This study compared the different parameters retrieved by two model Aethalometers (AE31 and AE33) based on one long-term observation. These parameters are important to study the environment and climate effects of absorbing aerosols. I recommend the publication of this paper after the following comments are solved.

Major comment.

This study only simply compares the measurement values of two model Aethalometers. The retrieved parameters are highly dependent on the transformation parameters. These transformation parameters are artificial, which has an important on the final comparison results. Therefore, the sensitivity test of these transformation parameters is needed. In addition, this study should give a conclusion about which model Aethalometer measures more accurately. Therefore, the comparison of two Aethalometers with other instruments (such as PAS) is needed to solve this problem.

Detailed comments.

Line 75. What is the special significance of one year of observation? The authors didn’t talk the intercomparing seasonality in the following discussion.

Line 100. The flow rate of AE33 is generally set as 5 Lpm. Whether this flow difference will cause a difference in the comparison result?

Line 105. What’s the RH range of dried sample flow? More information about the sample needs to be provided.

Eq. 8 and Eq. 15. The values of C_ref and C’ have an expected influence on the comparison result. Different studies used different empirical values. The sensitivity analysis of these values on the comparison result is recommended.

Eq. 20. Compared to maintain the consistency, it’s more important to acquire eBC accurately. This equation assumes the right measurement by AE31, and then correct the measurement by AE33. However, the measurement of AE33 maybe more accurate because of the technology development. Therefore, it’s more reasonable to correct AE31 data. More comparison of two Aethalometers with other instruments (such as PAS) is needed to solve this question.

Review of manuscript

“Field comparison of dual- and single-spot aethalometers: Equivalent black carbon, light absorption, Ångström” authored by Liangbin Wu et al.

General comments

After AE31, Aethalometer model AE33 is also joining in monitoring black carbon (BC) mass concentration and light absorption coefficient (b_abs) around the world. This implies a need of field intercomparison of the two models. In this study, the authors carried out a year-long collocated measurement comparison of the two models in urban Guangzhou. Considering that field intercomparisons of the two models are limited and the difference in secondary brown carbon light absorption estimation between the two models is largely unknown, this study could add knowledge about the instruments and applications. Scholars around the world could learn much from the results of this study. I recommend publication of this work in AMT once my concerns, major or minor, have been properly addressed.

Specific comments

1. To compare the performance of the two aethalometer models, AE31 and AE33, a collocated observation campaign should begin with a carefully prepared state. It is not clear here whether the two instruments were status-checked. Another concern relates to whether the two instruments were calibrated with a reference instrument of absorption measurement (such as PAX, PASS, or MAAP), or at least flowrate-calibrated with a standard flow meter. Informing readers of the starting point helps understand the comparison results in this study.
2. Line 31 and some other lines: In the manuscript, site-specific correction was again and again proposed possibly because the authors tend to believe that some differences are caused by differed composition or property of monitored aerosols from different sites. This is only a possibility instead of a certainty considering many model-related factors like hardware, mechanism, or filter tape, may all make differences in results. For example, filter tapes from different manufacturers with different materials may give substantially differed information in scattering coefficient, AAE, and so on. From this perspective, authors of this manuscript are proposed to be cautious whenever attributing differences between AE33 and AE31 to site-specificness unless properly and fully evidenced. Particularly when you used two different models of aethalometer in the same site in this study. Check through the manuscript to revise improper descriptions.
3. Equation (15) and the paragraph below: The authors used C′, C_AE33, and C_ref to represent different meanings for scattering correction. The denominator of Equation (15) was used to correct for the bias generated by filter scattering. In this sense, C′ is anyway not appropriate because you have employed C′ to represent “second correction factor”. I suggest the authors keep unambiguity and consistency in definition and application of the three forms of conversion factor and revise Equation (15).
4. Figure 3: Since you have customized the correction factor for filtering scattering (C_ref?) according to the differences in eBC between AE31V and AE33 so that equal eBC values from AE31V and AE33 could be yielded, it is not surprising that an extremely good correlation and slope (1:1) was observed after 2nd correction. For this reason, I don’t think Figure 3 is of high value. You may move it to Supporting Information.
5. Regarding toolkits: The authors mentioned a few tookits for convenient data processing; quite helpful, I think. But for readers, little is known about the toolkits unless they go to read the articles the authors cited. Therefore I ask the authors to give a brief description on individual toolkits where the toolkit and associated articles are cited so that readers could know something on the principle and mechanism helpful for understanding output results.
6. Lines 233-246: In this paragraph, the authors described LODs of eBC based on AE31 and AE33. Some of the results are easy to understand, e.g., longer time bases lead to lower LODs; but some results need a follow up explanation. For example, the LODs are wavelength-dependent, why? Another example, AE33 have better LOD than AE31, why? Are there any new technologies having been incorporated in AE33 and thus contributed to the improvement in LOD? Do you know what technologies that work?
7. Lines 284-285: “presumably because of the high loadings of light-absorbing aerosol particles at this urban site”, why? Can you argue for it?
8. Lines 354-355: “These results suggest that AAE_BC determination by AAE880/950 is suitable for AE33 data but not suitable for AE31 data”. Interesting, but why? Do you have some evidences (e.g., literatures) for this point?

Technical corrections

1. Line18-19: “However, field intercomparison of the two popular models, dual-spot (AE33) and single-spot (AE31) aethalometers, remain limited”. Note the disagreement between subject and predicate in number. Please check through the manuscript against similar errors (e.g., in lines 280-281).
2. Line 27: BrC_sec is an abbreviated form. Give complete form first. Please also check through the text against similar errors.
3. Line 88: Delete “was”.
4. Line 21, 66 collocated; Lines 82, 98, 383 colocated. Did you attempt to use “collocated” and “colocated” the same meaning?
5. Line 100: Suggest inserting a hyphen between single and spot. L131: Add “where” before “eBC_corrected”.
6. Line 197: Change “Text” to “Test”.
7. Lines 209-211: It seems better if you change “the BrC_sec is generated during the secondary aging process” to “the BrC_sec is secondarily generated during the aging process”.
8. Line 325: Attention, “fiiting” is ill-spelled.
9. Line 363: “those” should be “that”; similar cases can be found in lines 410, 414.
10. Line 378: obtain should be “obtained”.
11. Line 402: Revise “1.39∙ 2.1” to “1.39×2.1”.
12. Table 1: For AE31, there have been a column for “filter” scattering correction. why not add a column for loading correction?
13. Figure S1: Attention to “change of of sampling spot” in the title.