In this manuscript, the authors present a study of the light absorption and composition of water-soluble and -insoluble brown carbon (BrC) from filter samples collected in a highly polluted urban environment. The sample preparation and the spectroscopic and mass spectrometric analyses are carefully performed. The observations during the day and night are thoroughly discussed and support insights into the emission and evolution of BrC. For example, more polar, water-soluble components are more abundant during the day, when photochemical aging likely drives the functionalization of primary emissions. I think the manuscript is suitable for publication as a measurement report in ACP after minor technical revisions.

105 - Absorbance is unitless.

107 - What are the potential interferences for this subtraction (e.g., solvent and pH effects)? Was the pH of the water extracts measured or adjusted?

110 - I think it is clearer to write the units of Abs without a space (i.e., Mm^-1) here and throughout. In Figure 1a, the units of Abs also need to be corrected from m M^-1.

111 - Is there a precedent for the subtraction of A_700? If not, a brief justification should be provided here (e.g., BrC does not absorb at such long wavelengths, so a shift can be attributed to a change in the photons reaching the waveguide capillary cell).

166 - Standards, including 4-nitrocatechol and 4-nitrophenol, were used in the spectroscopic analysis - were standards also used in the MS analysis? In other words, how were the concentrations of the 38 identified components determined?

230 - I think a clearer explanation of the difference between nitrophenols and nitrocatechols should be presented. Both can be emitted from biomass burning, for example, so it may be surprising that they differ so much throughout this study.

365 - A more direct connection from polarity and absorptivity to functionalization could be incorporated here or earlier.

371 - Some primary emissions are also reduced during the night (e.g., transportation emissions). Are these changes reflected in the total mass concentrations or elsewhere?

394 - It is unusual to thank the co-authors. Their contributions are acknowledged by their authorship.

This paper attempts to identify the chromophores in brown carbon and show their diurnal patterns.  While there are many very nice aspects of the paper, I am deeply concerned about the big undefined assumptions that are made to link a high-resolution mass to specific structures.

Specifically, figure 3 includes the names of specific compounds that are traditionally analyzed using analytical standards however no description is given to describe that sort of targeted analysis.  Instead, the authors have described the analysis of BrC fractions using HPLC-PDA-HRMS and given a few details for assigning molecular formulas to masses with 3 ppm error.        

First, when 5 elements are allowed with a 3 ppm mass error for masses ranging up to 800 u, we can expect to have multiple plausible molecular formulas per mass measurement that meet those criteria.  So as a first step, we need to know how the plausible formulas were evaluated.  Then, perhaps we can calculate elemental ratios and determine the number of unsaturations, however, even the number of unsaturations will require an assumption about the oxidation state of N and S which should be defined. 

Second, structural analysis and structure confirmation require very deep study using MS/MS and NMR without specific analytical standards.  Then we would still want the structures to be defined as tentative until they can be confirmed with specific analytical standards.  

Third, I strongly doubt that PAHs can be observed using (+) ESI.  PAHs are simply too strongly nonpolar for ESI.  In fact, I wouldn't expect them to be in MeOH fractions.  I would believe that substituted PAHs could be present, but again that would need to be confirmed using analytical standards.  

In figure 2, the mass fractions of identified compounds are given but I do not recall seeing an explanation of how the analytical features were converted to mass fractions.  Again many assumptions were likely made that need to be explicitly described. 

Overall, the paper is quite interesting and there is great interest in the identity of BrC.  But, gaps in the scientific process must be filled with concrete steps or explicit rationale must be given for educated assumptions that are probably quite reasonable given the deep knowledge on this topic.