This manuscript presents an investigation of water soluble brown carbon aerosols in several provincial cities in northwest China. These regions are distinctive due to their contrasting meteorological conditions and emission sources compared to "hotspots" such as the Beijing-Tianjin-Hebei and Yangtze River Delta regions. Due to less exploration on aerosol chemistry in these cities, understanding of BrC aerosols in these urban areas has been limited so far. This manuscript presents many properties of BrC during wintertime. The most interesting point is that they provide a unique approach by combining BrC chromophores with WSOA factors, utilizing V-K diagrams and EEMs diagrams to illustrate chemical processes and optical changes. The results are useful for understanding of haze pollution and estimating of the climatic effects of BrC aerosols in energy-producing and heavy manufacturing cities in Northwest China. Overall, the manuscript fits well within the scope of ACP, and the results and presentation are reasonable and clear. However, there are a few issues that should be addressed before publication.

Major issues

1. This manuscript focuses on water-soluble brown carbon (WS-BrC); however, it is important to note that only a portion of BrC is water-soluble. Therefore, it is recommended to provide a more detailed description of water-soluble organic aerosols (WSOA) in the introduction section. Additionally, including relevant literature that discusses the molecular and chemical functional groups of WSOA would enhance the introduction section.
2. The evaluation of the radiative impact of WS-BrC is import for the radiative forcing of aerosol. In addition, the imaginary component of the complex refractive index of BrC is a critical input parameter for climate models. Numerous studies have employed the methodology proposed by Bikkina and Sarin (2019) to calculate the simple forcing efficiency (SFE) model of BrC to indicate the radiative forcing produced by a unit mass of PM on the atmosphere. These parameters could be considered.

Minor issues

1. Line 38: Typo in “including three humic-like substances (LO-HULIS, HO-HULIS1, and HO-HULIS2)”. Only two factors in section 3.5.
2. Line 112: Replace "PM2.5" with “PM_2.5”.
3. Some formulas in the method section appear error italics and normal font. 
4. 4. Line 136: water-soluble inorganic ions Commonly abbreviated as WSIIs.
5. In Figure 2e and 2f, data is missing at different sample ids. What is the reason?
6. 6. Unify the spaces before and after the mathematical operator, e.g. Missing space before "=" in line178,535,566 and Missing space before “+” in line 484, 515.
7. In Figure 6, it can be observed that the fluorescence peak position shifts significantly with the change of pH, so it is suggested to briefly discuss it.
8. Line 416: Replace "255nm" with “255 nm”.
9. Please unify to unit format, such as m²/g and μg m^-3
10. Line 413-416: “C5 was likely from primary sources such as coal burning and vehicle emissions, while C1 was secondary production.” Can you give a detailed explanation of how you came to this conclusion and please provide references.
11. What method is used to analyze the correlation between AMS and chromophores? Need to indicate the relative contribution or absolute strength of AMS factors or fragment ions used?

Bikkina, S. and Sarin, M.: Brown carbon in the continental outflow to the North Indian Ocean, Environ Sci Process Impacts, 21, 970-987, 10.1039/c9em00089e, 2019.

This study compared optical properties of BrC in wintertime aerosols in four capital cities of Northwest China. The authors also analyzed the chemical compositions by EEMs and HR-ToF-AMS, and discussed the difference in sources and chemical processes of BrC in the four cities. This study was well organized, and the paper was also well written.  I recommend the manuscript to be accepted with a moderate revision. Detailed comments were listed below.

Major comment:

In L16-19, and L86-94, the authors seems try to emphasis the important contribution of coal burning to the aerosols and BrC in the four capital cities of Northwest China. However, both optical properties and chemical compositions (such as AMS data) didn’t support it. In particular, in L 297-302, and L310-311, biomass burning would be the dominant source of BrC in this study. I suggest to revised the Abstract and Introduction for this point.

Minor comments:

1. L269-271, I would say WSOC/OC in Beijing (0.74) and Guangzhou (0.71) were obviously higher than those in this study.
2. L307 and 308, please give the full spelling of W-BrC and VW-BrC.
3. L361-363, Did’t you mean the absorbance at pH=10 increased …. relative to the pH=2?

4 L490, please give the full spelling of FF-POA.