This work investigated the impact of low temperature on the formation of strong BrC chromophores in HULIS and proposed two mechanisms, one is that the low temperature may lead to a non-liquid phase state of ambient particles, potentially introducing kinetic limitation on the diffusion of reactive species from gas phase into the bulk aerosol and second low temperature promoted the reaction of phenols with NOx radicals while inhibited the atmospheric oxidation of nitrophenols, thus facilitating the accumulation of BrC chromophores such as nitrophenols in HULIS. Overall, the paper was well-organized, and the results are of broad interest. I would recommend the paper be accepted after revision, as outlined below.

1 line 48-50. Please elaborate further on the assertion that HULIS is under low-temperature conditions for the majority of its atmospheric lifetime. This statement serves as a critical foundation for the research motivation and requires more evidenced or references to support it.

2 line 83-84 and Text S1. This semi-quantification strategy was based on the experiment experience that substances with similar structures and equal concentrations yield comparable signal intensities in mass spectrometry. Please specify the criteria for selecting the semi-quantification proxy compounds. Additionally, a thorough discussion on the uncertainties associated with this semi-quantification approach should be provided.

3 Move compound identification and quantification from SI to the main text and a discussion of semi-quantification errors, as suggested above, should be incorporated into the manuscript (either in the main text or the SI).

4 Section 3.2. In the discussion on the potential sources of HULIS, the authors selected two PM haze events instead of high HULIS episodes. What’s HULIS concentration during these two PM pollution events. Additionally, please include the mass spectra of HULIS samples from non-haze days for comparison with those in Event I and II. This would clarify whether the identified sources were specific to HULIS or more generally associated with PM2.5.

5 Section 3.3. The discussion of the effect of temperature on BrC chromophore formation is arguably the most important aspect of this study. As the author proposed, low temperature promoted the exothermic process (e.g., reaction of phenols with NOx radicals) and hindered the endothermic chemical reactions (e.g., atmospheric oxidation of nitrophenols), thus facilitating the accumulation of BrC chromophores in HULIS. In addition to this qualitative thermodynamic explanation, a more in-depth discussion is encouraged. Specifically, how does low temperature alter the overall atmospheric chemistry and also any comparable results/findings/ mechanistic insights from previous literature that support your conclusions.

This study collected atmospheric PM2.5 samples during wintertime in Changchun and identified the molecular structures in HULIS, providing a new insight into the potential sources and temperature effects on the light-absorption properties of HULIS. We have gained some new insights through this study, but there is still room for improvement.

Is the temperature division in Figure 3 based on the daily average temperature? Studying the impact of temperature on HULIS based on daily average temperature may be too crude. In addition, the temperature in the sampling area is generally low. How to compare the impact of high temperature on the formation of HULIS, because the so-called high temperature during the sampling period is far from the general high temperature, such as 20℃ or even 30℃ or above. Please clarify. Warm and cold need to be indicated the temperature range in the Graphical abstract.

The average concentration and standard deviation of PM2.5 during the entire sampling period need to be reported in order to reflect the rationality of the selection of two typical haze events. The PM2.5 concentration of Event II is not high. Please indicate the basis for selecting these two events. The daily meteorological data and mean values of the entire sampling process also need to be presented in the attachment.

Table S5 only shows the concentrations of K+ and SO2 for two pollution events and does not compare them with other non-polluting days. How can you know that these two pollution events were strongly contributed by biomass and coal combustion?

Can the mechanism of low temperatures reducing the photobleaching of brown carbon be further explored? And is it related to relative humidity?

This work investigates the molecular compositions and light absorption of HULIS in cold environments. The data is informative and interesting. However, some statements need more evidence and detailed in the main text. The following comments need to be addressed before publication.

Major comments:

Introduction: There have been many studies on revealing the molecular-level characteristics and compositions as well as the relation to HULIS light absorption. I may suggest the author review the literature on the HULIS molecular compositions in the introduction section. This would be helpful to gain a deeper understanding on the findings in this work.

Some important information in the experiment section is omitted. I strongly suggest moving some important information from the supplementary to the main text, especially for the molecular composition analysis of HULIS and the quantification of specific compounds. At least some of the key information should be briefly introduced in this section.

Lines 142-145: How did the authors choose the samples with "significant differences in MAE"? The criteria need to be detailed here.

Lines 169-171: More evidence should be provided to propose the compounds in Cluster II from secondary formation. In addition, the contribution and proportion of OSs among Cluster II need to be detailed here.

lines 189-199: My major concern: Have the authors checked the variation of EC concentrations as a function of ambient temperature? I may suggest plotting the EC concentration in Figure 3. If low ambient temperature is accompanied by high EC concentration, the increasing of combustion emissions could be the primary reason for the pattern in Figure 3. If not, the author should state in detail and exclude this possibility.

I suggest the authors to compare the quantification result of the BrC chromophores in Figure 3 with previous studies. Many nitro-aromatic compounds have been widely quantified in previous studies, and their contribution to organic aerosol concentrations and light absorption has been evaluated. The mass contribution of some nitro-aromatic compounds seems higher than previous studies. These should be explained, and the uncertainty of the quantification should be stated. In addition, the conversion from HULIS-C to HULIS mass should be described.

As some nitro-aromatic compounds have been quantified, I may suggest evaluating the light absorption contribution of some nitro-aromatic compounds with available standards. As the author stated, nitroaromatics are generally with high absorption capability. How much of the HULIS absorption is contributed by the quantified chromophores?

Specific comments:

Line 45: Change “elemental composition” to “molecular composition”.

Lines 116-120: Revise these sentences to be clear.

Line 125: Revise to be clear.

Figure 2: It seems strange plotting the mass spectra in positive mode from large to low molecular weight. The figure caption also needs to be revised.

Line 165: Nitro-aromatics could be formed via secondary formation. Please correct.

Lines 166-167: Revise to be clear.

lines 179-180: Revise to be clear.

lines 187-188: Revise to be clear.