From the journal Environmental Science: Atmospheres Peer review history

28-Jun-2023

Dear Dr China:

Manuscript ID: EA-ART-04-2023-000058
TITLE: Case study evaluation of size-resolved molecular composition and phase state of carbonaceous particles in wildfire influenced smoke from the Pacific Northwest

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

************

Reviewer 1

The manuscript using the micro-spectroscopy and high resolution mass
spectrometry analyses studied the interplay between particle size, phase state and chemical composition for aerosol influenced by a 2021 Pacific Northwest wildfire event. The author found that the particle compositions were not influenced by particle size and day/night cycle. They also found a slight increase in liquid-like character for smaller particles with no significant day/night dependency. The manuscript is of great scientific interest and this manuscript may have substantial implications for wildfire particle optical properties, transport, and atmospheric models. Overall the manuscript should be accepted after addressing the following minor issues.

1. Materials and Methods. The author needs to explain the reason for studying the particle sizes of stage 7, 8 and 9.
2. The author only collected particle samples for two days, and the account of the sample is relatively small. The authors should consider whether the results are convincing.
3. Page 9. The author has repeatedly mentioned that there is no diurnal difference in the chemical composition of wildfire-influenced particles. They also used STXM–NEXAFS, CCSEM – EDX and nano-DESI MS analyses to repeatedly verify this phenomenon. However, as described by the authors, nighttime nitrate oxidation typically results in varied composition, and consequently, distinct physical characteristic consequences for atmospheric particles, so they should explain how does this occurs in detail.

Reviewer 2

In their research paper, Vandergrift and colleagues examine the molecular structure, phase conditions, and evaporation of aerosol particles collected from a site that was thought to be impacted by wildfires. They employed a range of techniques such as microscopy and mass spectroscopy to scrutinize the samples of aerosols gathered during this period. They highlighted that smaller-sized particles might exhibit more liquid-like properties, with minimal variation in the phase state of aerosols collected during different times of the day. Studying the composition, phase state, and volatility of aerosols emanating from wildfires can yield critical understanding of air pollution dynamics and potential climate impacts. The manuscript itself is interesting but it is not very closely related to wildfires. After addressing the following issues, the manuscript is suitable to be published.


1. The methodology employed by the author to estimate evaporation kinetics, as depicted in Figure 4, seems to be lacking in important details. Initial model parameters, crucial for generating Figure 4, have not been adequately explained. For example, the Supplementary Information references Table S3 as the source for the initial particle diameter and mass fraction for each volatility bin, yet these factors are not listed in the table. Also, information on the volatility values for the 14 bins and the mass fraction of each bin is missing. This critical data needs to be added to either the main text or the SI. Furthermore, there's an absence of explanation on how the phase states of the aerosols were determined in Tables S4-S6. Comprehensive information on these estimates is required.

2. The back trajectory data, provided in Figure S1, doesn't seem to corroborate the theory that the aerosols originated from wildfires. Moreover, while the author notes an increased concentration of levoglucosan, there is no reference point given to determine what constitutes a "normal" level of levoglucosan in ambient aerosols. Would it not be more beneficial to steer clear of the "wildfires" theme and instead concentrate on the aerosols collected at this location during summer? I feel that will put the manuscript more interesting and avoid overstating the influence of the fire without concrete evidence.

3. Could you please clarify what is meant by the term "aspect ratio"? This term might not be familiar to those without a deep understanding of microscopy. Additionally, isn't the aspect ratio influenced by the size, even when particle compositions remain consistent?

4. The format of this manuscript is not particularly easy to read. For instance, there are no line numbers and all the texts were single-spaced.

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

Referee: 1

Comments to the Author
The manuscript using the micro-spectroscopy and high resolution mass spectrometry analyses studied the interplay between particle size, phase state and chemical composition for aerosol influenced by a 2021 Pacific Northwest wildfire event. The author found that the particle compositions were not influenced by particle size and day/night cycle. They also found a slight increase in liquid-like character for smaller particles with no significant day/night dependency. The manuscript is of great scientific interest and this manuscript may have substantial implications for wildfire particle optical properties, transport, and atmospheric models. Overall the manuscript should be accepted after addressing the following minor issues.

Author Response
We thank the Reviewer for their positive assessment of our manuscript. They additionally highlight several key areas for improvement and improved clarity of communication, which we have addressed below.

1. Materials and Methods. The author needs to explain the reason for studying the particle sizes of stage 7, 8 and 9.

Author Response
We specifically investigated these particle sizes (stages 7-9 from the MOUDI system) as the online SMPS data (Figure S3) confirms that these size ranges constitute the vast majority of the sample composition. While it is worth noting that the particle diameters from these two methods are defined/measured differently (aerodynamic diameter in the case of MOUDI, electrical mobility in the case of SMPS), the SMPS data may nonetheless serve as an approximate guideline for the range of diameters collected by the MOUDI system. We have added the following prose to the Materials and Methods section (page 4) to clarify this point.

“The SMPS data (Figure S3) was used to inform which particle diameters (i.e., stages) were representative of the sampling period, justifying the analyses of MOUDI stages 7-9.”

2. The author only collected particle samples for two days, and the account of the sample is relatively small. The authors should consider whether the results are convincing.

Author Response
We agree with the reviewer that the sample size is indeed relatively small, and as such, the results presented herein should not be construed as universally representative of wildfire-influenced smoke; we reassert the case-study nature of this study and note that it is reflected in the manuscript title as well. We are however confident in our observations for these specific samples due to the depth and variety of analyses employed and believe that this study may be informative to larger-scale campaigns and/or investigations of wildfire influence from different regions and seasons. To better communicate that we similarly agree that these results should not be construed as representative but rather a case-study, we have added additional prose to the Conclusion section as follows:

“As the observations described herein are limited to a case study, they should not be considered as a universal description of wildfire-influenced particles. However, the depth of multimodal analyses employed here for these selected samples nevertheless lends a high degree of confidence for the particular events investigated here, with many significant implications as described.”

3. Page 9. The author has repeatedly mentioned that there is no diurnal difference in the chemical composition of wildfire-influenced particles. They also used STXM–NEXAFS, CCSEM – EDX and nano-DESI MS analyses to repeatedly verify this phenomenon. However, as described by the authors, nighttime nitrate oxidation typically results in varied composition, and consequently, distinct physical characteristic consequences for atmospheric particles, so they should explain how does this occurs in detail.

Author Response
We appreciate that the reviewer shares an understanding of our conundrum: we agree that the results presented herein may not be what is expected from nighttime nitrate oxidation. While unexpected, we are still confident in our observations due to the comprehensive and multimodal nature of the experimental investigations. We agree with the reviewer that a detailed, mechanistic investigation into why this observation occurs would be of great interest to the atmospheric community. However, while we’ve hypothesized in the paper as to why this might be, we believe that our available data doesn’t permit an accurate, detailed assertion on this topic. Therefore, instead of presenting firm conclusions as part of this study, we believe such a pursuit is best done as part of future work. We have clarified this intention by modifying prose in the Conclusion as follows:

“While it may not be mechanistically investigated with the presented data, the highly specific examination of wildfire derived SOA evolution over time will also be evaluated in future.”

 
Referee: 2

Comments to the Author
In their research paper, Vandergrift and colleagues examine the molecular structure, phase conditions, and evaporation of aerosol particles collected from a site that was thought to be impacted by wildfires. They employed a range of techniques such as microscopy and mass spectroscopy to scrutinize the samples of aerosols gathered during this period. They highlighted that smaller-sized particles might exhibit more liquid-like properties, with minimal variation in the phase state of aerosols collected during different times of the day. Studying the composition, phase state, and volatility of aerosols emanating from wildfires can yield critical understanding of air pollution dynamics and potential climate impacts. The manuscript itself is interesting but it is not very closely related to wildfires. After addressing the following issues, the manuscript is suitable to be published.

Author Response
We thank the Reviewer for their positive assessment of our manuscript. They additionally highlight several key areas for improvement and improved clarity of communication, which we have addressed below.

1. The methodology employed by the author to estimate evaporation kinetics, as depicted in Figure 4, seems to be lacking in important details. Initial model parameters, crucial for generating Figure 4, have not been adequately explained. For example, the Supplementary Information references Table S3 as the source for the initial particle diameter and mass fraction for each volatility bin, yet these factors are not listed in the table. Also, information on the volatility values for the 14 bins and the mass fraction of each bin is missing. This critical data needs to be added to either the main text or the SI. Furthermore, there's an absence of explanation on how the phase states of the aerosols were determined in Tables S4-S6. Comprehensive information on these estimates is required.

Author Response
We agree with the reviewer that sufficient description of the evaporation kinetics was lacking. Additional text has been added to both the main manuscript and SI. A new table (Table S3) has been added to the SI containing information on the volatility bins (see below). A new figure (Figure S6) has also been added to the Supporting Information (see below), detailing the time-dependent evolution of the volatility distribution. Along with additional references, comprehensive description of the determination of the phase states listed in Tables S5-S7
(revised numbering is reflective of the resubmitted documents) has also been added to the SI. The phase state information in Table S7 was originally included in error, as such results were not presented/interpreted as part of the submitted manuscript. As a result, that section has been removed from the resubmitted manuscript.

 
Table S3: The initial particle diameter (Dp) and weighted mass spectrometry intensities for each volatility bin for stages 7, 8, and 9 of wildfire-influenced aerosol collected during periods of both day (August 2, 2021) and nighttime (August 3, 2021).
Daytime (August 2, 2021) Nighttime (August 3, 2021)
Stage 7 Stage 8 Stage 9 Stage 7 Stage 8 Stage 9

Initial Dp (µm) 0.44 0.25 0.14 0.44 0.25 0.14

C* (µg m-3) Parametrized volatilities weighted to MS signal intensities
10-9 0.151 0.149 0.113 0.139 0.125 0.087
10-8 0.061 0.058 0.054 0.055 0.053 0.048
10-7 0.067 0.062 0.060 0.060 0.060 0.054
10-6 0.070 0.069 0.073 0.065 0.068 0.060
10-5 0.083 0.080 0.081 0.076 0.080 0.072
10-4 0.096 0.093 0.095 0.092 0.098 0.093
10-3 0.092 0.092 0.099 0.096 0.098 0.102
10-2 0.085 0.089 0.093 0.092 0.094 0.098
10-1 0.090 0.091 0.096 0.094 0.095 0.103
100 0.091 0.092 0.101 0.097 0.098 0.116
101 0.071 0.079 0.084 0.081 0.082 0.103
102 0.027 0.029 0.033 0.033 0.031 0.041
103 0.015 0.017 0.017 0.018 0.017 0.023
104 0.001 0.001 0.001 0.001 0.001 0.001



Figure S6: Simulated volatility distributions of biomass burning OA at the end of 24 hours of evaporation in an organic gas-free environment at room temperature (solid lines with different colors for the stages 7, 8 and 9). Dashed lines show the initial volatility distributions for each stage.



 
2. The back trajectory data, provided in Figure S1, doesn't seem to corroborate the theory that the aerosols originated from wildfires. Moreover, while the author notes an increased concentration of levoglucosan, there is no reference point given to determine what constitutes a "normal" level of levoglucosan in ambient aerosols. Would it not be more beneficial to steer clear of the "wildfires" theme and instead concentrate on the aerosols collected at this location during summer? I feel that will put the manuscript more interesting and avoid overstating the influence of the fire without concrete evidence.

Author Response
We agree with the reviewer that we do not wish to overstate the influence of wildfires on the particles analyzed as part of this study. For this reason, we have consistently referred to the samples as ‘wildfire-influenced particles’ as opposed to claiming more direct, comprehensive wildfire origin. However, even though local sources surely contribute to the studied particles here, we still believe that the term ‘wildfire-influenced’ is appropriate. To further justify this point (complementing Figure S1), we have added a new supplementary figure (Figure S2) which displays air quality index contours for days leading up to, during, and after the sampling for this study (accessed from ‘AirNow’, a United States government resource that provides archives of official air quality index data; https://www.airnow.gov/about-airnow/). The geospatial origin of the intense signal is consistent with the wildfire locations shown in Figure S1. Furthermore, elevated air quality indexes (originating from wildfire locations) is observed during our described sampling periods from the Richland, WA site. The air quality index is significantly lessened in the Richland, WA area both before and after our sampling periods for this study. We have added supporting text for the inclusion of this figure in both the Materials and Methods as well as the Results and Discussion sections. As a result, we believe that Figures S1 and S2 allow for the confident assertion that the particles studied herein are at least wildfire-influenced. The newly added Figure S2 is shown below:


Figure S2. Air quality index (AQI) contours for July 23 through August 8, 2021. Data was accessed from the ‘AirNow’ resource.
For the sake of review only, we have also added a photo here taken of the Richland, WA area on August 2, 2021, in which a dense haze may be seen.


Figure R1: Hazy conditions observed during August 2, 2021 near the Richland, WA sampling location.

3. Could you please clarify what is meant by the term "aspect ratio"? This term might not be familiar to those without a deep understanding of microscopy. Additionally, isn't the aspect ratio influenced by the size, even when particle compositions remain consistent?

Author Response
We defined aspect ratio in section 2.2. of the submitted manuscript: “… aspect ratio (particle width to height ratio) of individual particles.” We have modified this though and added additional prose to clarify the definition and implementation as follows:

“Environmental scanning electron microscopy (ESEM; Quanta 3D, Thermo Fisher) with a FEI Quanta digital field emission gun (20 kV and 480 pA) and a tilted stage (75°) was used to probe the phase state via the aspect ratio measurements (particle width to particle height ratio) of individual impacted particles. While some of the experimental conditions used here may bias the end measurements against water content and volatile – semivolatile species (e.g., analyses are conducted at 293 K and ~2 × 10−6 Torr), this technique has been previously validated for phase-state measurements and is described in detail elsewhere. Additionally, the strategy used herein was importantly found to not be biased by particle size for the investigated particle size ranges here.31,32 Further description of these limitations and experimental parameters are described elsewhere.33”

The Review brings up an astute observation regarding the relationship of aspect ratio and size for particles of similar composition. It is likely that for increasing particle size, particle surface tension may eventually not be sufficient to counter the effect of gravity. In this scenario, increasingly large particles would have aspect ratios biased high, and consequently an overemphasized liquid-like character. While this specific limit has not been explored in detail, it is worth highlighting that this effect has not yet been observed (to our knowledge) for submicron aerosol (Cheng et al.; 10.1039/D1RA02530A).

However, it must also be considered that this here study describes increasing liquid like character for smaller particles, not larger ones. Because our observations trend in the opposite direction compared to the potential bias scenario described above, and because we have presented phase-state within the context of trends as opposed to absolute/specific evaluation, we believe that the data and interpretations as presented in the submitted manuscript are accurate. Further validation of the use of aspect ratios for submicron particles will be pursued in follow-up studies.

4. The format of this manuscript is not particularly easy to read. For instance, there are no line numbers and all the texts were single-spaced.

Author Response
This point is well taken. We have added line numbers to the resubmitted manuscript. For future submissions, we will be more diligent in providing accessible and easily read manuscripts.

28-Jul-2023

Dear Dr China:

Manuscript ID: EA-ART-04-2023-000058.R1
TITLE: Case study evaluation of size-resolved molecular composition and phase state of carbonaceous particles in wildfire influenced smoke from the Pacific Northwest

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Dr Tzung-May Fu
Associate Editor
Environmental Science: Atmospheres
Royal Society of Chemistry

Reviewer 1

I have no more questions