Potential of AOD Retrieval Using Atmospheric Emitted Radiance Interferometer (AERI)

Round 1

Reviewer 1 Report

The changes that the authors have made to the original version of this manuscript have helped to clarify some points that were, perhaps, not as clear as they could have been. Therefore the paper has been improved and I have no problems with it's publication.

Author Response

Thank you for your valuable comments. 
The manuscript has been more enriched based on the comments of other reviewers.

Reviewer 2 Report

The author's replies to my questions were sufficient

Author Response

Thank you for your valuable comments. 
The manuscript has been more enriched based on the comments of other reviewers.

Reviewer 3 Report

I have reviewed this manuscript.  The overall objective of retrieving aerosol optical depth using ground-based thermal infrared observations is extremely interesting and potentially quite powerful for many different scientific problems.  However, this current manuscript has some serious shortcomings.  I have spoken directly with two of the authors, and shared my concerns with them.  A couple points that need to be improved are:

*) additional details on the aerosol optical properties, aerosol physical properties (e.g., size distribution assumed), radiative transfer model, etc.  I am currently unable to reproduce these results without these details

*) an analysis of the uncertainty of the retrieved AOD as a function of atmospheric conditions (e.g., magnitude of the AOD, precipitable water vapor, cloud optical depth) is absolutely required.

I recommend major revisions are needed before this paper could be accepted for publication.  

Author Response

Thank you for your valuable comments. 
The manuscript has been more enriched based on your valuable comments.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

Thank you for your comments.

Please see the attachment. 

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This paper presents an algorithm that retrieves aerosol optical depth (AOD) from a ground-based infrared spectrometer (AERI).  The algorithm simultaneously retrieves the temperature and humidity profiles also.  An example from a single day of data collected on the west coast of Korea is used to illustrate the technique.

While I like the general intent and goal of the paper, unfortunately the paper has a fatal flaw and should not be published.  AOD is wavelength specific, so the comparison of the AOD retrieved from the AERI (8-13 µm) with the GAW-PFR (370 to 860 nm) in Figure 8, which is the culmination of this paper, convinces me that they are not in fact retrieving aerosol information from the AERI.  Assuming an Angstrom exponent of about 1 (typical values of this exponent range from 0.5 to 2.0), then a AOD of 0.5 at 500 nm (from the GAW-PFR) would translate into an AOD of 0.025 at 10 µm (from the AERI).  

This is reinforced by the aerosol information that they provide in Table 1 for the aerosols used in their calculations.  The size of their aerosols (Rmod) is very small, and using size parameter arguments (e.g., size parameter = 2 * !pi * wavelength / size) it can be seen that the absorption coefficient in the mid-infrared would be very small.  Also, those imaginary refractive indices in that table are almost certainly for the visible wavelength, not the infrared.  [And as a side note: I think that the scale heights used for the CA and UR aerosols is much too large – there is much evidence that the proper values for Z are more in the range of 1.5 to 3.0 km.]

So what is giving rise to the signal that they are sensitive to?  I believe, based upon the temporal variability seen in Figure 8, that they are seeing an optically thin cirrus cloud.

Reviewer 2 Report

The paper under review “Potential of AOD retrieval using Atmospheric Emitted Radiance Interferometer (AERI) “  demonstrates the potential sensitivity of using ground based hyperspectral radiometer (AERI) to estimate Aerosol Optical Depth. The paper discusses the basic inversion algorithms used to simultaneously retrieve Meteorological Properties as well as AOD simultaneously by a full spectral fitting and iterative procedure to account for the changes in Aerosol due to MET temperature and Water Vapor Mixing ratio. While more direct AOD ground retrievals can be obtained in the Day using Sky Radiometers, the Emission Approach can potentially perform night time AOD retrievals filling an important gap in Environmental and Climate observations

The paper is well written and structurally clear and does show on limited datasets and forward modeling that the downwelling emission spectrum is at some level sensitive to the aerosol amount and type of aerosol based on a mature Aerosol Modeling framework (OPAC). However, this paper as stated in the title is best a limited proof of concept and may be considered suitable for publication by introducing researchers to the potential and the extensive further work needed to show this approach can give good quantitative estimates of AOD.

While most of the future work can be developed in future papers, I do believe some level of sensitivity study concerning  the sensitivity of this approach is appropriate for this paper

• For example, using synthetic data for a given MET model and OPAC aerosol model, demonstrate AOD sensitivity as a function of the assumed (incorrect) OPAC model(s) for different MET backgrounds like Summer vs Winter, High Humidity vs Low Humidity etc.

• In addition, the authors should discuss the possibility of using the full spectral residual to see if the OPAC aerosol model can be determined

• If possible, the Authors should also discuss direct night time evaluation using possible collocated aerosol lidars,

Comments for author File: Comments.docx

Reviewer 3 Report

I have no significant comments regarding this paper. It is well-prepared and clear and the science appears to be sound and useful.