Modeling of Carbonyl/Ammonium Sulfate Aqueous Brown Carbon Chemistry via UV/Vis Spectral Decomposition

Round 1

Reviewer 1 Report

Review of MS titled as “ Modeling of Carbonyl/Ammonium Sulfate Aqueous 2 Brown Carbon Chemistry via UV/Vis Spectral 3 Decomposition” by Fan et al.,

In this study, authors have proposed to model the mass absorption cross-section (MAC) of  BrC formed by the reaction of a-dicarbonyls and typical inorganic ions using a single function representing their magnitude -dependent (M), width dependent (σ), and location-dependent (c)coefficient per line shape, yielding a total of 3n parameters to be fitted to a single given spectrum. From previous literature exploring the chromophoric properties of the CVOC/AS dark chemistry system used in this work, peak spread and approximate locations are found to remain roughly constant. Overall, the manuscript written very well and scintific soundness is very high. The proposed model by authors improve our current understanding on BrC absorption variability w.r.t. concentration from source emissions, divergent sources as well as aging factors .  Thus, if included in the climate models, the radiative forcing estimates will minimize the existing uncertainties.

L 181: Generally, solution absorbance increased with s,olution age and organic concentrations, with MG 181 changing the most and GA changing the least over the measured timeframe- in this sentence according to Figure1 , I could only see changes w.r.t. aging time or extent of aging. How it is justified for absorbance increase with increase in organic concentrations?- please clarify this part or am I missing something here.

L202: shown to show – correct for the grammar here, sentence is not clear.

 

Author Response

We thank the reviewer for their feedback. Please consult the attached file for specific responses.

Reviewer 2 Report

Overview

            Fan et al. decomposes absorption data from synthesized carbonyl/ammonium sulfate aerosol mimics. The study separates out absorption peaks through a first-order kinetic model that utilizes Gaussian line shape functions. As opposed to previous papers that use a single reference wavelength of absorption, the authors use a novel approach of modeling over a range of wavelengths. This modeling approach is robust for finding the kinetic coefficients for a specific system. However, I do not support publication of this paper in its current state due to manuscript structure, clarity, and atmospheric implication concerns. Due to clarity issues, the author conclusions and goals in this manuscript are often lost amongst the text. With major reorganization this paper can be a good contribution to the community.

 

            As the abstract and introduction of the manuscript reads now, it requires significant restructuring, word economy, and clarity. In the introduction, the authors bounce around between paragraphs, leaving the reader confused and it takes multiple reads to make connections. Non-relevant topics (e.g. imidazole) should be removed, while core topics (e.g. kinetic data) needs to be mentioned earlier in the manuscript. Further expansion upon the importance of dark chemistry techniques, and the significance of G/GA/MG (especially since MG has a low Henry’s Law Constant, resulting in minor concentrations in atmospheric aerosol) is needed.

 

            I am not confident the methods section conveys sufficient information to reproduce the experiments or findings of their work. I recommend an expansion of the experimental set-up. I would like the see the authors address the conditions in which aging occurred (any oxidants? Aged in presence of air or nitrogen? Etc.).

 

            I have concerns about the atmospheric implications of this model. Air parcels spend on the order of ~10 minutes in a cloud. I would like the authors to discuss where this model would be applicable, under what conditions it would be applicable, and specific limitations of the model. The authors briefly address the role of their model in the last sentences of the manuscript, but this needs to be addressed earlier.

 

             

 

Specific Comments

            A writing aspect that contributes to the confusion in the introduction is the large number of long sentences. Examples include lines 75-79 and lines 85-89 – each is a 5 line long sentences. These long sentences are especially troublesome when followed by common nouns, since it is unclear which particular aspect of the previous sentence is being referred to. Notably, the authors start many sentences with the word “these,” which is unclear.

 

            In areas of the manuscript the authors change the tense and pluralization of words. This is most notable in the abstract. An example is the first sentence where aqueous brown carbon is referred to as “its” (singular), and “these” plural, in the same sentence.

 

            Lines 302-305: “As a spectral decomposition method can regresses value using an entire spectrum, a properly constructed set of lineshape functions may be used to infer kinetic information in a more holistic and broadly informed fashion for a given set of time-resolved data.” Regress should not be pluralized.

 

            There are areas in the introduction were further literature research should be done, and where references are missing. An example of a missing reference is at the end of the sentence on line 47 (“…aqueous aerosol.”).

 

            Line 98: “Here, we present an alternative, top-down spectral decomposition approach to the prediction of UV-visible absorbance properties in CVOC/AS aerosol mimics.” It is unclear what the alternative approach the authors refer to is.

 

            Consider stacking the three panels in Figure 1 and Figure 2 to allow for the individual panels to be larger.

 

            It is unclear how many fitted curves (2, 3, 4) best fit the system, and whether the number of fitted curves remained constant throughout the different analysis processes.

 

            Lastly, the authors have many sentences in which they are lacking an article. Examples include:

• Line 131: “…take place within the”
• Line 143: “The models discussed in this work aim to decompose the MAC spectra…”.
• Lines 252-253: “Pyrazines have also been proposed as G/AS dark chemistry products that absorb in this region, though they also contribute to absorbance in the 255-275 nm range.”

 

 

 

Author Response

We thank the reviewer for their feedback. Please consult the attached file for specific responses.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors made significant additions and restructuring changes to the manuscript, resulting in a more clear and improved paper. Specifically, the authors made major changes to their introduction, experimental set-up, and atmospheric implications section. 

The authors offer non-specific location answers in their response to  comments and it is difficult to track things down in a straightforward way.

The authors response to my concern about atmospheric implications, and where the model would be applicable, demonstrate they understand complex chemistry, but maybe not interactions during physical transport of air parcels vertically in the atmosphere through boundary layer clouds. The authors say that the lifetime of air parcels is not expected to affect the ability of their model. I do not see sufficient detailed argument for that claim. 

Author Response

We thank the reviewer for their additional comments. Please see the attachment for our response.

Author Response File: Author Response.pdf