This manuscript presents interesting field measurements in the Qinghai–Tibet Plateau (Yangbajing) and urban Guangzhou during July 2022 with established measurement techniques including multi-wavelength Aethalometer and online aerosol mass spectrometer (AMS and ACSM) measurements to quantify black carbon (BC), brown carbon (BrC), and organic aerosols. Source apportionment (positive matrix factorization, PMF) and multilinear regression analysis were conducted to provide further insights on BrC source-specific mass absorption cross-sections (MAC), and a simple forcing efficiency (SFE) for individual OA factors. This work clearly falls within the scope of ACP, focusing on aerosol light absorption properties, radiative forcing, and aerosol composition. In addition, the manuscript provides a valuable comparison between a polluted city environment and a “pristine” environment (i.e., YBJ site), for which very few datasets have been reported so far.

However, the Method sections should include more technical details to further support the results and interpretation. And some results should be discussed further to extract more value from the dataset hence to improve the manuscript’s relevance and importance. I recommend publication after the points outlined below are addressed.

A few general comments. It would be useful to provide a broader discussion of the impact of BrC on radiative forcing in both regional and global contexts. There are some comments in the introduction and conclusion sections but they are rather scattered. This could be difficult for readers not familiar with aerosol speciation and its connection to regional BrC.

SP-AMS was referred in Section 2.3 (L154) and SI. However, insufficient operational information were provided. With the results and information provided by the SI, I would assume the instrument was operated with tungsten vaporizer. In that regard, the instrument is typically referred to as the “high-resolution aerosol mass spectrometer (HR-AMS or HR-ToF-AMS)”. If dual vaporizer mode was used, please provide the relevant details, as this would also affect the interpretation of the PMF results (e.g., refractory BC should be included in the PMF analysis).

For AMS and ACSM PMF analysis, only limited justification is provided in the SI and I have several questions:

1. For the AMS PMF, In Fig. S2a, the Q/Q_exp dropped below 1.0 after the 2-factor solution. Could you provide more justification for factor separation? All of your measurements peak at 0800 but not 2200 LT while the above 3 factors have similar diurnal profile, and some overlapping spectra. Could you provide more evidence?

1. Following the above question, rBC and metal detected by the dual vaporization mode can help separate sources further (Bibi et al., 2021; Ma et al., 2025; Rivellini et al., 2020). It is relevant for the context since the HOA and BBOA have very similar diurnal profile. If the instrument measured with dual vaporization mode, PMF should be evaluated with refractory components.

1. For Fig. S4a, it is somewhat confusing to label peaks using m/z values since they are high-resolution PMF results.

1. For the ACSM PMF, please provide the rationale for constraining BBOA.

1. Changes in final PMF factors will affect MLR analysis.

For the assumption of AAE_BC value, could you elaborate more on the decision to use 0.8-1.2 range? To be specific, L203 stated “In this study, the value of AAE 1.4 lead to most of the BrC light absorption coefficient to be negative values…” However, some studies have shown that thermal denuded BC_AAE values can be higher than 1.4 (e.g., Török et al., 2018). I may miss the justification of BrC existence at both sites.

Minor comments

A few sentences could be reorganized to improve clarity, particularly at L38-41,L87-90,  and L289-292.

L289: revise to …”where BrC contributes the most”

L 445-448. Do you mean some of the HOA is transported? It is interesting the se, at YBJ site, HOA increased at night time while NO stay flat or decreased. What are the sources to the strong morning NO spike at YBJ and GIG sites?

L525: …”Singapore during winter” is misleading as Singapore do not have winter season based on temperature.

Reference

Bibi, Z., Coe, H., Brooks, J., Williams, P. I., Reyes-Villegas, E., Priestley, M., Percival, C. J., and Allan, J. D.: Technical note: A new approach to discriminate different black carbon sources by utilising fullerene and metals in positive matrix factorisation analysis of high-resolution soot particle aerosol mass spectrometer data, Atmospheric Chem. Phys., 21, 10763–10777, https://doi.org/10.5194/acp-21-10763-2021, 2021.

Ma, M., Rivellini, L.-H., Zong, Y., Kraft, M., Yu, L. E., and Lee, A. K. Y.: Advances in characterization of black carbon particles and their associated coatings using the soot-particle aerosol mass spectrometer in Singapore, a complex city environment, Atmospheric Chem. Phys., 25, 8185–8211, https://doi.org/10.5194/acp-25-8185-2025, 2025.

Rivellini, L. H., Adam, M. G., Kasthuriarachchi, N., and Lee, A. K. Y.: Characterization of carbonaceous aerosols in Singapore: insight from black carbon fragments and trace metal ions detected by a soot particle aerosol mass spectrometer, Atmospheric Chem. Phys., 20, 5977–5993, https://doi.org/10.5194/acp-20-5977-2020, 2020.

Török, S., Malmborg, V. B., Simonsson, J., Eriksson, A., Martinsson, J., Mannazhi, M., Pagels, J., and Bengtsson, P.-E.: Investigation of the absorption Ångström exponent and its relation to physicochemical properties for mini-CAST soot, Aerosol Sci. Technol., 2018.