignment does not look likely. However, what both space and earth studies have in common is that both use optical approaches (modeling and observations) to discuss observed polarization and particle alignment. Yet, this introduction does not mention why an oriented particle would have a distinctive optical signal. For example, what is the connection and consequence of not having particle random orientation and the observed optical properties? A number of the studies cited do dwell into this physical mechanism and this is important, because one of the basic assumptions of these studies is that the optical relationships between observed polarization in interstellar dust also apply to dust in suspension in the Earth atmosphere.  

In other words, I think the introduction is bringing in concepts and explanations that have been used in space physics to explain atmospheric observation in Earth. Some of these concepts and explanations may or may not be directly applicable in our atmosphere. So, it would be very useful to acknowledge this fact with some text tweaking and text additions (probably much less text than what I just wrote here).

Also, in my search of bibliography of direct sun measurements of DLP, I have not found many. It appears these recent papers provide a measure of comparison although they focus on all sky polarization (where direct sun observations are a subset of them). Also, they report DLP with very different instruments than SolPol. However, they also report RT simulations of skylight DLP and compare with their observations. So, there is useful information the authors of the present study to compare with the SolPol observations or at least help to put more context or interpret the observations :

 Shuai Li, Rui Wang, Congming Dai, Wenqing Xu, and Jie Zhan, "Impact of aerosols on the polarization patterns of full-sky background radiation," Opt. Express 31, 19918-19930 (2023)

  Peifeng Pan, Xin Wang, Tian Yang, Xiankun Pu, Wenli Wang, Changhao Bao, and Jun Gao, "High-similarity analytical model of skylight polarization pattern based on position variations of neutral points," Opt. Express 31, 15189-15203 (2023)

   Le Guan, Shiqi Li, Liyuan Zhai, Sheng Liu, Hui Liu, Wei Lin, Yan Cui, Jinkui Chu, and Huikai Xie, "Study on skylight polarization patterns over the ocean for polarized light navigation application," Appl. Opt. 57, 6243-6251 (2018)

Also, an important classic study on this subject is the book by Coulson with an extensive discussion on atmospheric polarization as seen from the ground. I highly advise to take a look at Chapter 4 for a thorough discussion of the sources of polarization of the clear sky atmosphere with model and observations commented.   Chapters 5-7 are also very relevant:

Coulson, Kinsell L.. “Polarization and Intensity of Light in the Atmosphere.” (1989). A. Deepak Pub., Hampton, Va.,

Line 294 : Gassó et al, reference is wrongly listed. Check the proper reference.

Line 316 : note the Kemp et al, paper also reports a non-zero V component in observed solar flx (of the order ~10-6)

Line 365-366: Not a clear sentence, I do not understand what you are traying to say. Please rewrite/clarify as needed.

Lines 335-337 :"This is our first consistent indication that preferentially - vertically or horizontally - aligned dust particles could be present in the observed layers and cause the dichroic extinction of transmitted sunlight through the layer."

and

Lines 379-381 : "The segregation between reference days and dust driven days, along with the distinct behaviour of DOLP with increasing SZA 380 is consistent with the findings from stellar polarimetry (Bailey et al., 2008; Ulanowski et al., 2007) and provides the first indications of particle preferential orientation with a solar polarimeter."

I think these are strong statements with respect to probing particle alignment. What I do see is consistency with what was reported in those studies, but those studies are different than this one in a number of ways. For example:

Bailey et al and Ulanowski report observations a nighttime conditions, that is when multiple scattering is not present and it is clearly present in the current study. Also, both studies reported observations at higher wavelengths (625 nm for Bailey et al and  780nm for Ulanowski et al) than this study (500nm) . The latter is a wavelength for which Rayleigh scattering (and related polarization) can play role. Certainly, it more relevant that the two quoted studies. These two facts warrant additional analyses as MS and molecular polarization can play an unknown role and are a consideration that probably were not necessarily in the cited studies.

Line 438-440 "The observed excess in LP is linearly proportional to the increasing airmass, which could be attributed to the particle viewing geometry for fixed AODs or the number of aligned particles for a fixed viewing geometry as DOLP 440 increases more rapidly for days with larger AODs (Figure 8b)."

Can you reference this? This quite a speculation. How can you be sure that this LP vs AirMass relationship is related to particle orientation? Other considerations maybe be at play here such changes in atmospheric loading and size distribution, variability in dust composition or variability of multiple scattering through the day due to loading changes.

Section 5.3 : This test is a good idea

Line 510 : can you add a reference where this statement is explained? I mean why the mean orientation should be 60 degrees? .