The study performed by A. Almansa et al. presents an extension of the current operative method for calibrating the field Cimel instruments from AERONET by calibration transfer, allowing the existence of differences between channels wavelengths from primary and secondary instruments. This is not only useful for improving the transfer when small differences exist between the two channels to be transferred, but also to transfer the calibration from PFR-GAW radiometers, contributing to the traceability of measurements. Also, it allows to apply the cross calibration method to same Cimel models with different nominal channels, such as those from the AERONET-OC type. The results have been validated with standard Langley plots showing good results, and the issues raised have been also addressed.

English usage is also clear to my understanding, so I would not propose further need for native English revision.   

My general recommendation for this manuscript is to be accepted, with minor changes.

General comments:

- The study would be an extension of a previous work from Fargion (2001), at least for the OC case, but I think the method has also some ideas in common from an older paper from Soufflet (1992). I think it merits to check for it, if the authors didn't do before.   

 

Specific comments:

- Why air mass is limited to minimum 2? Interval 2-5 is common for standard Langleys, but why limiting to air mass 2 in case of cross calibration? For this case, data around noon should be good, even if the airmass is smaller than 2 and some turbulence could make the measurements have higher variability, if this is the reason. Anyway, a comment could be included. 

- Page 2, line 27: I think Campanelli et al (2012) would be more meaningful than Campanelli et al. (2004) reference here.  

- Page 2, line 49: why some associated stations of PFR instruments are not part of GAW? What is the difference with full PFR-GAW stations?

- Page 3, line 81: it would be interesting to state main factors causing the higher uncertainty in the ratio cross-calibration.

- Page 3, line 86: please add a brief meaning of the CE318-TV12-OC model as it has been introduced here for the first time in the paper.

- Page 4 line 97: not sure the expression "the detectors are filtered" is correct in this case. 

- Page 4, line 103: do the collimator minimize stray light only when the sky radiance is measured? 

- Page 8, line 214: In the standard Langley method, constant tau is assumed; variations of tau are caused by variations of aerosol burden mainly. In the LR, constant delta_tau is now assumed; what is the main factor for variations of delta_tau during the LR process? I assume AE is a main factor, but a short comment could be useful here. 

- Page 8, equation 6: has been ancillary data used in this equation? or AERONET derived terms for the master instrument? (both tested sites).

- Page 8, line 229-230: I think it would be clearer if in this sentence it is stated that tau_F,a is estimated using the Angstrom law using data from the master AOD spectrum (or I assume it is how it has been done). 

- Page 9, line 267-268: Then no postcalibration and interpolation has been used to get the V_0,SL for the two photometers?

- Page 14, line 340: "tracking problems" make me think of technical problems appearing during tracking. I thknk the authors refer to the general lack of continuous tracking during the measurement sweep, ending on UV wavelengths. Maybe the authors should reformulate somehow the expression, for example "tracking limitations"? 

- PAge 15, line 357: what is the measurement time required for a triplet/individual sweep when the pointing is adjusted before each single specxtral measurement? 

References suggested:

V. Soufflet, C. Devaux, and D. Tanré. Modified langley plot method for measuring the spectral aerosol optical thickness and its daily variations. Appl. Opt., 31(12):2154–2162, 1992.

M. Campanelli et al. (2012) Monitoring of Eyjafjallajökull volcanic aerosol by the new European Skynet Radiometers (ESR) network, Atmospheric Environment 48, doi:10.1016/j.atmosenv.2011.09.070. 

                  This paper entitled present the Langley Ratio method for optimizing the calibration constants between two sun photometer that do not have the same spectral bands, although differences must be minimum. The method is great in advancing the optimization of sun-photometry, particularly between two different networks such as AERONET and GAW-PFR. Authors present the potentiality of the method and its applicability for detecting instrumental drifts. I recommend its publication, but before I have some issues that I would like the authors answer:

• Authors claim the importance of different field-of-view (FOV) of the instruments. Can you quantify of this affect the Langley Ratio method.
• I miss an intercomparision between the Langley Ratio and the classical Langley methos. It could have been possible with instruments at Izaña.

Technical comments

• Abstract: In lines 1-2 you refer to ‘photometer’. I propose to specify ‘sun photometer’.
• Line 44. Can you give the link to GAW-PFR network? Are the data publicly available?
• Line 56. What is ACTRIS? Can you give the link?
• Line 65. ‘Langley calibration technique’, a reference is needed.
• Line 66. What is the acronym ‘SI?
• Instrumentation: Adding a table that summarizes the main characteristics of each instrument would be ideal. Actually, the authors use different CIMEL versions that have different bands. The reader might find easier the importance of the Langley Ratio technique.
• Lines 110-112: There are more inversion techniques for obtaining aerosol microphysical properties. For example, check GRASP algorithm.
• Lines 113-114: A reference is needed for the statement about AOD uncertainties.
• Lines 114-117: Will the technique be used for ocean-color applications?
• Lines 186-187: I do not understand why Langley technique requires long observational periods of one/two months. The same statement is in the introduction (Lines 76-77). Theoretically with one day of measurements during very clean and stable conditions at high altitude you have Langley calibration.
• Equation 3: Is difficult to follow unless you define each of the variable. Same happens for Equation 4.
• Equation 5: It is not clear to me how do you compute the differences in aerosol optical depth.
• Section 5.1. It is important to know the ranges of AODs you have during the measurement periods.
• Line 263: Why limiting to airmasses 2-5
• Figure 1: How do you explain the outliers in the Figure? Particularly those above 2%. Why positive values predominate?
• Section 5.3. I do not understand relative differences if you are using the standard Langley calibration. Who is your reference?

The authors present a new method for transferring calibration from a reference photometer, using a synergetic approach when master and field instruments have different spectral bands. This new method, so called Langley Ratio method, was first applied between a PFR and a CE318-TS photometer, because these two photometers have different optics, sun-tracking systems and spectral bands. The campaign and validation at Izaña Observatory (IZO) and Valladolid showed the very low relative differences and standard deviations in the calibration constant transferred in Izaña from PFR to Cimel, up to 0.29 % and 0.46 %. This is really a satisfactory result, and the following studies vitrificated that the Langley Ratio method is a robust and suitable tool for transferring calibrations, detecting and correcting possible instrumental issues.

In summary, the paper is well-written and logically organized. I think it provided a useful way to conduct the calibration of sun photometer in a more efficient pattern, which is important thing for the observation network all over the world . So, I recommend this paper to be published in AMT after revision, but I still have some question that the authors should take into consideration as below:

1, Line 49. I think it is unnecessary to emphasize the Valladolid site is not a part of GAW.

2, Line 66. The authors should explain more about “SI”, as we don’t know what is the “SI”.

3, Line 70. I think the LR method is useful for the sun photometers in different/same spectral bands. However, the sentence here implies it just for the different spectral bands. The authors should check this.

4, Line 90. I think a table should be more useful here, to highlight the different spectral bands of device in this paper. So that to avoid much too long title in your Figures, repeating the bands.

5, Line 108. Maybe “every 15 minutes (in default)” is more accurate. This is an adjustable option in control box of CE318.

6, Line 195. The variable in equation 2 is undefined, please check.

7, Line 416. In conclusion part, the authors should give us some advice that the shortage of LR ratio method, or the un-suitable case, to avoid the calibration uncertainty.