The manuscript by Fan et al. isolated low- and high- molecular weight HULIS from ambient samples collected at Anhui, China. The HULIS samples were measured for molecular size distribution, UV-VIS light absorption, fluorescence spectra, infrared absorption, and high-resolution mass spectra. Technical approaches of the sampling and chemical analysis procedures sound. Discussion of the data is occasionally speculative. The reviewer has a concern about the interpretation of the mass spectra, as detailed below.

The data shown in the manuscript would add some information to the existing literature of HULIS studies. The manuscript looks like a collection of data, rather than discussing novel findings in detail. The reviewer thinks that this manuscript may be more suitable to be considered as a measurement report, though the final decision should be made by the editor and authors.

Major comment

The size-exclusion chromatograms demonstrated that the dominant portion of HULIS have molecular weights of larger than 1000 Da. On the other hand, the mass spectrometer (orbitrap, ESI negative mode) measured the mass ranges of m/z = 60 ~ 900.  The authors showed the mass spectra for the range of m/z 100 ~ (approximately) 450. The reviewer is confused how to interpret the data. The reviewer only has the knowledge on analyzing relatively small molecules using the ESI mass spectrometers. The reviewer’s understanding is that fragmentation of molecules is minimal in the ESI negative mode. If the idea is also applied to the current dataset, the mass spectra measured by the study would only cover a relatively small portion of HULIS. On the other hand, if the mass spectrometer were to be measuring most of components in HULIS due to fragmentation during ionization processes, the meanings of the whole discussion in section 3.3 is unclear. The reviewer suggests the authors to clarify this point in the revised manuscript, and update the descriptions in section 3.3 if necessary.

Minor comments

Abstract

‘This observation suggests the possibility of small molecules assembling to form the HMW HULIS through intermolecular weak forces.’

To the best of my knowledge, small molecules that are assembled by intermolecular forces are not called as high-molecular weight species. They are simply clusters or aggregates of molecules. If the authors interpretation were to be right, the reviewer is not sure if HMW HULIS should really be called as ‘HMW.’ This point is related with the above-mentioned major comment. The reviewer suggests the authors to clarify the point.

L87 Previous studies have shown that aerosol WSOM, also known as brown carbon…

WSOM and BrC are obviously different concepts. If WSOM and BrC are really considered as equal, please cite references to support the statement.

Figure S1

It would be better to show the location of the observation site in the map.

L258 ‘Therefore, the higher proportions of large-size chromophores and resulting larger apparent molecular size of HMW HULIS may indicate their possible secondary formation nature.’

It is not clear how this statement could be supported by discussion in other parts of the manuscript. The statement seems to be a hypothesis, rather than what the authors can convincingly tell from their results.

L330 ‘Therefore, it can be speculated that LMW HULIS are more susceptible to enrich the by-products resulting from the degradation and oxidation of BrC during processes like photooxidation and photobleaching.’

Are there any other possibilities in differences (e.g., emission sources)? The reviewer suggests the authors to discuss potential sources and atmospheric processes in more detail.

L338 ‘humic-like fluorophores’

I am confused about this statement. All the fluorophores in humic-like substances could be called as humic-like fluorophores.

The authors characterized High (HMW) and Low Molecular Weight (LMW) using a combination of analytical techniques to differentiate the molecular, functional and optical properties between samples collected during summer and winter in Anhui (China). The manuscript is well written and the link between seasonal sources, HULIS functional and molecular level of information are well linked with the optical properties. The results are a bit long in some sections, and perhaps some of the material (less novel results) could be moved to the Supplementary information to avoid losing the readers and keep the manuscript clear and concise. I would recommend this manuscript for publication after addressing the major comments.

General comments:

In section 2.6, are there any biases due to the use of the negative mode with ESI-MS? Can the authors estimate the fraction of HMW and LMW HULIS lost during the ESI characterization, or do they assume that most of the HMW HULIS will end up in a multiple charge state or “disassemble” (based on the statement page 32 line 596)? The idea of HMW HULIS being aggregates of smaller molecules should be developed in more details throughout the discussion instead of only mentioning it toward the end.  

Would the contribution of small molecules at 570Da (peak i) present in HMW chromatogram (similarly the fraction of HMW in the LMW, peak ii ~2200 Da) as seen in Figure 1, influence the subsequent observation, in particular the FTIR spectra?

Could Tar Balls influence the samples and absorption measurement?

Minor comments:

Page 6 line 109 and 110: “low-LMW” change to low-MW, and “MW MSOM” do you mean WSOM here?

Page 10 line 193: “with a scanning speed of 12,000 nm/min”.

Page 13 Figure 1:

Page 13 line 252: “lager” did the author mean larger?

Page 18 line 343: “were divided five fluorescence regions” to “were divided in five fluorescence regions”

Page 13 Figure 1: Would the small percent of LMW in HMW samples and HMW in the LMW solution influence the subsequent observation?

Page 20 line 396: “HMW HULIS generally exhibit more intense at 1721” rephrase

Page 23 line 436 : “an strong” please correct.

Page 33 line 613: “HMW HULIS contain amounts of carboxylic functional groups,

reduced nitrogen species (e.g., amines) and aromatic species than LMW HULIS”, contain more/higher?

 Supplement:

Figure S4: Use different type of markers between the summer and winter values of AAE vs MAE for the current study.