Brown carbon (BrC) is a poorly characterized component of particulate matter that absorbs solar radiation and may contribute significantly to global warming. Some of the factors responsible for the current knowledge gap on BrC are the lack of understanding of its chemical composition, light-absorption properties, and contribution to total particulate mass and light absorption, which are likely subjected to significant spatial and seasonal variations. The measurement report by Dong et al. focused on the physicochemical characteristics and sources of BrC in Tianjin, northern China. In particular, the light absorption and excitation emission matrix fluorescence of both water-soluble and water-insoluble but methanol-soluble BrC in PM_2.5 were measured off-line using a three-dimensional fluorescence spectrometer. The measurements were performed for one year and the seasonal variations were investigated. The results showed clear seasonal differences both in the BrC light-absorption and the type of constituting chromophores, an association with the potential sources, and a significant contribution of BrC to climate warming. The methodology used was appropriated for the described investigation, and the obtained results were well presented and of high relevance to the field of atmospheric science. Therefore, I recommend for publication after the following comments are addressed: 

Major comments:

P6L242-The authors claim that “The higher Abs365 in winter indicates that the light absorption of BrC in PM2.5 may have more significant effect on the climate and the photochemical reactions in the atmosphere over Tianjin in winter than in other seasons”. However, the effect of aerosols on climate are complex and photochemical reactions depend as well on the type and amount of atmospheric oxidants, precursor VOCs, and many other factors. Therefore, these conclusions are difficult to estimate based on a higher light absorption at a specific wavelength.

P7254-In this sentence it is mentioned that the absorption coefficient of WI-MSBrC was always greater than that of WSBrC across the shorter wavelengths. However, In Fig. 2a the absorption coefficient of MSBrC was lower than the one of WSBrC from autumn to spring, except at a wavelength of about 280nm.  

P11L301-The results show a higher correlation between Abs365 and K+ in spring and autumn, especially for WSBrC, which is opposite to what was observed for Cl- where the correlation was higher in winter. Shouldn´t the correlation with K+ be higher in winter as well when biomass burning is usually higher? Is there any reason why biomass burning at the measured site would not be higher in winter?

P11L303-In this sentence, it is mentioned that the correlation between WSOC and K+ in autumn was stronger than that between MSOC, confirming that most of the chromophores generated by biomass burning were water-soluble. However, in P7L258 it says that most chromophores, including PAHs from biomass burning, were soluble in methanol. Furthermore, in Sect. 3.4 it is mentioned that “MeOH-soluble OC was much higher than that in WSBrC, which indicated that chromophores were (more?) abundant in WI-MSBrC than in the WSBrC.” Can the authors clarify these differences?

P13L342-It is mentioned that the AAE of BrC increases with the polarity of constituents. However, the AAE of MSOC was similar to the one of WSOC. Shouldn´t the AAE be higher for WSOC?

P14L370-The fluorescent properties of WSBrC are dominantly described this chapter. For example, in the first paragraph of Sect. 3.3.1 the proportion of water-soluble chromophores was presented and discussed contrary to the ones of water-insoluble but methanol soluble chromophores. However, the methanol-soluble chromophores had actually contributed the most to the fluorescence volume of aerosol particles (e.g. Fig. 6). Is there a reason for focusing the discussion of this paragraph on WSBrC when the contribution of MSBrC was much more significant?

P14394- In this paragraph it is mentioned that “the higher molecular weight and aromatic organic compounds contribute more to WSBrC in summer and autumn while the contents of WI-MSBrC (winter > autumn > spring > summer) were opposite”. Can the authors clarify in more detail the results that supported this conclusion?

Minor comments:

P1L33-The word “However” should be replaced by “For example,” since that sentence does not contradict the previous one.

P2L41-Do the authors mean nitrogen-containing aromatic compounds?

P2L48-Automobile exhaust is a consequence of fossil fuel combustion. Could these be combined?  

P2L55- Not only ultraviolet spectroscopy but ultraviolet-visible spectroscopy is commonly used to study the light absorption of brown carbon.

P2L67-Can the authors clarify this sentence? In particular, are the authors comparing the sensitivity of EEM with the one from UV-vis spectroscopy? What type of classification are the authors referring to and what “shape of absorption spectra” means in this case?

P2L73-This sentence could be changed to “quantitative measurement of light-absorbing organic components”. The structural properties of those compounds are also important since they determine both the light-absorption and the potential health effects.

P2L74- There are currently ways to differentiate between light absorption of black and brown carbon. For example, this differentiation has been done using AE33 measurements and employing a method based on the wavelength dependence of AAE (WDA analysis). However, it is true that this separation is challenging since both components absorb light over the entire UV-vis range and some assumptions are made to separate their light absorptions.

P4L133-Can the extraction method be described in more detail? For example, was the entire filter used for the extraction or a part of it (1.0 cm by 1.5 cm in size?) was taken for the OC/EC analysis? Were the filters weighted prior and after particle sampling? Where were the quartz filters inserted during ultrasonic extraction? The PTFE filter was used to remove undissolved particles, right? Currently, it is mentioned that it was used to remove water-insoluble compounds. The majority of those should remain in the extraction vessel/tube for subsequent extraction with methanol. Was the extraction efficiency determined in this study?

P4L145-Should this section be part of the chemical analysis? I understand that the used instrument relies on the measurement of a physical property, but it is an analytical chemistry instrument. Or Sect. 2.2. could be changed e.g. to “Physical-chemical analysis”.

P4L154-These samples were also analysed by the HORIBA Aqualog optical spectrometer, right? In that case, there is no need to mention “a fluorescence spectrometer” since the blank samples were analysed by the same instrument.

P4L170-The AAE can both indicate that the BrC has a greater of smaller contribution to aerosol absorption, depending on its value. The last part of the sentence can be removed.

P6L219-The units can be removed from the title.

P6L227-The decomposition of BrC constituents during summer, likely due to photobleaching, is induced both by solar intensity and oxidants present in the atmosphere.

P6L229-This is true, but a decrease in atmospheric oxidation likely plays a role as well.

P7L261-The nitroaromatics compounds do not always contribute 60% to the absorbance. The sentence needs to be rewritten to e.g. “In another study, nitroaromatics have contributed 60 % to the total absorbance in the 300-400 nm range”

P8L282-The authors mean on summer holidays? This paragraph can be combined with the previous one because it continues that discussion.

P8L283-And photobleaching as mentioned earlier?

P12L316- The references are missing.

P13L320-Can the authors clarify this sentence?

P13L343-The large MAE365 during winter is a consequence of air pollution. Therefore, the sentence should be modified to “which is a result of severe air pollution in the mentioned period”.

P13L45-This sentence can be deleted or moved to the Methods section.

P13L347-Change to UV-Vis range.

P13L350-What were the values for MSBrC?

P13L355-Are the presented values the SFE range?

P13L359-Change to UV-Vis range.

P21L514-Can the authors clarify this paragraph? Is this the total FV of SOC (WSBrC+WIBrC) or the correlation of FVs of WSBrC and WIBrC with SOA?

P21L520-The second sentence can be removed/modified since Fig. 10 refer to the NFVs and not to the overall optical properties of the different samples.

P22L540- N-containing substances were soluble in water. but how about in methanol?

P2L5553-Do the authors mean that Tianjin PM2.5 contained more polar BrC than other cities of China as shown by the higher AAE values?

P23L556-And photobleaching?

P23L568.The polarity of water is higher than the one of methanol. Since WIBrC contributed the most to the fluorescence of aerosols, can the authors conclude that this indicates that there were more polar BrC substances in the collected aerosol samples?

p23l571-How about biomass burning?

Review of "Measurement Report: Optical Characterization, Seasonality, and Sources of 2 Brown Carbon in Fine Aerosols from Tianjin, North China: Year-round Observations" by Dong et al.

Review prepared by: Dr. Taveen Kapoor and Dr. Rajan Chakrabarty

This study reports the optical properties of water-soluble and methanol-soluble brown carbon (BrC) in fine aerosols (PM2.5) sampled over a year in Tianjin, North China. The authors employ a relatively new BrC characterization technique, three-dimensional fluorescence spectroscopy measure the seasonal variations in optical properties of BrC and their chromophore constituents. They evaluated the  relationship between BrC and chemical composition in PM2.5 and the possible sources of BrC over the sampling region. Overall, the manuscript needs revisions before it can be considered for publication as a measurement report in ACP.

Major comments:

1) The novelty of this study is in the use of relatively new characterization techniques of BrC. Because of the relative newness, there is a need for justification and more context as to why the authors chose this technique over previously established ones.

The excitation-emission matrix technique is used to understand the fluorescence spectra of organic carbon compounds. The reason for studying the fluorescence of organic compounds is not clear. Also, a range of expected values for the different chromophore groups before indicating the measured values will help the reader to interpret the results better. For example, (L405) a fluorescence index < 1.4 indicates higher aromaticity but later it is said that BrC contain mainly aromatic compounds (L415) despite values of up to 2.23 (L412).

The PARAFAC analysis is used to identify chromophore groups. Three groups have been identified, but information about why there are three groups and not more (or less) has not been mentioned. The selection procedure should be mentioned.

Information on how the humidification index, δ¹⁵N_TN and δ¹⁵C_TC are calculated is missing from the manuscript.

2) While considerable effort has been put into generating and summarizing the data, the study fails to connect the measurements to provide a coherent picture from the results of the various measured properties (BrC concentrations, absorption, fluorescence, fluorescence, humidification, etc.). Some attempts are made to correlate the two sets of properties using linear correlations, but these do not necessarily lead to consistent results. For example, L302 says that biomass burning is a major source of BrC in the autumn months, but L409 says that terrestrial organic matter is the major source. These apparent contradictions make the manuscript difficult to understand, which may be avoided by providing appropriate context to the measurements being made, as highlighted in the previous comment. A discussion of the interrelationships between the measured properties, with a special emphasis on the new findings from the new measurements will be of great benefit to the scientific community.

3) L400 and L415 make strong statements about aging of BrC based on the measured humidification index and fluorescence index. The authors are requested to substantiate their claims, as arguments do not seem convincing in their present form, i.e., without any direct measurements of aging.

4) A lot of information provided in the figures and tables may be moved to the supplementary material as the information they provide and the text discussing them is disproportionate. Figures 3, 4, and 11 show scatter plots amongst various measured properties. But most of the discussion on these figures is around just the correlation coefficients and not the actual values (which are summarized in Table 1 already). The authors may consider moving some of the figures to the supplementary material or use a more concise figure to summarize the same information. Similarly, Figures 1, 5, and 7 show time series but the discussion is restricted to seasonal variations.

Table 1 reports the mean and standard deviations of the measured values and the range of values. Providing both sets of information seems redundant, and one set may be moved to supplementary material. Some of the information in these tables is also repeated in Figure 2.

Figure 8 has two kinds of plots, but the set of line plots is not labelled, and it is unclear what they represent. These likely show the emission and excitation spectra, which are already shown in the figure set of three-dimensional figures above.

5) Differences are reported between the parameters measured during the different seasons, but the statistical significance of the differences are not discussed. These should be added to make the discussion more robust.

Since the authors are checking for associations between the variables, the R value should be reported instead of the R² value (used as a measure for model predictability). Also, the significance of the correlations reported in L487-489 do not seem correct (R²= 0.01/0.06, having significant correlations, p < 0.05). These need to be re-checked.

Minor comments:

• Abstract could include context on the need for the measurements.
• L120: “The blank filters were left in the filter hood for 10 minutes” - not clear what this means and why this was done.
• L125: OC and EC are not spelled out before first use here.
• L120: Should be “thermal-optical carbon analyzer”
• L144: How was the concentration of WSOC determined? In the equation it is implied that all the organic compounds that are water insoluble OC are methanol soluble. While this may be a fair assumption, the authors may acknowledge that there may be some chromophores that are also methanol insoluble (Shetty et al., 2019).
• L159: Please provide a reference for this equation.
• L170: Line is unclear and needs to be rephrased.
• L174: ‘C’ is a constant and not the concentration of extract
• L185: Please provide a reference for the equation.
• L202: PARAFAC and SOLO are not defined before first use here. Please also provide a link or reference to the code here.
• L214: What is basis of selecting the values of constants in the in the equation to calculate the SFE? Also, a value for backscatter coefficients is provided but is not present in the equation. Why is the mass scattering efficiency being ignored?
• L230: Comparison of measured absorption those reported at sites in the USA, but these are likely to be influenced by very different source. There are several other studies reporting BrC absorption in areas with biomass and coal combustion sources that may offer a fairer comparison.
• L254: “...absorption coefficient of WI-MSBrC was always greater than that of WSBrC across the shorter 256 wavelength...” - this is not true for spring, summer, and autumn months!
• L303: Is there a known source of biomass burning during the autumn season that can corroborate this result?
• L308: not clear what is meant by “dust in spring”
• L330: The finding that AAE_WSOC and AAE_WI-MSOC are similar is a bit surprising since the water insoluble portion is expected to have a stronger absorption and weaker wavelength dependence (please see Saleh, 2020, and references therein).
• L358: The results demonstrate the radiative forcing from BrC absorption. Whether or not they contribute significantly to radiative forcing depends on the overall radiative forcing magnitude – please rephrase. The same also needs to be edited in the abstract and summary sections.
• L373: It is not clear how a comparison between water soluble and water insoluble would lead to seasonal differences in the remaining part of the sentence.
• L376: “more water-soluble chromophores” - relative to what?
• L382: Was SOA calculated or was this a finding from a previous study? Please provide details or a reference.
• L392: Seems to be a typo here – please rephrase.
• L416: What are terrestrial organics?
• L443-445: What is meant by core consistency and unexplained residues?
• L455: PLOM not defined before its first use.
• L471, 475: Please provide a reference.
• L278: SOC not defined before its first use.
• L554: It is unclear how a similar AAE of WS and WI BrC implies that that the more polar.
• L563: What is meant by color clusters?
• L567: reference?