This manuscript investigated diurnal variations of BrC in a megacity in Northeast China. The studied region is distinct, because it has quite different meteorological conditions and emission sources compared to well-known hotspots such as Beijing and surrounding regions. So far, however, aerosols in this region remained poorly understood with limited studies, e.g., regarding their chemical compositions, physical properties, sources and impacts. In this context, the authors conducted field measurements during two distinct seasons in a “largely unexplored” megacity in Northeast China, and traced the diurnal variations of BrC back to the changes of aerosol sources. The unique light absorption spectra of BrC observed during open burning episodes are especially interesting. Therefore, I think this manuscript represents a valuable contribution to better understanding of haze pollution in Northeast China, and my overall assessment is that it could be considered for publication in ACP after addressing the comments below.

Major point. MAE, which can be converted to the imaginary part of the complex refractive index of BrC, is an important parameter for climate models. In addition to a summary of the observational results, implications of this study should also be involved in the Conclusions section. To my understanding, although the winter is much colder in Northeast China compared to Beijing, MAE did not show apparent difference between these two regions. This is a potentially important point for the spatial distribution of MAE, but was completely ignored by the authors. In addition, the authors should make clear recommendations regarding whether the diurnal variations of MAE need to be considered in climate models.

Specific Points.

1. Lines 34-35. Suggest towing down this statement.
2. Line 75. Typical temperatures during winter in Beijing should also be given for comparison.
3. Line 96. Why did the daytime and nighttime samples have different sampling durations?
4. Line 112. I think sonication could increase the extraction efficiency of BrC.
5. Line 185. Suggest adding “compared to residential burning of crop residues” after “levels”.
6. Line 277. Suggest adding “robustly” before “unfold”.
7. Line 348. Is it necessary to introduce another indicator for open burning episodes?
8. Line 363. This sentence is unclear, rewrite it.
9. Lines 405-407. I would like to see a scatter plot showing the dependence of r on F or K.
10. Line 738. Suggest changing “another” to “the other”.
11. Line 752. Suggest changing “the same” to “a common”.
12. Line 761. I think it is better to clarify again that LG/OC involved in the equation was on a basis of carbon mass and was in %.
13. Table S2. Re-write the note as: AAE were not provided due to the frequent occurrences of agricultural fires, which could result in distinct peak at ~365 nm for the light absorption spectra of BrC.

The manuscript of Cheng et al. reports the diurnal variations of brown carbon (BrC) investigated during two distinct seasons in the northernmost megacity of China. Authors discussed drivers of diurnal BrC variations observed in two seasons, i.e., a cold winter (January 2021) and an agricultural fire-impacted spring (April 2021), relying on indicators of various sources.

This paper is well written, the experimental part is well presented and, along with citing the relevant literature, the experimental approach is well described. However, my main concern is directed to data presentation, interpretation, and drawing the conclusions as will be indicated later. Considering the importance of the topic that is the focus of this article, my overall assessment is that this paper should be considered for publication in ACP, but after major revision since there are some issues that need to be addressed to improve this work. 

General comments  

The authors hypothesized on more absorbing BrC at night, based on comparison of mean nighttime and daytime MAE₃₆₅ values in winter. However, I do not see that this difference is statistically significant. Furthermore, authors attempted to explain the drivers of observed “diurnal variations”, but have not reached a clear conclusion, which is not surprising since it is double if the diurnal difference even exists. In fact, authors discussed that the predominant influencing factor for MAE₃₆₅ is vehicle emissions, especially those from nighttime HDDTA transport, based on the lower average R_S/N observed at night (0.5 ± 0.1) compared to R_S/N for the daytime samples (0.7 ± 0.2). The problem here is again that the average R_S/N valuesobtained for the nighttime and daytime samples were not statistically different and such a conclusion is overstated.

The authors should first test the statistical significance of the MAE₃₆₅ difference between night and day in winter. Furthermore, the discussion and conclusions should be based on statistically reliable data, and rigorous arguments need to be added to this paragraph. I suggest rewriting this paragraph, including changing the title.

Diurnal variations of MAE₃₆₅ in spring (averaging 0.98 ± 0.31 and 1.69 ± 0.65 m²/gC) should also be disused based on statistically proven difference between the day and night samples.

Specific comments

L240-L241 MAE₃₆₅ exhibited a negative dependence on R_S/N for nighttime samples? Please explain.

L325 Please explain how Fig 2b is created. Are there cumulative fire events present for January and April? Please indicate this in the figure caption.

L335 Is there any evidence of more frequent/intense nighttime burning from NASA/NOAA Suomi National Polar-orbiting Partnership (S-NPP) satellite, and/or the Fire Information for Resource Management System?

L390-L393 I agree that aromatic compounds with nitro-functional groups are good representatives of BrC related to biomass burning emission. I suggest not referring specifically to methylnitrocatehols, but rather to aromatic compounds with nitro-functional groups in general.

L439-L441 Based on my general comment above, please rewrite this part of the conclusion about the higher MAE₃₆₅ observed at night in winter samples.

L453-L455 Please rewrite the sentence since in its current form one could read that your work also involves chromophore absorption spectra and molecular measurements.