General comments:

The study investigates the optical properties and molecular characteristics of atmospheric brown carbon (BrC) in Shenzhen, China, focusing on how different sources impact Earth’s radiation budget. BrC absorbs light in the UV-visible range, and its properties vary depending on the source. Molecular analysis revealed that biomass burning species from the CHON family exhibited the strongest absorption. These findings may help improve the understanding of BrC's radiative impact and enhance climate model accuracy. Overall, the paper is very clear and can be published after a minor revision.

Specific comments:

1. Line 77-79: For the comparison of online and offline measurements, please also refer to Chen et al. 2022a.

2. Line 99-102: This sentence needs to be rewritten because of the following comments.

(1) “Nitrophenols, nitrobenzene, and their derivatives” is not an appropriate description. Nitrobenzene itself, unfortunately, only has weak absorption in the tropospheric wavelength range (wavelength above 290 nm) (Frøsig et al., 2000). Strong light absorption mainly relies on the further substitution of carbonyl or hydroxyl groups (i.e., nitrobenzaldehyde, and nitrophenol, as the author mentioned). It would be preferred to replace “Nitrophenols, nitrobenzene, and their derivatives” with “Nitroaromatics”.

(2) Benzene is not the only backbone for BrC molecules; pyrrole, naphthalene, and indole backbones can also be substituted by nitro groups and become nitroaromatic “BrC molecules” (Jiang et al., 2019; Mayorga et al., 2022; Baboomian et al., 2023; Cui et al., 2024; Dalton et al., 2024). Please also mention this information in the sentence.

(3) Actually, “BrC molecules” is not a commonly used phrase, it would be better to replace it with “BrC chromophores”.

3. Line 216-225: Please provide some technical details about the offline preparation (e.g., filter storage temperature, filter extraction procedure).

4. Line 222: “L (m)” should be “L (cm)” or just “L”, according to the “1 cm” in Line 223.

5. Line 314-315: Technically, the offline MAC calculated by Equation (8) may not be directly compared with the online MAC because of the Mie effect (Liu et al., 2013; Zeng et al., 2020; Zeng et al., 2021; Chen et al., 2022a). If possible, please calculate the Mie-converted offline MAC based on the reference; otherwise, if the Mie model cannot be accessed for this calculation, please provide some descriptions in the manuscript to highlight this uncertainty.

6. Line 256-357: Please also highlight that the light-absorbing CHO compounds may come from biomass burning smoke (Desyaterik et al., 2013; Chen et al., 2022b; Zhou et al., 2022; Chen et al., 2023)

7. Line 366-369: CHON from biomass burning may also involve pyrrole-derived and indole-derived species, including C4H4N2O2 (nitropyrroles), C4H3N3O4 (dinitropyrroles), C4H2N4O6, C4H3NO2 (maleimide), C4H3NO3, C4H3NO4, C4H5NO3, C4H5NO4, C8H5NO2 (isatin), C8H6N2O2 (nitroindole), C8H7NO4, C16H10N2O (indoxyl red), and C16H10N2O2 (indigo dye) (Montoya-Aguilera et al., 2017; Jiang et al., 2019; Mayorga et al., 2022; Chen et al., 2022; Baboomian et al., 2023; Chen et al., 2023; Chen et al., 2024; Dalton et al., 2024; Jiang et al., 2024). Did the authors also see these compounds?

8. Since this paper emphasizes the “molecular characteristics” at the title, please (if possible) provide a table that summarize the major BrC molecular chromophores identified in the field samples.

References:

Baboomian et al., Light absorption and scattering properties of indole secondary organic aerosol prepared under various oxidant and relative humidity conditions. Aerosol Sci. Technol., 2023, 57(6), 532–545. https://doi.org/10.1080/02786826.2023.2193235 

Chen et al., Solvent effects on chemical composition and optical properties of extracted secondary brown carbon constituents. Aerosol Sci. Tech., 2022a, 56(10), 917–930. https://doi.org/10.1080/02786826.2022.2100734

Chen et al., Effects of Nitrate Radical Levels and Pre-Existing Particles on Secondary Brown Carbon Formation from Nighttime Oxidation of Furan, ACS Earth Space Chem. 2022b, 6, 11, 2709–2721. https://doi.org/10.1021/acsearthspacechem.2c00244 

Chen et al., Contribution of Carbonyl Chromophores in Secondary Brown Carbon from Nighttime Oxidation of Unsaturated Heterocyclic Volatile Organic Compounds, Environ. Sci. Technol. 2023, 57, 48, 20085–20096. https://doi.org/10.1021/acs.est.3c08872 

Chen et al., Relative Humidity Modulates the Physicochemical Processing of Secondary Brown Carbon Formation from Nighttime Oxidation of Furan and Pyrrole, ACS EST Air 2024, 1, 5, 426–437. https://doi.org/10.1021/acsestair.4c00025 

Cui et al., Chemical Composition and Optical Properties of Secondary Organic Aerosol from Photooxidation of Volatile Organic Compound Mixtures, ACS EST Air, 2024, 1, 4, 247–258. https://pubs.acs.org/doi/10.1021/acsestair.3c00041 

Dalton et al., Isomeric Identification of the Nitroindole Chromophore in Indole + NO3 Organic Aerosol, ACS Phys. Chem Au 2024, 4, 5, 568–574. https://pubs.acs.org/doi/10.1021/acsphyschemau.4c00044 

Desyaterik et al., Speciation of “brown” carbon in cloud water impacted by agricultural biomass burning in eastern China, J. Geophys. Res. Atmos., 2013, 118, 7389–7399, doi:10.1002/jgrd.50561

Frøsig et al., Kinetics and Mechanism of the Reaction of Cl Atoms with Nitrobenzene, J. Phys. Chem. A, 2000, 104, 48, 11328–11331. https://pubs.acs.org/doi/10.1021/jp002696o 

Jiang et al., Brown Carbon Formation from Nighttime Chemistry of Unsaturated Heterocyclic Volatile Organic Compounds, Environ. Sci. Technol. Lett. 2019, 6, 3, 184–190. https://pubs.acs.org/doi/10.1021/acs.estlett.9b00017

Jiang et al., Molecular analysis of secondary organic aerosol and brown carbon from the oxidation of indole, Atmos. Chem. Phys., 2024, 24, 2639–2649, https://doi.org/10.5194/acp-24-2639-2024 

Liu et al., Size-resolved measurements of brown carbon in water and methanol extracts and estimates of their contribution to ambient fine-particle light absorption, Atmos. Chem. Phys., 2013, 13, 12389–12404, https://doi.org/10.5194/acp-13-12389-2013

Mayorga et al., Chemical Structure Regulates the Formation of Secondary Organic Aerosol and Brown Carbon in Nitrate Radical Oxidation of Pyrroles and Methylpyrroles, Environ. Sci. Technol., 2022, 56, 12, 7761–7770. https://pubs.acs.org/doi/10.1021/acs.est.2c02345 

Montoya-Aguilera et al., Secondary organic aerosol from atmospheric photooxidation of indole, Atmos. Chem. Phys., 2017, 17, 11605–11621, https://doi.org/10.5194/acp-17-11605-2017 

Zeng et al., Global Measurements of Brown Carbon and Estimated Direct Radiative Effects, Geophys. Res. Lett., 2020, 47, e2020GL088747, https://doi.org/10.1029/2020gl088747

Zeng et al., Assessment of online water-soluble brown carbon measuring systems for aircraft sampling, Atmos. Meas. Tech., 2021, 14, 6357–6378, https://doi.org/10.5194/amt-14-6357-2021

Zhou et al., Molecular Characterization of Water-Soluble Brown Carbon Chromophores in Snowpack from Northern Xinjiang, China, Environ. Sci. Technol. 2022, 56, 7, 4173–4186. https://doi.org/10.1021/acs.est.1c07972 

“Source-Dependent Optical Properties and Molecular Characteristics of Atmospheric Brown Carbon” describes measurements of the composition and optical properties of BrC aerosols sampled during the summer in Shenzhen, China. Different sources of BrC are proposed based on the measurements that have different mass absorption cross-sections (MAC). BrC is a complex topic that is of importance due to its role in understanding the radiative budget. Understanding real-world sources of BrC and constraining its optical properties is an area of active investigation that would greatly benefit from more data. While the underlying data from this manuscript may be able to contribute to and advance our understanding of BrC, it is my opinion that the analysis is insufficiently developed. Substantial additional analysis is required and, in my opinion, publication at this time is premature. Below I outline the main reasons. 

Main comments

• In my opinion, the main contribution that this manuscript attempts to make is in providing source specific MAC and AAE for three different BrC sources. However, the attribution of the measurements to these specific sources in tenuous and requires substantial further justification. The sources are identified based on classification of 3 cases. From what I can tell, the classification was made based on only a few parameters (for Case 1, ozone, particulate nitrates, wind speed, and duration; for Case 2; wind direction, wind speed, duration; Case 3; concentration of particulate potassium, duration). This limited number of variables is insufficient for source attribution, particularly in a complex environment and if the goal is to provide profile information that is representative of a given source. There may be sufficient data already collected (i.e. the mass spectral measurements) to do a more accurate source apportionment.
• In real-world samples, multiple BrC sources are likely to be mixed together. This fact and how it affects the results is insufficiently described. I am not convinced by the data presented that each period could be attributed to a single BrC source. How mixing of various sources affects the retrieved BrC properties is essential for accurately reporting profiles. Without consideration of the mixing of BrC sources, the results (in their current state) are of limited use to the community.
• To improve the understanding of BrC sources, it is important to consider how representative the results of a case study such as this are. Knowing the variability of BrC characteristics for a given source is critical for achieving the outcomes hat the manuscript identifies as the driving science questions (e.g., improving models, etc.). The failure to show the variability could be impacted by the limited duration of the measurement, but the broad averaging approach taken for assigning BrC sources (rather than more specific source apportionment) also contributes. It may be possible to understand more of the variability in the data set and thus improve the impact of the work by applying statistical analysis of the data and considering more of the days.
• The manuscript insufficiently discusses how aging (browning or bleaching) would affect the results. What are the timescales of aging are anticipated? How do those compare to what is known about rates of browning/bleaching?
• It appears that MAC values are reported based on organic carbon mass rather that organic aerosol mass. The implications of that difference and how it affects comparison between these results and results from the literature should be clearly stated to avoid the misuse of the results by future studies.

Minor comment

The manuscript does not clearly communicate the length of the measurement period (~2 weeks judging by Fig. 1). The wording in the abstract (line 27 “summer”), introduction (line 114 “summer”), and methods (line 125 “August to September”) all imply that the measurement period is much longer than it was. These statements should be updated to not mislead the reader.

Summary: The underlying data presented seems potentially useful. However, further analysis is required for the results to be of use/application beyond this one study. Given the data collected, I think a more nuanced analysis of the measurements is potentially possible and such results could be potentially publishable.

The atmospheric conditions are complex and changeable, considering both the emissions and meteorological conditions. So it could be considered “super lucky” to capture three different feature events in field measurements which is only 12 days long. However, based on what the authors currently show in the paper, the determination of different events, at least Case 3, is vague. I think this study could contribute to the research on BrC, if the authors could address the following comments properly.

Section 2.2 is too long. It is suggested that the section 2.2 could be split into two parts, i.e., online and offline determination of BrC, respectively, to make it easier to read.

Figure 1: the axis label of left axis in Figure 1c is suggested to revise. The current expression may lead to some understanding, e.g., the ratio of nitrate/sulfate.

Lines 250-263: K+ may also come from sources other than biomass burning, e.g., fireworks, so the determination of Case 3 should be with caution. Since the authors have collected filters, the authors are suggested to determine the abundances of other biomass burning tracers to help support the identification of Case 3.

Section 3.1: since the title is “light absorption of BrC”, it would be more appropriate to add some discussions on results from water-soluble absorption as well.

Figure 3: it is suggested to use different expressions of AAE370-550 for online and offline results.

For the chemical characterization, as the collection period for each filter is 24 hours, does it mean that the compounds determined were only from one or two samples for each Case? Then is it representative for this Case?