Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range transported smoke over the southeast Atlantic

Fakoya, Abdulamid A.; Redemann, Jens; Saide, Pablo E.; Gao, Lan; Mitchell, Logan T.; Howes, Calvin; Dobracki, Amie; Chang, Ian; Ferrada, Gonzalo A.; Pistone, Kristina; Leblanc, Samuel E.; Segal-Rozenhaimer, Michal; Sedlacek III, Arthur J.; Eck, Thomas; Holben, Brent; Gupta, Pawan; Lind, Elena; Zuidema, Paquita; Carmichael, Gregory; Flynn, Connor J.

doi:https://doi.org/10.5194/egusphere-2024-3197

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https://doi.org/10.5194/egusphere-2024-3197

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13 Jan 2025

| 13 Jan 2025

Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range transported smoke over the southeast Atlantic

Abdulamid A. Fakoya, Jens Redemann, Pablo E. Saide, Lan Gao, Logan T. Mitchell, Calvin Howes, Amie Dobracki, Ian Chang, Gonzalo A. Ferrada, Kristina Pistone, Samuel E. Leblanc, Michal Segal-Rozenhaimer, Arthur J. Sedlacek III, Thomas Eck, Brent Holben, Pawan Gupta, Elena Lind, Paquita Zuidema, Gregory Carmichael, and Connor J. Flynn

Abstract. Biomass burning aerosols (BBA) from agricultural fires in southern Africa contribute about one-third of global carbonaceous aerosol load. These particles have strong radiative effects in the southeast Atlantic (SEA), which depend on the radiative contrast between the aerosol layer in the free troposphere (FT) and the underlying cloud layer. However, there is large disagreements in model estimates of aerosol-driven climate forcing due to uncertainties in the vertical distribution, optical properties, and lifecycle of these particles.

This study applies a novel method combining remote sensing observations with regional model outputs to investigate the aging of the BBA and its impact on the optical properties during transatlantic transport from emission sources in Africa to the SEA. Results show distinct variations in Ångstrom exponent (AE) and single scattering albedo (SSA) as aerosol age. Near the source, fresh aerosols are characterized by low mean SSA (0.84) and high AE (1.85), indicating smaller, highly absorbing particles. By isolating marine contributions from the total column during BBA transport across the SEA, our analysis reveals an initial decrease in BBA absorptivity, with mean FT SSA of 0.87 after 6–7 days, followed by increased absorptivity with mean FT SSA of 0.84 after 10 days, suggesting enhanced absorption due to chemical aging.

These findings indicate that BBA becomes more absorbing during extended transport across the SEA, with implications for reducing model uncertainties. Our remote sensing-based results agree well with previous in situ studies and offer new insights into aerosol-radiation interactions and the energy balance over the SEA.

How to cite. Fakoya, A. A., Redemann, J., Saide, P. E., Gao, L., Mitchell, L. T., Howes, C., Dobracki, A., Chang, I., Ferrada, G. A., Pistone, K., Leblanc, S. E., Segal-Rozenhaimer, M., Sedlacek III, A. J., Eck, T., Holben, B., Gupta, P., Lind, E., Zuidema, P., Carmichael, G., and Flynn, C. J.: Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range transported smoke over the southeast Atlantic, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3197, 2025.

Received: 15 Oct 2024 – Discussion started: 13 Jan 2025

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Journal article(s) based on this preprint

25 Jul 2025

Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range-transported smoke over the southeast Atlantic

Atmos. Chem. Phys., 25, 7879–7902, https://doi.org/10.5194/acp-25-7879-2025,https://doi.org/10.5194/acp-25-7879-2025, 2025

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Interactive discussion

Status: closed

CC1: 'Comment on egusphere-2024-3197', dorothy lsoto, 13 Jan 2025

This is a very excellently written abstract and must congratulate the team on a very important research project. It would help to replace "Near the source" with the actual source you are referring to if space allows. This might help keep the reader reminded of what they are still reading about especially for those not very familiar with the science. Overall, this study is very well done from just reading about the novel methodology used.

Citation: https://doi.org/10.5194/egusphere-2024-3197-CC1
CC2: 'Comment on egusphere-2024-3197', Anoruo chukwuma, 14 Jan 2025

The present form of the draft is not clear enough and several paragraphs are not complete. Some terms were used in a strange way, for example "450 science...".

The method section not clear and several instruments (data) used not mentioned. In general, the draft should be logically linked to tell nice story about method and how AOD from different instruments were subset. For example, subsection 2.6 line 295.

I have doubt about use of level 1.5 (not screened) complemented with level 2.0.

Citation: https://doi.org/10.5194/egusphere-2024-3197-CC2
RC1: 'Comment on egusphere-2024-3197', Anonymous Referee #2, 14 Jan 2025

The manuscript titled "Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range transported smoke over the southeast Atlantic" provides an in-depth analysis of impacts of aerosol aging on the single scattering albedo (SSA) and Ångstrom exponent (AE) by using the observation network composed of AERONET (AErosol RObotic NETwork), 4STAR(Spectrometers for Sky-Scanning, Sun-Tracking Atmospheric Research), etc. through the long-range transported smoke over the southeast Atlantic. The manuscript deals with important scientific themes, however, there are several points that should be addressed to improve its rigor and clarity:

1. In the lines 107 - 110, why single scattering albedo (SSA) increasing indicating their ability to absorb sunlight?

2. In the lines 144 and 530, unclear "OA" and "ASI". define them.

3. In the lines 159 - 169, the introduction of the structure of the paper is not needed.

4. In the Figure 2, We are more concerned about the flights used in the paper. Could they be marked out?

5. In Section 2 "Data and Methods", the introduction is overly lengthy. For example, lines 322 - 332 can be briefly introduced.

6. In the lines 398 - 399, "In Figure 3(b), the vertical distribution of aerosol extinction revealed maximum extinction below 1 km", 1km or 10km? The description doesn't match the figure. Does "aerosol extinction" refer to that of a certain layer or cumulative extinction?

7. In the lines 437 - 438, "We do this by integrating the available collocated AERONET, STAR and WRF-AAM output datasets from September 2016, August 2017, and October 2018." Does the data integrated by this method exclude the impact of annual variations on the data?

8. In the Figure 5, Whether the SSA of the two data points of 4STAR that are lower than 0.75 in 8 - 10 days are abnormal data or not, and whether the final results will be affected if they are removed.

9. In the Figure 7, Is it because the standard deviation (std) is too small that there are no error bars for the data points in the 2 - 4 days and 4 - 6 days, or is it that there is only one data point in each of these two time periods, making it impossible to calculate the error bars?

10. In the Figure 8 and Figure 9, It is recommended to keep the order of the labels on the x-axis consistent with that of Figure 7 to improve readability. Moreover, Figure9and Figure 8 can be combined into one figure.

11. In Section 3.3.3 Free Tropospheric Single Scattering Albedo, explain why the SSA in the free troposphere (FT) is lower than that in the total column (TC).

12. In the lines 551 and 553 - 554, "continually decreases from 0.87", "begins at 0.84(0-2 days), increases to 0.857 (2-4 days), 0.862 (4-6 days), and peaks at 0.871(6-8days), before declining to 0.84 (8-10 days) and 0.845 (10-12 days)", increase from8- 10 days to 10 - 12 days is in contradiction with "continually decreases".

Citation: https://doi.org/10.5194/egusphere-2024-3197-RC1
RC2:
'Comment on egusphere-2024-3197', Anonymous Referee #3, 03 Feb 2025
Fakoya et al. show a method to predict the evolution of optical properties and vertical aerosol composition upon atmospheric aging in the southeast Atlantic (SEA) region. They used remote sensing data combined with a regional model. They found that biomass-burning aerosol absorption first decreases and then increases with aging. Overall, this well-designed study can help reduce the current uncertainties in aerosol climate effects. I have a few minor comments to help improve this paper. Please see my comments below:
General comments:
It is unclear how you developed the AE threshold. Did you do any statistical tests to verify your threshold? Why do you choose these four values to test?

I suggest using EAE for Extinction Angstrom Exponent across the whole manuscript instead of AE since AE might be confused with AAE (Absorption Angstrom Exponent).

How did the AE vary along with the aerosol absorption properties?

Fig. 4 is unnecessary since the same information is shown in Fig. 6.

Have you considered lensing enhancement since SOA can condense on soot and tar balls?

Specific comments:
For Fig. 1, it is not clear what each green line represents. Are they from different original days? Moreover, it might be good to add labels of back days or transport directions on the path. It will be good to add vertical transport as well. Also, I suggest labeling flight directions on the red line. For panel b, I recommend adding the y-axis of altitude. Also, please increase the font size. They are too small.

It is unclear which tracer gases you used to estimate the aging time. Did you use CO? Then, it should be singular, not plural. Which emission inventory did you use for your model simulation?

Equation 4. It should be R_FT, not R_BL, on the left side.

Please label wavelengths when you discuss or show anything related to optical properties such as SSA, AOD, and AE.

L308, please define FT when it first appears in the main text.

L548-551, “These distributions … SSA in the region.” I might miss something, but it is unclear to me how you come up with this conclusion. Could you explain a little bit more?

L564-566, “This decrease in … of larger particles.” This is also unclear to me.

L578, “These processes happen rapidly … first few days,” Needs citations.

L579, “heterogeneous oxidation repartitions some of the aerosols …” Oxidation refers to a chemical transformation, whereas repartition suggests a physical or phase-based process. It would be helpful to clarify whether the oxidation itself leads to repartitioning through chemical breakdown, or if the repartitioning occurs as a result of phase equilibrium shifts independent of oxidation.

L581, it is unclear what is the concurrent change you mentioned here.
Citation: https://doi.org/10.5194/egusphere-2024-3197-RC2
AC1: 'Comment on egusphere-2024-3197', Abdulamid Fakoya, 01 Apr 2025

We thank the two anonymous reviewers for their valuable feedback and also appreciate the comments from Dorothy Lsotho and Anoruo Chukwuma. The responses to all reviewer and community comments are provided in the attached PDF

Citation: https://doi.org/10.5194/egusphere-2024-3197-AC1

Interactive discussion

Status: closed

CC1: 'Comment on egusphere-2024-3197', dorothy lsoto, 13 Jan 2025

This is a very excellently written abstract and must congratulate the team on a very important research project. It would help to replace "Near the source" with the actual source you are referring to if space allows. This might help keep the reader reminded of what they are still reading about especially for those not very familiar with the science. Overall, this study is very well done from just reading about the novel methodology used.

Citation: https://doi.org/10.5194/egusphere-2024-3197-CC1
CC2: 'Comment on egusphere-2024-3197', Anoruo chukwuma, 14 Jan 2025

The present form of the draft is not clear enough and several paragraphs are not complete. Some terms were used in a strange way, for example "450 science...".

The method section not clear and several instruments (data) used not mentioned. In general, the draft should be logically linked to tell nice story about method and how AOD from different instruments were subset. For example, subsection 2.6 line 295.

I have doubt about use of level 1.5 (not screened) complemented with level 2.0.

Citation: https://doi.org/10.5194/egusphere-2024-3197-CC2
RC1: 'Comment on egusphere-2024-3197', Anonymous Referee #2, 14 Jan 2025

The manuscript titled "Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range transported smoke over the southeast Atlantic" provides an in-depth analysis of impacts of aerosol aging on the single scattering albedo (SSA) and Ångstrom exponent (AE) by using the observation network composed of AERONET (AErosol RObotic NETwork), 4STAR(Spectrometers for Sky-Scanning, Sun-Tracking Atmospheric Research), etc. through the long-range transported smoke over the southeast Atlantic. The manuscript deals with important scientific themes, however, there are several points that should be addressed to improve its rigor and clarity:

1. In the lines 107 - 110, why single scattering albedo (SSA) increasing indicating their ability to absorb sunlight?

2. In the lines 144 and 530, unclear "OA" and "ASI". define them.

3. In the lines 159 - 169, the introduction of the structure of the paper is not needed.

4. In the Figure 2, We are more concerned about the flights used in the paper. Could they be marked out?

5. In Section 2 "Data and Methods", the introduction is overly lengthy. For example, lines 322 - 332 can be briefly introduced.

6. In the lines 398 - 399, "In Figure 3(b), the vertical distribution of aerosol extinction revealed maximum extinction below 1 km", 1km or 10km? The description doesn't match the figure. Does "aerosol extinction" refer to that of a certain layer or cumulative extinction?

7. In the lines 437 - 438, "We do this by integrating the available collocated AERONET, STAR and WRF-AAM output datasets from September 2016, August 2017, and October 2018." Does the data integrated by this method exclude the impact of annual variations on the data?

8. In the Figure 5, Whether the SSA of the two data points of 4STAR that are lower than 0.75 in 8 - 10 days are abnormal data or not, and whether the final results will be affected if they are removed.

9. In the Figure 7, Is it because the standard deviation (std) is too small that there are no error bars for the data points in the 2 - 4 days and 4 - 6 days, or is it that there is only one data point in each of these two time periods, making it impossible to calculate the error bars?

10. In the Figure 8 and Figure 9, It is recommended to keep the order of the labels on the x-axis consistent with that of Figure 7 to improve readability. Moreover, Figure9and Figure 8 can be combined into one figure.

11. In Section 3.3.3 Free Tropospheric Single Scattering Albedo, explain why the SSA in the free troposphere (FT) is lower than that in the total column (TC).

12. In the lines 551 and 553 - 554, "continually decreases from 0.87", "begins at 0.84(0-2 days), increases to 0.857 (2-4 days), 0.862 (4-6 days), and peaks at 0.871(6-8days), before declining to 0.84 (8-10 days) and 0.845 (10-12 days)", increase from8- 10 days to 10 - 12 days is in contradiction with "continually decreases".

Citation: https://doi.org/10.5194/egusphere-2024-3197-RC1
RC2:
'Comment on egusphere-2024-3197', Anonymous Referee #3, 03 Feb 2025
Fakoya et al. show a method to predict the evolution of optical properties and vertical aerosol composition upon atmospheric aging in the southeast Atlantic (SEA) region. They used remote sensing data combined with a regional model. They found that biomass-burning aerosol absorption first decreases and then increases with aging. Overall, this well-designed study can help reduce the current uncertainties in aerosol climate effects. I have a few minor comments to help improve this paper. Please see my comments below:
General comments:
It is unclear how you developed the AE threshold. Did you do any statistical tests to verify your threshold? Why do you choose these four values to test?

I suggest using EAE for Extinction Angstrom Exponent across the whole manuscript instead of AE since AE might be confused with AAE (Absorption Angstrom Exponent).

How did the AE vary along with the aerosol absorption properties?

Fig. 4 is unnecessary since the same information is shown in Fig. 6.

Have you considered lensing enhancement since SOA can condense on soot and tar balls?

Specific comments:
For Fig. 1, it is not clear what each green line represents. Are they from different original days? Moreover, it might be good to add labels of back days or transport directions on the path. It will be good to add vertical transport as well. Also, I suggest labeling flight directions on the red line. For panel b, I recommend adding the y-axis of altitude. Also, please increase the font size. They are too small.

It is unclear which tracer gases you used to estimate the aging time. Did you use CO? Then, it should be singular, not plural. Which emission inventory did you use for your model simulation?

Equation 4. It should be R_FT, not R_BL, on the left side.

Please label wavelengths when you discuss or show anything related to optical properties such as SSA, AOD, and AE.

L308, please define FT when it first appears in the main text.

L548-551, “These distributions … SSA in the region.” I might miss something, but it is unclear to me how you come up with this conclusion. Could you explain a little bit more?

L564-566, “This decrease in … of larger particles.” This is also unclear to me.

L578, “These processes happen rapidly … first few days,” Needs citations.

L579, “heterogeneous oxidation repartitions some of the aerosols …” Oxidation refers to a chemical transformation, whereas repartition suggests a physical or phase-based process. It would be helpful to clarify whether the oxidation itself leads to repartitioning through chemical breakdown, or if the repartitioning occurs as a result of phase equilibrium shifts independent of oxidation.

L581, it is unclear what is the concurrent change you mentioned here.
Citation: https://doi.org/10.5194/egusphere-2024-3197-RC2
AC1: 'Comment on egusphere-2024-3197', Abdulamid Fakoya, 01 Apr 2025

We thank the two anonymous reviewers for their valuable feedback and also appreciate the comments from Dorothy Lsotho and Anoruo Chukwuma. The responses to all reviewer and community comments are provided in the attached PDF

Citation: https://doi.org/10.5194/egusphere-2024-3197-AC1

Peer review completion

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

AR by Abdulamid Fakoya on behalf of the Authors (01 Apr 2025) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (10 Apr 2025) by Quanfu He

I want to take this chance to say thanks to the reviewers for the valuable comments. I also appreciate the authors being serious about the comments. After reading all of the materials, I have some additional suggestions for this manuscript:
1. This manuscript is generally lengthy, especially the method part already reaches line 372. Though the text helps to demonstrate the broad background knowledge of the authors, it does not help to keep readers focused on the important scientific findings. Here are several specific suggestions for your consideration: 1) Delete the text regarding the introduction of the structure of the paper, 2) The first few sentences introducing the measurement modes of the 4STAR. You are not using the other two modes at all. Start directly from “This study…”, 3) The Second paragraph introducing the WRF-CAM5 is a good summary of the model, but I don’t think it is necessary here unless you have done something unique to the model. As a method part, the first and last paragraphs are enough. 4) There is a long introduction on why to separate the BL contribution from TC. The sentence “Dang et al. (2022) showed that BBA sampled during ORACLES dominate… ” is good enough, and all of the text before can be potentially removed.
2. RC2 5 and text in the revised manuscript: There are several problems in this revised paragraph. 1) Why emphasize flaming here? Smoldering is more efficient in producing rBC and brown carbon. 2) Organic coating is the condensation of organic vapors on the existing particles, while nucleation produces new particles. Don’t mix them. 3) Many lab experiments have shown that the biomass burning BrC has a lifetime of hours to 2 or 3 days. This bleaching process might be the main reason for the increase in SSA during transport. 4) Increasing OA does not mean increasing SSA. It’s true only when the OA is not absorbing or less absorbing. 5) During 6-8 days, aging probably already causes organic loss in the particles. Therefore, the lensing effect is weaker, which increases SSA. But due to the loss of less absorbing species (compared to black carbon), the mass absorption efficiency increases and size decreases, both lead to decreased SSA. The authors need to carefully change the text.
3. The abstract must be one single paragraph.
4. The authors used indentation for starting some new paragraphs, but not to all. Please be constant.

Hide

AR by Abdulamid Fakoya on behalf of the Authors (18 Apr 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (28 Apr 2025) by Quanfu He

AR by Abdulamid Fakoya on behalf of the Authors (08 May 2025) Manuscript

Journal article(s) based on this preprint

25 Jul 2025

Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range-transported smoke over the southeast Atlantic

Atmos. Chem. Phys., 25, 7879–7902, https://doi.org/10.5194/acp-25-7879-2025,https://doi.org/10.5194/acp-25-7879-2025, 2025

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Tiny atmospheric particles from wildfire smoke impact climate by interacting with sunlight and clouds, the extent of which is uncertain due to gaps in understanding how smoke changes over time. We developed a new method using remote sensing instruments to track how these particles evolve during atmospheric transport. Our results show that the ability of these particles to absorb sunlight increased as they travel. This discovery could help improve predictions of future climate scenarios.


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