The work of Santiago Gassó and Kirk David Knobelspiesse on the “Circular Polarization in Atmospheric Aerosols” addresses the not-extensively-researched theme of the remote sensing of aerosols in the atmosphere using circular polarization measurements. The discussion is quite interesting for a remote sensing scientist, involving a literature review on the subject along different scientific fields, and highlighting the importance of circular polarization measurements for monitoring biogenic and oriented non-spherical particles in the atmosphere, as well as monitoring aerosols in multiple-scattering mediums. This work was a pleasure to read and review and it should be published in ACP.

That being said, some parts of the manuscript need explaining and better phrasing, as indicated below.

I should also note that my expertise does not cover the field of radiative transfer calculations, thus my review on Section 6 may miss possible problematic points. 

Lines 22-28, “Current techniques… this subject.”: Include also the reasoning for using circular polarization measurements for better typing/characterization at low concentrations.

Lines 39-42, “As a… Wei et al., 2020)”: Please rephrase.

Lines 50-51, “While existing… (BOA)”: Line 49 states that the “remote identification is still inadequate…”. Then you state that the “existing methods can identify smoke etc..”. Please rephrase, or provide a measure of uncertainty for the results of the “existing methods”.

Line 71, “...such biological...”: Replace with “...such as biological...”.

Line 75, “…important in public health…”: Replace with “…important for public health…”.

Lines 73-74, “These are… Levy et al., 2013)”: Move this phrase to the previous paragraph.

Line 82, “adequate pixel size”: Do you mean “non-adequate pixel size”?

Lines 86-88, “Like the intensity-only… loading conditions”: Please provide reference(s) for this statement.

Line 89, “…remote sensing sensors using polarization technologies…”: Replace with “…remote sensing sensors onboard satellite platforms using polarization technologies…”

Lines 139-141, “The difference in… of traversing light.”: Replace with “The difference in propagation speeds in each projection of the travelling wave results in changes in phase and magnitude of each of the observed components of the propagating light. The result of these differences is the elliptical polarization of light, with linear and circular polarization to be particular cases. Polarization thus results from the differential speeds of the wave components of the traversing light.”

Lines 143-144, “…CP can arise …. and non-spherical.”: Provide reference.

Lines 145-146, “In addition… orientation.”: Replace with “In addition, unpolarized light incident upon an ensemble of particles can result in outgoing CP if multiple scattering occurs in the medium regardless of the particle type, shape, and orientation.”

Line 147, “… or any identifying observation of the system.”: What do you mean with the phrase “or any identifying observation of the system”? Please clarify and/or rephrase.

Lines 149-151, “A polarized beam… medium as:”, Replace with “A polarized beam is represented by the Stokes column vector. The Stokes vectors for the incident and outgoing light are related by the 4x4 scattering matrix (S) that represents the scattering medium as:”.

Lines 160-161, “The elements… can be used as…”: Replace with “The elements S_4j (bottom row in equation 1) and S_i4 (right most column) are associated to the circular polarization of the scattered light and can be used as ….”.

Lines 168-171, “For non-spherical particles,… particle shapes.”: Include discussion on the numerical solutions as well (e.g. ADDA calculations for dust particles in Gasteiger et al. (2011)).

Lines 189-192, “However,… Section 6).”: Rephrase, referring to this light as multiple-scattered light.

Lines 199-201, “Similarly… in matrix 2.”: Provide the scattering matrices for the ensemble of chiral particles, and the ensemble of chiral particles that is made up of an equal number of left- and right-handed particles.

Lines 238-239, “Exact solutions… (Bohren, 1975).”: This is confusing… Can chiral particles have spherical shapes? Please clarify here.

Line 263, “Rosenbush et al., 2007”: This work is not included in the list of references. Please add.

Line 286, “Two of the main … while airborne.”: Include also the marine particles (e.g. Haarig et al., 2019) and pollen.

Line 297-298, “In this case,… electrical field.”: Provide reference.

Lines 301-304, “Further… 90 degrees.”: Kolokolova and Nagdimunov (2014) also proposed a methodology for differentiating aligned particles from optically-active particles. Please include this info here, as well.

Lines 308-309, “In these cases… a fire.”: Provide reference, or explain more, to support for this statement.

Lines 312-313, “This includes… (Daskalopoulou et al., 2021).”: The studies of Harrison et al. (2018) and Daskalopoulou et al. (2021) do not prove that the particles passively “remain charged” far from the sources. Instead, Harrison et al. (2018) refers to “an active charging process” and provides the triboelectrification as a possible cause. Please include more info here from the respective papers.

Lines 320-322, “Since… as well.”: Replace with “Since similar mechanisms of particle orientation seem to be present in the Earth’s atmosphere, and it may well be possible that these oriented particles scatter circular polarized light as well.”

Lines 320-321, “Since… atmosphere.”: Support this statement with more info.

Lines 325-326, “Lidars… Hu et al., 2003).”: Include also the polarization lidars discussed in Paschou et al. (2022) and Tsekeri et al. (2021).

Lines 331-333, “Recent… aerosols.”: There is no “Martin et al. (2010)” in the reference list. If you mean the work of Martin et al. (2016), they do not provide measurements of mineral aerosol. Please correct.

Line 350, “ …by two different spores as reported by…”: Replace with “ …by two different spores illuminated with linearly-polarized light, as reported by…”.

Lines 351-352, “Further… matrix”: Replace with “Further this is not the only non-zero element S_4j or S_i4 of the scattering matrix.”.

Line 357, “Additional studies…”: Do you refer to laboratory studies? Please clarify.

Lines 360-366, “For example, … overlooked”: Please rephrase.

Caption of Fig. 6: Provide more details on what we see in the plots. At which heights the “particle concentrations are rather high” and the “aerosol composition is consistent with organic aerosols”? Discuss the effect of particle hydration below 1-1.5km.

Lines 391-394, “Figure 6… (Lee et al., 2018).”: The circular polarization is low in the PBL, probably due the high RH and the hydration of the aerosol particles. Moreover, if I understood correctly form the plots, high values of CP are found for low particle concentrations (higher up). Re-write this part, being more specific on what we see in Fig. 6.

Lines 399-400, “While these two… circular polarization.”: Please provide more explanations on why the study of Cao et al. (2011) showed no optical activity for bioaerosols (e.g. due to noisy measurements?). These results contradict a big part of the discussion in the manuscript, thus a thorough explanation should be provided here.

Lines 405-406, “…there are no studies… the atmosphere”: There is the study of Petaja et al. (2016). Please rephrase.

Lines 418-420, “…as well as… matrix.”: Replace with “as well as consideration of the effect of chiral or oriented particles on the extinction matrix, resulting in the dichroic extinction of the propagating light.”

Lines 441-448, “The size distribution… to be spherical.”: Do these aerosol models correspond to specific aerosol types? Please discuss.

Lines 496-497, “However, … by a factor of 2-3.”: Provide explanation why.

Line 504, “… this is the range of concentrations most globally prevalent…”: I do not think this is necessarily true for all aerosol types. Please provide reference to support this statement.

Line 615, “… in Sij is assumed and omitted for simplicity.”: Replace with “… in Sij is assumed and omitted for simplicity in Eq. A2 and A3.”

Line 632, “… resulting in:”: Replace with “… resulting in Eq. A3.”

Line 634: Number the equation as Eq. A3.

Given that this a manuscript is primarily a review article I approached it as a guide that could be used to justify and drive future developments in the field of polarizations measurements, in general and circular polarization, in particular. Althuogh the majority of my experience is with in situ sensor technology, the measurenments of polarized, scattered light is of particular interest to me for all the reasons that the authors highlight.

Reviewer #1 has done a stellar job of identifying a number of areas that need clarification or correction, so there is no point in my reiterating those points, especially since the reviewer caught quite a few that I had missed.

I am mostly writing to encourage the authors to address two questions that I think are relevant to this discussion.

1) There is much information that can be extracted from both LP and CP measurements, but such information seems to be predicated on knowing a prori if the particles are spherical with homogeneous composition. In the active sensing realm of measurements, measuring the LP components, the polarization ratio is sensitive to the complex refractive index (RI) as well as the asphericity but changes in the ratio due to asphericity tend to be larger than changes due to the CRI.  For measurements of CP in the current application, how can you differentiate changes due to shape compared to changes due to chemistry?

2) Athough this review paper specifically discusses CP produced from interactions of unpolarized light with particle ensembles, I think it might have been useful to take some examples from the lidar community who already employ CP to show how these measurements are already highlighting how CP measurements offer complementary information to LP.

I think that the authors have done a very comprehensive job of promoting the benefits of CP measurements and this will be a very useful review that will be used for developing future measurement capabilities.

Summary:

The paper presents an literature review about circular polarization. It explains how circular polarization is generated and which aerosols in the atmosphere produce circular polarization. A few radiative transfer simulations are carried out to investigate the sensitivity of circular polarization on aerosol optical thickness and on aerosol composition. The conclusion is that measurements of circular polarization from space could provide additional information on aerosol compared to measurements of intensity and linear polarization only. The literature review is very interesting to read but it does not include new scientific results. The simulation part is very short and also seems to be not correct (see comment below). For these reasons I can not recommend to publish the paper in its current status, in my opinion major revisions are required.

General:

•  It is claimed that technical advances have demonstrated the feasibility to measure the full Stokes vector from space. However, this is not discussed in the text in more detail. The literature review and also the simulations show, that circular polarization (CP) is several orders of magnitude smaller than linear (LP). The authors should provide more information about planned passive instruments that could measure CP with such a high precision from space.
• The RT simulations are not convincing, because they do not include absorption by trace gases (e.g. ozone in UV). The results show higher linear polarization for a scattering angle of 160.8° than for a scattering angle of 112°, which is not expected because the maximum polarization for Rayleigh scattering is at 90° scattering angle. The relative sensitivity of CP to aerosol composition is large, but since the magnitude is so small it will still be very difficult to measure it.

Specific comments:

l. 165 ff: "For example, exact solutions can be found for 2pr/l << 1 and |mr/l|<< 1 (Rayleigh scattering) ..."-> you are talking here about the scattering matrix elements S_4j and S_i4, which describe circular polarization. For Rayleigh scattering they are exactly 0, because Rayleigh scattering does not cause any circular polarization.

Fig. 3 (caption): "right panel" should be "left panel" and reference to "right panel" is missing

l. 298: "This concept has been applied in astronomy studies where the observations of linear and circular polarization in comets and interstellar dust" -> What is the observed degree of linear/circular polarization in interstellar dust?

l. 325: "Lidars with CP detection capabilities have been proposed for cloud phase" -> How can cloud phase be detected with circular polarization? Please explain.

l. 329: "... (Gilbert and Pernicka, 1967; Lewis et al., 1999) and foggy atmospheres (van der Laan et al., 2017). These studies highlighted the fact that in high- and low-density particle environments, the propagation of circular polarization does not degrade as quickly as linear polarization." -> Why should linear polarization degrade quickly in low-density particle environments? Why does circular polarization degrade slower than linear?

l. 395: "inagreement" -> "in agreement"

l. 395: "Interestingly the distinctive CP found by lidar is inagreement with the theoretical modelling study by Kolokolova & Nagdimunov, (2014) where optically active particles were shown to have non-zero CP and zero linear polarization in the backscattering direction. However, a controlled study (Cao et al., 2011) measuring the degree of LP and CP in pollen backscattering found that both scale with each other following the predictions of Mishchenko & Hovenier, (1995). That study concluded there is no additional aerosol information by measuring both LP and CP. While these two offer somewhat conflicting conclusions, both highlight that indeed biogenic aerosols do produce circular polarization" -> What are the different assumptions in the studies by Kolokolova and Nagdimunov 2014 and Mishchenko&Hovenier 1995? I assume these are theoretical studies, so there should be a simple explanation for the conflicting conclusions?

l. 429: Eq. 4 (circular polarization after 2 scattering events) -> This equation is not very special (included in all VRT codes handling circular polarization). Why is this equation derived and written down here, it is not used at all in the discussion of the results?

l. 447: "The particle shape is assumed to be spherical." -> In the discussion you highlighted the importance of particle shape and orientation for circular polarization. Then, in the model simulations it is neglected. At least randomly oriented aspherical particles can be handled in most state-of-the-art VRTE models...

l. 449: "The atmospheric column only contains air (i.e., no trace gases)" -> Why modelled without trace gases? Calculations without O3-absorptions are very unrealistic in the UV-range. To my understanding "air" normally includes also the trace gases.

Figs. 7/8: What is the definition of the scattering angle? Normally it is given by cos(theta_s)= cos(n_inc * n_sca), where n_inc and n_sca are incoming and scattered directions, respoectively? I doubt that Fig. 7  is for scattering angle of 160.8 degrees. LP by Rayleigh scattering has a maximum around 90° scattering angle, so I would expect much higher LP in Fig. 8 (scattering angle 112°) than in Fig. 7 (160.8°, backscattering direction).

In the discussion of the modelling results, please discuss also the dependence of LP and CP on scattering angle.

l. 522 "Overall, these plots suggest there is a sensitivity to differences between coarse and fine mode dominated aerosols in both linear and circular polarization." -> Since CP is scaled with 10⁴, I assume that even if the relative sensitivity is higher in CP, it can still be better measured in LP? I can not see the benifit of measuring CP.

Fig.9, labels in right panels:  U->V

What is the scattering angle of the simulations shown here?

Author contribution: What is the contribution of KK?