From the journal Environmental Science: Atmospheres Peer review history

29-May-2023

Dear Dr Shetty:

Manuscript ID: EA-ART-05-2023-000067
TITLE: Brown Carbon Absorptivity in Fresh Wildfire Smoke: Associations with Volatility and Chemical Compound Groups

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Reviewer 1

In this study, the authors conducted high-resolution chemical and optical measurements on four major wildfires from three different locations. The field observations showed that extremely low volatility organic compounds had a strong correlation with the light absorption of BrC in smoke aerosols. The non-water-soluble components of BrC were found to be related to the concentration of elemental carbon, indicating that they were simultaneously released from the wildfires. Furthermore, they also found that polycyclic aromatic hydrocarbons, monoheterocyclic oxygen-containing aromatic compounds, and nitrogen-containing aromatic compounds were good predictors of the observed coefficient of BrC. The findings are valuable for the better understanding of biomass burning origin BrC and the light absorption properties. The conclusions are likely solid, and some discussions are not supportive. however, it is still worth of publication after addressing my follow comments.

Introduction
"...indicating that different mechanisms lead to the formation of BrC with different optical Properties." The diversity of values does not only indicate the presence of distinct mechanisms, but could also be attributed to various sources.

The authors suggested that combustion conditions are a better predictor of BrC optical properties than individual chemical groups. Does this means that the volatility determines the BrC optical properties? what do the combustion conditions refer to? more detailed information should be included here to make it clear.

I suggest a more comprehensive summary of light absorbing compounds in the biomass burning aerosols.

Methods
The molecular information was also used to determine aerosol volatility using the parametrizations from Li et al. It would be better to give a brief introduction of the parametrizations herein.

Results and Discussion
I am perplexed about why only Abs405 values were applied to calculate the light absorption coefficient of BrC. It would be more beneficial to obtain a more comprehensive overview of the light absorption across the measured wavelength, for both WS BrC and MeS BrC. By doing so, the light absorption of WI BrC could also be calculated across the measured wavelength, allowing for a more accurate representation of the contribution of WI BrC to MeS BrC across the measured wavelength.

Section 3.3: The paragraph is somewhat hard to understand.
Table 1 presents the correlation coefficient values obtained from a linear correlation between MeS and WS BrC Abs405 and various chemical compound classes that were identified as significant chromophores in previous studies. I donot quite understand that Terpenoid and sugar were previously identified as significant chromophores.

The authors stated that nitroaromatics are primarily generated during nighttime oxidation chemistry. Is this taken from previous studies or is there evidence from the observation?

"The correlation coefficients for WS BrC Abs405 were better than or comparable to the MeS BrC Abs405. The unidentified compounds are likely to constitute the MeS fraction of BrC, and consequently, the measured correlations are better for the WS component of BrC." complicated expression?

The result first showed that "In contrast to previous studies, PAHs and NOCs demonstrated weaker correlations with BrC light absorption coefficients", but later in the multivariate regression analysis, the result showed "The three compound groups—PAHs, aromatics, and NOCs—identified as good predictors of BrC light absorption coefficients have been shown to be efficient light absorbers". How does this make sense?

Reviewer 2

This manuscript by Shetty et al. presents results of wildfire brown carbon (BrC) light absorption properties and their dependence on aerosol chemical composition and volatility. Smoke aerosol filter samples were collected from 3 wildfires as part of the 2019 FIREX-AQ campaign. Chemical composition was obtained using GCxGC coupled with HR ToF-MS. Volatility distributions were estimated using 2 methods: parametrization based on HR ToF-MS molecular assignments and thermal-optical (ECOC) analysis. Light absorption properties of methanol- and water-extracts were obtained using UV-vis spectroscopy. The analyses identified major compound groups that were associated with BrC absorption and yielded correlations between absorption of methanol-soluble / water-insoluble BrC and the abundance of low-volatility compounds / elemental carbon. The manuscript is well-written and the data is of interest to the atmospheric science community. I believe that the manuscript is suitable for publication in ES: Atmospheres after the following comments are addressed.

Comments:
1) Figure 1: The results in Figure 1 are interesting, but the way they are presented makes it hard to compare the volatility distributions from the two methods (TOA and ToF-MS). I suggest the following:
1.a) Present the ToF-MS identified vs unidentified fractions separately.
1.b) Present the EC fractions from TOA separately.
1.c) Show pie charts of volatility distributions of the identified organics from ToF-MS vs TOA volatility distributions (without EC). This will make the discussion of the comparison between the two methods easier. Also, I would lump the IVOC and SVOC from ToF-MS into one bin and make a note in the legend that OC1 contains both SVOC and IVOC (if SVOC are assumed to be all in OC1, then OC1 definitely also includes IVOC).
1.d) Are the ToF-MS volatility distributions based on OC or OA? If OA/OC is significantly different for the different volatility bins, the ToF-MS distributions need to be converted to OC-basis so that the comparison with TOA is apples-to-apples.

2) Methanol-soluble, water-soluble, and water-insoluble fractions.
2.a) As the authors point out, some BrC is not methanol-soluble. Therefore, the BrC fraction that is referred to as water-insoluble (WI) should be methanol-soluble water-insoluble (MeS-WI).
2.b) Line 240: the description of how WI BrC light absorption was estimated should be moved to Section 2.3.
2.c) Figure 2: What about WS BrC?
2.d) Graphical ToC: Shouldn’t the blue arrows be pointing in the opposite directions? WI BrC increases with increasing EC.

Minor comments:
1) Line 54-56: Because k of BrC is wavelength-dependent, this statement on the range of k values requires specifying the wavelength.
2) Line 153-154: Remove this statement. This information is presented in Section 2.5.
3) Title of Section 3.2 and captions of Figure 2 and Figure 3: ‘Change’ should be replaced with something like ‘dependence.’ The way it reads now gives the impression that the results are for the same BrC that evolves (changes).
4) Line 294: replace ‘substitutes’ with ‘substituted.’

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

We thank the reviewers for their constructive comments which have helped improve the
manuscript. Our point-by-point response to the comments are listed below. The reviewer
comments are in italics, our responses are in red font, and the changes made to the manuscript
are highlighted yellow in red italics.
Referee: 1
Comments to the Author
In this study, the authors conducted high-resolution chemical and optical measurements on four
major wildfires from three different locations. The field observations showed that extremely low
volatility organic compounds had a strong correlation with the light absorption of BrC in smoke
aerosols. The non-water-soluble components of BrC were found to be related to the
concentration of elemental carbon, indicating that they were simultaneously released from the
wildfires. Furthermore, they also found that polycyclic aromatic hydrocarbons, monoheterocyclic
oxygen-containing aromatic compounds, and nitrogen-containing aromatic compounds were
good predictors of the observed coefficient of BrC. The findings are valuable for the better
understanding of biomass burning origin BrC and the light absorption properties. The
conclusions are likely solid, and some discussions are not supportive. however, it is still worth of
publication after addressing my follow comments.
Introduction
"...indicating that different mechanisms lead to the formation of BrC with different optical
Properties." The diversity of values does not only indicate the presence of distinct mechanisms,
but could also be attributed to various sources.
We agree with the reviewer. The statement now reads as follows: “The imaginary component of
BrC refractive index (indicative of particle light absorption) spans a wide range of values from
10-3 to 0.3 at ultraviolet and lower visible wavelengths. The wide range of refractive index
values indicates that different mechanisms and fuels lead to the formation of BrC with distinct
optical properties.”
The authors suggested that combustion conditions are a better predictor of BrC optical
properties than individual chemical groups. Does this means that the volatility determines the
BrC optical properties? what do the combustion conditions refer to? more detailed information
should be included here to make it clear.
We thank the reviewer for this question and for helping us clarify our argument. We are referring
to combustion conditions such as higher temperatures and fuel-to-oxygen ratios. Rather than the
volatility of the molecules constituting BrC directly determining their optical properties, we
argue that the aforementioned combustion conditions could lead to the formation of molecules
with both stronger light-absorption and lower volatilities. It is known that high temperatures
could lead to the formation of large highly-conjugated molecules due to increased collisions
between molecules at those temperatures1, 2. These large molecules are expected to have lower
volatilities as well. While the phenomenon of increasing molecular size with temperature has
been previously observed in laboratory-based studies, a detailed analysis of the chemical
composition of the resulting BrC aerosols is still lacking. Following the reviewer’s suggestion,
we have added the following lines to the text:
“Through laboratory experiments, they demonstrated that organics darken as their volatility
decreases, and they demonstrated that controlling combustion conditions, such as temperature
and the fuel-to-air ratios, can result in BrC with predictable optical properties3. Increased
molecular collisions during high temperature combustion can lead to the formation of large,
highly-conjugated molecules with lower volatilities1, 2. These low-volatility molecules are likely
to strongly absorb ultraviolet and visible light4, 5. However, studies supporting this hypothesis
from real-world biomass burning emissions are scarce6. Furthermore, a comprehensive
understanding of the relationship between the chemical composition of these low volatility
compounds and BrC optical properties is still lacking.”
I suggest a more comprehensive summary of light absorbing compounds in the biomass burning
aerosols.
We thank the reviewer for their suggestion. We have added the following passage to the
introduction section with a more detailed summary of BrC in biomass burning aerosols:
“Pöschl7 initially proposed a classification system for BrC, relating optical and thermochemical
classes of carbonaceous particles to various molecular groups. A comprehensive review by Laskin
et al.8 provides current knowledge on the chemistry and of BrC, including both optical and
chemical characterization methods. A powerful technique for chemical characterization of
biomass burning BrC involves combining high-performance liquid chromatography with
photodiode array detection and high-resolution mass spectrometry9, 10. This approach has
successfully identified nitroaromatics and oxygenated aromatics as important BrC chromophores.
However, it may overlook high molecular weight compounds that are not extractable in the solvents
used for analysis. The importance of molecular size in determining BrC light absorptivity can be
highlighted size-exclusion chromatography measurements11-14. Online measurements of BrC using
aerosol mass spectrometers have identified oxygenated organics15-17, nitroaromatics18-20, and low-
volatility oxygenated organics21 as dominant light-absorbing groups in different regions. The wide
range of compounds observed in BrC across various studies underscores the complexity of BrC
composition and emphasizes the need for a more holistic metric to relate BrC optical properties to
chemical composition.”
Methods
The molecular information was also used to determine aerosol volatility using the
parametrizations from Li et al. It would be better to give a brief introduction of the
parametrizations herein.
We had provided details regarding how the parametrizations were used in the Supporting
Information. However, following the reviewer’s comments, we have added some details to the
passage. It now reads as follows:
“Li et al.22 analyzed over 30000 compounds using a “molecular corridor” approach and
developed parametrizations to predict the saturation mass concentration of organic compounds
from their elemental composition. The parametrizations consist of a semi-empirical equation with
different parameters as inputs based on the elements present in a particular molecule. We used
these parametrizations with the data obtained from the HR ToF-MS as a secondary technique to
predict the volatility distributions of the aerosols. Further details on how the parametrizations
were used are provided in the Supporting Information.”
Results and Discussion
I am perplexed about why only Abs405 values were applied to calculate the light absorption
coefficient of BrC. It would be more beneficial to obtain a more comprehensive overview of the
light absorption across the measured wavelength, for both WS BrC and MeS BrC. By doing so,
the light absorption of WI BrC could also be calculated across the measured wavelength,
allowing for a more accurate representation of the contribution of WI BrC to MeS BrC across
the measured wavelength.
We used Abs405 values as a proxy for BrC light-absorption as the data depicted in Figures 2 and 3
would be hard to represent using absorption coefficients from multiple wavelengths. Such
representations are typical for comparing BrC light absorption to different metrics as seen across
different studies20, 23-25. While we appreciate the reviewer’s suggestion, a comparison across the
measured wavelengths is beyond the scope of this work.
Section 3.3: The paragraph is somewhat hard to understand.
Table 1 presents the correlation coefficient values obtained from a linear correlation between
MeS and WS BrC Abs405 and various chemical compound classes that were identified as
significant chromophores in previous studies. I donot quite understand that Terpenoid and sugar
were previously identified as significant chromophores.
We thank the reviewer for pointing this out. The passage now reads as follows:
“Table 1 presents the correlation coefficient (R2) values obtained from a linear correlation
between MeS and WS BrC Abs405 and various chemical compound classes that had good
correlations with biomass burning BrC or were identified as significant chromophores in
previous studies.”
The authors stated that nitroaromatics are primarily generated during nighttime oxidation
chemistry. Is this taken from previous studies or is there evidence from the observation?
The statement is based on the fact that atmospheric processing of smoke plumes is dominated by
NO3· radical chemistry in the absence of light. Consequently, NO3· radicals lead to the formation
of nitroaromatics in dark plumes or during nighttime. Additionally, a study based on aircraft
measurements during the FIREX-AQ campaign supports this statement26. We acknowledge that
this may not be common knowledge and have added the appropriate citations. We thank the
reviewer for pointing out that the argument had citations missing. The lines are changed to the
following:
“Nitroaromatics are primarily generated in dark plumes or during nighttime oxidation
chemistry26, 27. Since the wildfire emissions in this study were sampled within an hour of being
emitted, it is unlikely that they had undergone substantial atmospheric processing to produce
large amounts of nitroaromatics28.”
"The correlation coefficients for WS BrC Abs405 were better than or comparable to the MeS BrC
Abs405. The unidentified compounds are likely to constitute the MeS fraction of BrC, and
consequently, the measured correlations are better for the WS component of BrC." complicated
expression?
Thank you for pointing out the complicated sentence structure. The sentence is restructured to
read the following:
“The linear fits using the WS BrC Abs405 exhibited correlation coefficients that were equal to or
greater than those obtained with the MeS BrC Abs405. The MeS fraction of BrC is likely to be
composed of compounds that remained unidentified using our chemical characterization method.
Consequently, the identified compound groups would be better predictors for the WS component
of BrC than the MeS component.”
The result first showed that "In contrast to previous studies, PAHs and NOCs demonstrated
weaker correlations with BrC light absorption coefficients", but later in the multivariate
regression analysis, the result showed "The three compound groups—PAHs, aromatics, and
NOCs—identified as good predictors of BrC light absorption coefficients have been shown to be
efficient light absorbers". How does this make sense?
A multivariate regression analysis considers that the dependent variable (BrC light absorption
coefficients in this case) can be influenced by multiple separate independent variables (the
different compound classes). So, while the individual contributions of the PAHs and NOCs to the
BrC light absorption coefficients were not significant, the multivariate regression estimates the
combined contribution of all the tested variables. For example, there might be a sample where
the NOC concentrations are low, but BrC Abs is high due to the predominant contributions from
the PAHs and aromatics. In another case, the aromatics and PAHs are in lower concentration, but
BrC Abs is still high, likely due to an abundance of NOCs. Such effects would be difficult to
understand using simple linear regressions which underscores the importance of the multivariate
regression analysis. We have added the following lines to the text to better present our argument:
“To determine the combined contribution of the different compound groups to BrC Abs405, we
conducted a multivariate regression analysis using all 11 quantified compound classes. A
synergistic combination of PAHs, aromatics, and NOCs was found to be a significant (p < 0.05)
predictor of BrC light absorption after we removed groups which were linearly correlated.”
“While the PAHs and NOCs were individually poor predictors of BrC Abs405, a combination of
PAHs, aromatics, and NOCs was good at predicting BrC light absorption coefficients. The
aforementioned three compound groups have been shown to be efficient light absorbers29-31, so
the poorer correlations with individual groups are likely a result of different concentrations of
the compound classes in the sampled plumes.”
Referee: 2
Comments to the Author
This manuscript by Shetty et al. presents results of wildfire brown carbon (BrC) light absorption
properties and their dependence on aerosol chemical composition and volatility. Smoke aerosol
filter samples were collected from 3 wildfires as part of the 2019 FIREX-AQ campaign.
Chemical composition was obtained using GCxGC coupled with HR ToF-MS. Volatility
distributions were estimated using 2 methods: parametrization based on HR ToF-MS molecular
assignments and thermal-optical (ECOC) analysis. Light absorption properties of methanol- and
water-extracts were obtained using UV-vis spectroscopy. The analyses identified major
compound groups that were associated with BrC absorption and yielded correlations between
absorption of methanol-soluble / water-insoluble BrC and the abundance of low-volatility
compounds / elemental carbon. The manuscript is well-written and the data is of interest to the
atmospheric science community. I believe that the manuscript is suitable for publication in ES:
Atmospheres after the following comments are addressed.
Comments:
We thank the reviewer for these insightful comments. We have modified the figure such that it is
now easier to make comparisons between the TOA and ToF-MS measurements.
1) Figure 1: The results in Figure 1 are interesting, but the way they are presented makes it hard
to compare the volatility distributions from the two methods (TOA and ToF-MS). I suggest the
following:
1.a) Present the ToF-MS identified vs unidentified fractions separately.
Instead of presenting the unidentified fractions separately, we have moved them to the start of the
pie chart when going clockwise. This makes direct comparisons between the two methods easier.
We are still keeping the unidentified fractions in the plots as removing them disproportionately
increased the fraction of SVOCs identified using ToF-MS.
1.b) Present the EC fractions from TOA separately.
The EC fractions are now presented separately in a bar next to the pie plots.
1.c) Show pie charts of volatility distributions of the identified organics from ToF-MS vs TOA
volatility distributions (without EC). This will make the discussion of the comparison between the
two methods easier. Also, I would lump the IVOC and SVOC from ToF-MS into one bin and make
a note in the legend that OC1 contains both SVOC and IVOC (if SVOC are assumed to be all in
OC1, then OC1 definitely also includes IVOC).
The previous version of the manuscript did acknowledge that OC1 would likely contain IVOCs
which cannot be identified using our methodology. However, following the reviewer’s
appropriate suggestion, we have combined IVOC and SVOC into a single bin and noted the same
in the legend. We thank the reviewer for this insightful comment.
1.d) Are the ToF-MS volatility distributions based on OC or OA? If OA/OC is significantly
different for the different volatility bins, the ToF-MS distributions need to be converted to OC-
basis so that the comparison with TOA is apples-to-apples.
We thank the reviewer for their astute observation. The ToF-MS distributions in the previous
figure were based on OA mass. The OC/OA mass was substantially lower for the LVOC
compounds. Consequently, their OC concentrations were lower. The OA masses have
subsequently been converted to OC mass such that the comparison is “apples-to-apples”. We
believe that the OA and IVOC distributions are still important, so a figure with these data has
been added to the Supplementary Information.
Below is the modified version of Figure 1:
2) Methanol-soluble, water-soluble, and water-insoluble fractions.
2.a) As the authors point out, some BrC is not methanol-soluble. Therefore, the BrC fraction that
is referred to as water-insoluble (WI) should be methanol-soluble water-insoluble (MeS-WI).
We agree with the reviewer that the water-insoluble fraction of BrC should be referred to as
MeS-WI; however, a prior version of the manuscript with that notation was clunky and harder to
read which prompted the change to just WI BrC. We have added a line explicitly stating this
detail in the methods section as shown in response to the comment below.
2.b) Line 240: the description of how WI BrC light absorption was estimated should be moved to
Section 2.3.
As suggested by the reviewer, we moved the section in line 240 to Section 2.3 and added the
following details:
“The MeS BrC Abs405 and WS BrC Abs405 values were estimated by measuring the absorbance of
organics extracted in methanol and water, respectively. The WI BrC Abs405 values were estimated
by subtracting WS BrC Abs405 from MeS BrC Abs405. The calculations assume that WS BrC are a
component of the MeS BrC32, 33. However, recent studies have indicated that a significant
fraction of highly light-absorbing BrC may be insoluble in both methanol and water34, 35.
Therefore, it should be noted that the WI BrC Abs405 values reported here represent the MeS-WI
fraction of BrC. For clarity and readability, we will refer to the MeS-WI fraction of BrC as
simply WI BrC throughout the manuscript.”
2.c) Figure 2: What about WS BrC?
The WS BrC increased with EL/LVOC concentrations as well, but the increasing slope was
steeper for MeS BrC which led to the observed correlation with WI BrC. We have now added
plots with WS BrC in the Supporting Information.
2.d) Graphical ToC: Shouldn’t the blue arrows be pointing in the opposite directions? WI BrC
increases with increasing EC.
The arrows were depicting WS BrC and were hence pointing in the denoted directions; however,
the reviewer’s comment is well received. Because we have investigated the dependence of WI
BrC in this study, we have modified the figure such that the arrows are pointing in the opposite
direction and now depict the dependence on WI BrC. Below is the modified ToC graphic:
Minor comments:
1) Line 54-56: Because k of BrC is wavelength-dependent, this statement on the range of k
values requires specifying the wavelength.
The sentence has been modified to state the following:
“The imaginary component of BrC refractive index (indicative of particle light absorption) spans
a wide range of values from 10-3 to 0.34, 15, 36-38 at ultraviolet and lower visible wavelengths.”
We thank the reviewer for pointing this out.
2) Line 153-154: Remove this statement. This information is presented in Section 2.5.
The sentence has been removed.
3) Title of Section 3.2 and captions of Figure 2 and Figure 3: ‘Change’ should be replaced with
something like ‘dependence.’ The way it reads now gives the impression that the results are for
the same BrC that evolves (changes).
We have modified “change in" with “dependence of” throughout the manuscript where
appropriate. We thank the reviewer for pointing this out.
4) Line 294: replace ‘substitutes’ with ‘substituted.’
The word has been replaced.
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16-Jul-2023

Dear Dr Shetty:

Manuscript ID: EA-ART-05-2023-000067.R1
TITLE: Brown Carbon Absorptivity in Fresh Wildfire Smoke: Associations with Volatility and Chemical Compound Groups

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Reviewer 2

The authors have adequately addressed my comments.

Reviewer 1

The authors have addressed all my concerns.