Review of ‘Chemical composition, optical properties, and oxidative potential of water and methanol-soluble organic compounds emitted from the combustion of biomass materials and coal’ by Cao et al.

The manuscript describes the experimental data of aerosol particles generated by laboratory combustion of coal and biomass. The study mainly focuses on chemical characteristics of organic aerosols. The authors separated organic aerosol to three categories (water soluble, HULIS, and methanol soluble). The segregated samples were measured for UV-VIS, EEM, NMR, and DTT assay. It seems to the reviewer that the measurements were carefully conducted. Compared with the previous studies from the group, the major novelty of the manuscript seems to be the addition of DTT assay. However, the novelty was not clear without checking the references from the group. The importance/significance/novelty of the DTT assay data of the study was not clearly discussed. The reason why UV-VIS, EEM, and NMR measurements need to be concurrently conducted with the DTT assay was not shown. As a result, the manuscript looks like a collection of data, rather than a document which provides a coherent story. A significant improvement will be needed for the manuscript to be accepted by the journal.

The introduction starts with the general importance of brown carbon. Subsequently, the authors describe recent studies on brown carbon from biomass burning and coal combustion, especially those from China. However, it was not clearly discussed what the current knowledge gap is. The authors mention that ‘However, few studies have directly investigated the oxidative potential (OP) of BrC emitted from combustion processes.’ If there are already some studies on the topic, it would have been better to discuss the contributions/limitations of the previous research explicitly.

Section 3 (results and discussion) describes the measurement results, and demonstrates that the values were comparable to the previous studies. Although it is always good to see the agreements of the present data with previous studies, the novelty/uniqueness of the present study was unclear. In addition, the section does not discuss the data deeply, although the data obtained by the authors were compared with that from previous studies.

The manuscript contains grammatical issues. It would have been better if a native speaker of English checked it.

Specific comments

L184 ‘Approximately 5 mg of the BrC fractions (i.e., HULIS, WSOC, and MSOC) derived from BB and CC were dissolved in 500 μL deuterium oxide and then transferred to a 5 mm NMR tube’

MSOC should contain some water-insoluble fraction. I wonder if all the components of the sample have successfully been measured.

L529 ‘These results suggested that the primary smoke from BB and CC in this study had a weaker ROS generation capacity than ambient aerosols, which was likely due to the differences in the chemical composition of WSOC in BB and CC smoke particles and ambient aerosols’

I think that it is an important point. Do the authors have any ideas/hypothesis on how the NMR or EEM data could contribute to discussing this point?

The manuscript by Cao et al. titled, “Chemical composition, optical properties and oxidative potential of water- and methanol-soluble organic compounds emitted from the combustion of biomass materials and coal” describes the characterization of various physicochemical properties of aerosols emitted from biomass and coal combustion. While the work here is important, as it provides necessary data that is currently missing in the literature, from BrC light absorptivity and oxidative potential values with corresponding chemical composition information, there is critical information missing that hinders the ability to review this manuscript at this time. As such, major revisions to the manuscript need to be done before it can be further reviewed for publication. Also, a more in-depth discussion of the author’s observations is warranted to clearly highlight the novelty of this work.

Specific Comment:

One of the major conclusions re: the DTT assay work is that the ROS activity is “weaker” compared to previous studies, likely due differences in chemical composition. This point should be further supported by comparing the chemical composition observation from this current study and other cited studies of ambient aerosol. This adds to the novelty of this study.  

Detailed information about the methodology re: the TOC content analysis was supposed to be in the supplementary, but in the supplementary file, this “detailed measurement method is provided in the SI file” (line 133-134 of SI). This information needs to be presented, as some of the results presented are mass-normalized. However, it is unclear if “ug” is referring to the mass of carbon, or the mass of the WSOC, or the mass of the PM (e.g., the conversion of the mass of OC to organic PM concentration uses a conversion factor (Turpin and Lim, Aerosol Science and Technology, 2001, 35(1) 602-610). This lack of information makes it difficult to assess if the direct comparison of mass-normalized results as reported in the current manuscript (e.g., Figure 6) is applicable.

L184: For the ¹H NMR work: What does it mean that the BrC fractions (e.g., dissolved in water, recovered from the SPE cartridges with solvent, and methanol extracted) were dissolved in D₂O? Effectively, this was a liquid-liquid extraction? I am not sure if the all the MSOC components dissolved in the D₂O (and which from my understanding is the portion of the sample that leads to the ¹H NMR signal), and as such, I am uncertain if the authors can equate the proton-NMR data are measurements of BrC in the MSOC BrC fraction. Why not use deuterated methanol instead?

L224: How is HULIS a hydrophobic fraction of water-soluble organic carbon? Perhaps you mean it is less polar components of the WSOC?

L125: It is not clear as to how these blank filters can be used to correct the mass of smoke, optical signal, and DTT consumption by BrC. Were these blank filters placed into the filter collection system behind the first filter that contains most of the aerosol (e.g., this is typically done for quartz filter for breakthrough, such as correction for semi-volatile organic carbon).

Minor comments:

L218: It is not clear which parameter was used to infer “average contribution of WSOC to wood smoke PM2.5” from Table 1 (is it WSOC-C/PM)?

Figure 5: I recommend the authors abbreviate “typical biomass burning” as something else other than “WS” as it is confusing when “WS” is commonly used to refer to water-soluble.

It is also useful to provide the DTT values that are normalized by volume of air, these values would be useful for the calculation of exposure. Can the authors provide these analogous values in the supplementary?

The volumes and concentrations of reagents used in the current study are not as described by most of the papers cited by Bates et al., 2019 and Verma et al., 2012. The general approach is certainly identical, but the specific details are not. I highly recommend the authors to specify the differences, as this may be important for comparison to other literature value.

L56: The Lin and Yu, as well as Ma et al. papers, looked at the generation of ROS by HULIS - these paper did not actually report whether the HULIS is also BrC (e.g., it is still not known if all HULIS can be considered as BrC and vice versa). On a related note, the next sentence is written in a manner that suggests that all HULIS is BrC.

Technical Corrections:

The Ma et al. reference has some typos/weird characters.

L77: I don’t understand the following sentence: “However, these studies only focused on the BrC fractions emitted from BB or CC, and therefore the comprehensive characterization and full understanding of the BrC fractions from combustion processes are still required.” Is the author referring to the fact that the cited studies only focused on certain fractions of BrC (e.g., water-soluble BrC?).

L138: the Cheng et al. (2016) reference was included twice.

L206: It is not clear what the acronym DTPA stands for, it has not been explained prior.