the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Jan 2025
Atmospheric processing and aerosol aging responsible for observed increase in absorptivity of long-range transported smoke over the southeast Atlantic
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.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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.- Preprint
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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". -
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 RFT, not RBL, 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
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