Measurement Report: Atmospheric aging of combustion-derived particles: impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media

Runberg, Heather L.; Majestic, Brian J.

doi:https://doi.org/10.5194/egusphere-2023-4

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https://doi.org/10.5194/egusphere-2023-4

Preprints

30 Jan 2023

| 30 Jan 2023

Measurement Report: Atmospheric aging of combustion-derived particles: impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media

Heather L. Runberg and Brian J. Majestic

Abstract. Environmentally persistent free radicals (EPFR) are a pollutant found on fine atmospheric particulate matter (PM_2.5), particularly on PM_2.5 formed from combustion processes. EPFR are organic radicals that can endure in the environment for days to years. Interest in the toxicity of EPFR has increased significantly in recent years, as it has been shown to have substantial ability to form reactive oxygen species (ROS), but little is known about how its characteristics change as PM_2.5 ages in the atmosphere. Here, we exposed newly produced hexane-generated soot to simulated sunlight for 24 hours. Changes to the EPFR characteristics of the particles were measured by electron paramagnetic resonance (EPR) spectroscopy. The soot was then added to water and a second exposure to light was used to measure hydroxyl radical (OH) formation from both photo-aged and dark-aged soot. There were no changes to EPFR characteristics (spin concentration, g-factor, peak width, or lineshape) due to the exposure to simulated sunlight, however the soot’s ability to form OH was greatly reduced by photo-aging. Photo-aged soot resulted in an almost 60 % reduction in OH formation over soot which had been aged in the dark for the same amount of time.

Received: 10 Jan 2023 – Discussion started: 30 Jan 2023

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Journal article(s) based on this preprint

30 Jun 2023

Measurement report: Atmospheric aging of combustion-derived particles – impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media

Heather L. Runberg and Brian J. Majestic

Atmos. Chem. Phys., 23, 7213–7223, https://doi.org/10.5194/acp-23-7213-2023,https://doi.org/10.5194/acp-23-7213-2023, 2023

Short summary

Heather L. Runberg and Brian J. Majestic

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-4', Guorui Liu, 18 Feb 2023
Comments on the manuscript entitled “Measurement Report: Atmospheric aging of combustion-derived particles: impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media”.
EPFRs character change and ability to induce OH formation under influence of light and water were studied to simulate the environmental fate of EPFRs in this research. EPFRs were characterized fully by many instrumental or fitting methods. The results are valuable. In general, after minor revision, the manuscript can be considered for publication on EGUsphere.
Have you verified that materials you used could not interference the results during the experiment? Transition metals were reported to influence formation and stabilization of EPFRs. The disposable aluminum baking dish and aluminum foil used in this research might have influences. Please clarify that.

Section 2.2, you mentioned there was a storage period of the EPFR samples. Considering the inevitable decay and high reactivity of free electron (even though in EPFRs), please provide the storage time of each sample and try to give evidence for not change of EPFRs during the storage.

Line 134, though the g value in samples A-G showed no obvious change, it still could not conclude whether there was no radical type change. There was no hyperfine splitting information in the EPR spectra (Figure A1), making it difficult to deduce the structure of EPFRs. In addition, except for the influence of temperature, there were probably more than one kind of radical in each of the samples to give a single, broad peak. The sentence “Changes in g-factor indicate a change in radical type, which was not observed in this study.” was suggested to be modified.

Line 145, please add discussion about possible composition change of samples based on the different ΔBp-p data.

Could you give possible EPFR structures based on the characterization data? And if possible, try to give the formation mechanism.
Citation: https://doi.org/10.5194/egusphere-2023-4-RC1
- AC1: 'Reply on RC1', Brian Majestic, 16 May 2023
  
  We thank the Reviewer for their insightful comments. Please see the attached document for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-4-AC1
RC2:
'Comment on egusphere-2023-4', Xiao-San Luo, 24 Feb 2023

Comments for egusphere-2023-4

This manuscript investigated the impacts of atmospheric aging of combustion-derived particles on the EPFRs concentration and its ability to produce ROS in aqueous media. This issue and findings are significant, the experimental design and measurements are also ingenious. Some comments of improvements are suggested for considering as follows:

General comments:
Revisions are needed to present results and support discussions appropriately. There are three main points worthy to notice.
1. The conclusion and significance of this study should be carefully summed up. For examples:
First, the author emphasized the importance of soot in PM2.5 (Line 41), but there was no particle size analysis for samples in this study.
Second, the soot generated from hexane in this study as model particle is not the main byproducts of wildfire (Line 245). It's more likely from the fuel combustion, as the hexane is widely used in the fuel and industry.
Third, the EPFRs in soot from real combustion process was verified to be EPFRs-metal conjugate, which might pose potentially more health effects.

2. The materials and methods section seems uninformative to readers. Since many techniques have been used in this study, detailed and clearly descriptions of these methods are strongly recommended to support the coming results. For examples:
Line 60: How was the “final soot concentration was 45 ppm (m/v) ” prepared? Any micro-balance used for soot weight quantification should be told?
Line 89: Why this concentration of SBA used in this study? How to determine the concentration of SBA used? Were there any preliminary experiments for supports?
Line 90: Why 24 h for soot aged but 16 h for photoreactions here?

3. The results and discussion of OH production and measurement are not solid and convincible. This part should be paid cautious consideration.
Line 181: How were the controls done? It seems that there was no OH produced by soot slurries exposed to light and photo-aged previously. The results could come from SBA photo degradation, not only from the reaction with OH which was produced by soot samples.
Line 183-184: “This is consistent with a recent study reporting a decrease in OH concentrations in wildfire smoke plumes downfield from the point source, vs the same plume measured nearer the source (Akherati et al., 2022)”. But soot with dark-aged and then exposed to light can produce higher OH.
Line 185-188: That study (Gehling, 2014) measured the OH from dark-aged EPFRs containing-PM suspension, which is different from current study.
Line 228: “only EPFR on the surface of the particles are available for OH formation”, but the samples were well mixed and stirred in this study.
Line 233-234: “The lack of changes to chemical composition of the soot upon irradiation, as seen in the GC-MS and UV-Vis analysis, support the hypothesis that only EPFR on the surface of the particle are available for OH formation”. Doubt. The OH formation in this study seems more affected by the light.

Citation: https://doi.org/10.5194/egusphere-2023-4-RC2
- AC2:
  'Reply on RC2', Brian Majestic, 16 May 2023
  
  We thank the Reviewer for their helpful comments. Please see the attached document for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-4-AC2
  - AC3: 'Reply on AC2', Brian Majestic, 16 May 2023
    
    Please use this document instread of the previous one.
    
    Citation: https://doi.org/10.5194/egusphere-2023-4-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-4', Guorui Liu, 18 Feb 2023
Comments on the manuscript entitled “Measurement Report: Atmospheric aging of combustion-derived particles: impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media”.
EPFRs character change and ability to induce OH formation under influence of light and water were studied to simulate the environmental fate of EPFRs in this research. EPFRs were characterized fully by many instrumental or fitting methods. The results are valuable. In general, after minor revision, the manuscript can be considered for publication on EGUsphere.
Have you verified that materials you used could not interference the results during the experiment? Transition metals were reported to influence formation and stabilization of EPFRs. The disposable aluminum baking dish and aluminum foil used in this research might have influences. Please clarify that.

Section 2.2, you mentioned there was a storage period of the EPFR samples. Considering the inevitable decay and high reactivity of free electron (even though in EPFRs), please provide the storage time of each sample and try to give evidence for not change of EPFRs during the storage.

Line 134, though the g value in samples A-G showed no obvious change, it still could not conclude whether there was no radical type change. There was no hyperfine splitting information in the EPR spectra (Figure A1), making it difficult to deduce the structure of EPFRs. In addition, except for the influence of temperature, there were probably more than one kind of radical in each of the samples to give a single, broad peak. The sentence “Changes in g-factor indicate a change in radical type, which was not observed in this study.” was suggested to be modified.

Line 145, please add discussion about possible composition change of samples based on the different ΔBp-p data.

Could you give possible EPFR structures based on the characterization data? And if possible, try to give the formation mechanism.
Citation: https://doi.org/10.5194/egusphere-2023-4-RC1
- AC1: 'Reply on RC1', Brian Majestic, 16 May 2023
  
  We thank the Reviewer for their insightful comments. Please see the attached document for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-4-AC1
RC2:
'Comment on egusphere-2023-4', Xiao-San Luo, 24 Feb 2023

Comments for egusphere-2023-4

This manuscript investigated the impacts of atmospheric aging of combustion-derived particles on the EPFRs concentration and its ability to produce ROS in aqueous media. This issue and findings are significant, the experimental design and measurements are also ingenious. Some comments of improvements are suggested for considering as follows:

General comments:
Revisions are needed to present results and support discussions appropriately. There are three main points worthy to notice.
1. The conclusion and significance of this study should be carefully summed up. For examples:
First, the author emphasized the importance of soot in PM2.5 (Line 41), but there was no particle size analysis for samples in this study.
Second, the soot generated from hexane in this study as model particle is not the main byproducts of wildfire (Line 245). It's more likely from the fuel combustion, as the hexane is widely used in the fuel and industry.
Third, the EPFRs in soot from real combustion process was verified to be EPFRs-metal conjugate, which might pose potentially more health effects.

2. The materials and methods section seems uninformative to readers. Since many techniques have been used in this study, detailed and clearly descriptions of these methods are strongly recommended to support the coming results. For examples:
Line 60: How was the “final soot concentration was 45 ppm (m/v) ” prepared? Any micro-balance used for soot weight quantification should be told?
Line 89: Why this concentration of SBA used in this study? How to determine the concentration of SBA used? Were there any preliminary experiments for supports?
Line 90: Why 24 h for soot aged but 16 h for photoreactions here?

3. The results and discussion of OH production and measurement are not solid and convincible. This part should be paid cautious consideration.
Line 181: How were the controls done? It seems that there was no OH produced by soot slurries exposed to light and photo-aged previously. The results could come from SBA photo degradation, not only from the reaction with OH which was produced by soot samples.
Line 183-184: “This is consistent with a recent study reporting a decrease in OH concentrations in wildfire smoke plumes downfield from the point source, vs the same plume measured nearer the source (Akherati et al., 2022)”. But soot with dark-aged and then exposed to light can produce higher OH.
Line 185-188: That study (Gehling, 2014) measured the OH from dark-aged EPFRs containing-PM suspension, which is different from current study.
Line 228: “only EPFR on the surface of the particles are available for OH formation”, but the samples were well mixed and stirred in this study.
Line 233-234: “The lack of changes to chemical composition of the soot upon irradiation, as seen in the GC-MS and UV-Vis analysis, support the hypothesis that only EPFR on the surface of the particle are available for OH formation”. Doubt. The OH formation in this study seems more affected by the light.

Citation: https://doi.org/10.5194/egusphere-2023-4-RC2
- AC2:
  'Reply on RC2', Brian Majestic, 16 May 2023
  
  We thank the Reviewer for their helpful comments. Please see the attached document for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-4-AC2
  - AC3: 'Reply on AC2', Brian Majestic, 16 May 2023
    
    Please use this document instread of the previous one.
    
    Citation: https://doi.org/10.5194/egusphere-2023-4-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Brian Majestic on behalf of the Authors (16 May 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (17 May 2023) by Thomas Berkemeier

The authors addressed all reviewer concerns satisfactorily. I would like to raise two minor points before the manuscript can be accepted for publication.

[1] I disagree with the use of "OH concentrations" in the manuscript (e.g. l. 93, l. 189 etc.). The authors do not measure in situ concentrations of OH (which are much lower at any given point in time), but rather use a probe to estimate time-integrated OH production. I suggest to replace "OH concentration" with "OH production" were applicable.

[2] I suggest to add more disclaimers to the comparison with the work by Akherati et al. (2022). The authors already formulate with care ("... is consistent with ..."), but the brevity of the statement might lead a reader to overinterpret the findings. It could be made clearer that (i) not all gas-phase OH in an air pollution plume comes from photochemistry of soot in aqueous environment, (ii) OH produced in this way might not even enter the gas phase, and (iii) OH concentrations in a biomass burning plume will likely decrease naturally over time due to dilution. Alternatively or in addition, the authors can generalize the statement more, e.g. that a reduced capability of OH production after a period of photo-irradiation is also seen in more complex systems.

References

Akherati, A., He, Y., Garofalo, L. A., Hodshire, A. L., Farmer, D. K., Kreidenweis, S. M., Permar, W., Hu, L., Fischer, E. v.,
Jen, C. N., Goldstein, A. H., Levin, E. J. T., DeMott, P. J., Campos, T. L., Flocke, F., Reeves, J. M., Toohey, D. W., Pierce, J. R.,
and Jathar, S. H.: Dilution and photooxidation driven processes explain the evolution of organic aerosol in wildfire plumes,
Environmental Science: Atmospheres, 2, 1000–1022, https://doi.org/10.1039/D1EA00082A, 2022.

Hide

AR by Brian Majestic on behalf of the Authors (23 May 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (25 May 2023) by Thomas Berkemeier

AR by Brian Majestic on behalf of the Authors (31 May 2023)

Journal article(s) based on this preprint

30 Jun 2023

Measurement report: Atmospheric aging of combustion-derived particles – impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media

Heather L. Runberg and Brian J. Majestic

Atmos. Chem. Phys., 23, 7213–7223, https://doi.org/10.5194/acp-23-7213-2023,https://doi.org/10.5194/acp-23-7213-2023, 2023

Short summary

Heather L. Runberg and Brian J. Majestic

Data sets

Data Set for "Atmospheric Aging of Combustion-Derived Particles: Impact on Stable Free Radical Concentration and Its Ability to Produce Reactive Oxygen Species in Aqueous Media" Runberg, Heather and Majestic, Brian https://digitalcommons.du.edu/chemistrydatasets/2/

Heather L. Runberg and Brian J. Majestic

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Environmentally persistent free radicals (EPFR) are an emerging pollutant found in soot particles. Understanding how these change as they move through the atmosphere is important to human health. Here, soot was generated in the laboratory and exposed to simulated sunlight. The concentrations and characteristics of EPFR in the soot was measured and found to be unchanged. However, it was also found that the ability of soot to form hydroxyl radicals was stronger for fresh soot.


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