the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Aug 2023
Evidence of a dual African and Australian biomass burning influence on the vertical distribution of aerosol and carbon monoxide over the Southwest Indian Ocean basin in early 2020
Abstract. The pristine atmosphere of the southwest Indian Ocean (SWIO) basin underwent significant perturbations during the 2020 austral summer. This study documents the complex variability of aerosols and carbon monoxide (CO) over this remote oceanic region and identifies the processes governing it in the upper troposphere – lower stratosphere (UT-LS). Aerosol profiles exhibit a multi-layer structure in the tropical UT-LS in January and February 2020. The numerical models (FLEXPART and MIMOSA) showed that the modulation of the aerosol content in the lower stratosphere is due to the intense and persistent stratospheric aerosol smoke layer generated during the 2019–20 extreme Australian bushfire events. One part of this stratospheric aerosol smoke layer was advected zonally by the prevailing easterly winds and its passage over Reunion was recorded by increased aerosol extinction profiles on 27th and 28th January. The analysis of the advected potential vorticity highlights an isentropic transport of air masses containing Australian biomass burning aerosol from extra-tropical latitudes to Reunion at the 400 K isentropic level, on 28th January. Interestingly, our results show that the biomass burning activity in eastern Africa, weak during this season, contributed to modulate (up to 90 %) the vertical distribution of CO and aerosols in the upper troposphere over the SWIO basin. The simultaneous presence of African and Australian aerosols smoke layers has been recorded by ground-based observations at Reunion. This study highlights for the first time the influence of the African emissions from biomass burning to the CO and aerosols distribution in the upper troposphere over the SWIO basin during the convective season. The results show that besides PyroCb -driven injection of biomass burning products to the stratosphere, an alternative pathway may exist during the regular deep-convection season in the tropics.
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RC1: 'Comment on egusphere-2023-1946', Anonymous Referee #1, 23 Oct 2023
Review of "Evidence of a dual African and Australian biomass burning influence on the vertical distribution of aerosol and carbon monoxide over the Southwest Indian Ocean basin in early 2020" by Bègue et al.
Overall comments
This is a solid piece of work that adds to the field of knowledge related to the exceptionally large atmospheric perturbations resulting from the 2020 Australian New Year's fires and other biomass burning events. The authors make good use of ground-based and spaceborne measurements, in conjunction with models, and provide useful information on the origins of pollution in the Southwest Indian Ocean basin during this period.
The work is scientifically strong, and well described by the text and figures in the paper. I'm happy to recommend it for publication pending some generally minor corrections (typically wording suggestions) and clarifications detailed below.
Specific comments
Page 2
Line 16: "modulate" -> "modulating". Insert "by" before "up to"?
Line 20: "aerosols" -> "aerosol"
Page 3
Line 1: Two commas in a row
Line 5: Perhaps add "typically" or "usually" before "considered"?
Line 30: "aerosols" -> "aerosol"
Page 4
Line 15/16: There were many more papers than those cited here that discussed. Suggest you add Schwartz et al., 2020 (doi:10.1029/2020gl090831), Santee et al., 2022 (doi:10.1029/2021gl096270), Solomon et al,. 2023 (doi:10.1038/s41586-022-05683-0) among others.
Page 5
Line 33: "Aerosols" -> "Aerosol"
Page 6
Line 18: "needed" -> "necessary"
Line 19: "The background profile" -> "This profile"
Page 10
Line 11: It looks like you give the URL for an older version of the MLS document, describing v4.2. Update
Line 19: I'm not sure about "physico-chemical" perhaps simply "microphysical and chemical"?
Page 11
Line 1: "on" -> "to"
Line 9: "taking" -> "taken". Also suggest deleting "Then"
Line 22: "discuss on" -> "evaluate"
Page 16
Line 31: "worthwhile" -> "worth"
Page 18:
Line 24: Add comma after "significant" and delete the "and" that follows. Also insert "The" or "A" before "Time-averaged"
Page 19
Line 7: Anomalies from what? Is it some kind of map, or is it a broader regional average. Specifically, if it's a map, does that mean that a region with an anomaly near zero could still be convective, reflecting a climatological situation? Why not just show OLR rather than anomalies?
Line 12: Why is this not shown? You have the space for it – it's a 2x2 figure with nothing in the bottom right corner.
Page 21
Line 17: Delete ", only"
Line 18: Delete comma after "fires)"
Line 19: Add "information" after "inventory"
Line 25: "A possible" -> "One possible"
Line 26: Delete "due to"
Line 27: Is "address" the right word? Might "test" be better? Also, in the remainder of this sentence, perhaps remind us what the original model approach had been.
Page 22
Line 3: "2" -> "two"
Line 7: "on" -> "to"
Line 10: "have" -> "has"
Line 27/28: Delete "One can observe that"
Line 28: Comma needed after "Africa"
Line 29: "21th" -> "21st"
Line 32: Perhaps add "a process" before "taken into account"
Page 23
Line 4: "to trigger" -> "for triggering"
Line 23: "modulate" -> "modulation of"
Line 25: "suggested" -> "suggests". Add comma after "that"
Page 24
Line 20: "by the funding of" -> "by funding from"
References: There is inconsistency between the citations here. Some have journals in italics, others do not. Also, Clain et al., is missing a journal, Hu et al., Ohneiser et al., and Pizzo et al., seem to be printed in a grey font.
Figures: I suggest you remove all the black boxes around multi-panel figures. They are overly distracting (and may not be allowed by the journal in any case.
Figure 1: The "axes" in 1a are way too thick. Fonts in 1b are way too small
Figure 2 caption:
Line 3: "on" -> "to"
Line 4: Add comma after "(2013)" and replace" that" with "which".
Line 5: "on" -> "of". Add comma after "properties". Change "Namely" to "Specifically"
Figure 3 caption, line 3: Insert "the" before "SKYNET"?
Figure 7: Overly tight/cramped layout. Make fonts in color bars bigger (perhaps only label every other level)
Figure 8: White points and black crosses too hard to see. Make them bigger?
Figure 12: Delete "in" in caption line 4
Figure 13, line 5: Call it "OMPS-LP", rather than just OMPS?
Citation: https://doi.org/10.5194/egusphere-2023-1946-RC1 -
AC1: 'Reply on RC1', Nelson Bègue, 16 Feb 2024
Dear all,
First of all, the authors acknowledge referee 1 and the editor for the time spent to review this manuscript and also for their constructive comments. Our responses are in the attached file (supplement). Futhermore, The modifications are indicated by red bold fonts in the revised manuscript.
Best regards,
Nelson BEGUE
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AC1: 'Reply on RC1', Nelson Bègue, 16 Feb 2024
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RC2: 'Comment on egusphere-2023-1946', Anonymous Referee #2, 05 Jan 2024
The study “Evidence of a dual African and Australian biomass burning influence on the vertical distribution of aerosol and carbon monoxide over the Southwest Indian Ocean basin in early 2020” by N. Bègue et al. presents ground and space based remote sensing measurements of biomass burning aerosol particles and CO after the severe Australian bush fires in 2019/2020. The long range transport of the bush fire plume to Reunion is shown. Lagrangian backward trajectories were then used to study the origin of the air masses with enhanced aerosol extinction and enhanced CO column density in the column over Reunion.
General comments
The major issue I have with this manuscript is that I don't know what is the key research question and finding of this study.
The first part of the manuscript (until page 16) presents measurements of the wildfire plume from 7 different instruments. Stratosphere, upper troposphere and lower stratosphere (UTLS), and troposphere are included in the data (Figs. 1-6), but it is not clear if there is a focus on a specific altitude range. The results are presented together with comparisons to literature, which make it difficult to distinguish between potentially new findings in this study and already existing knowledge from previous studies (Kablick et al., 2020; Khaykin et al., 2020, Kloss et al., 2021).The uncertainty in investigated altitude range becomes evident, in the comparison of the CO observations between 9-30 km in this study with Khaykin et al. (2020) analysing stratospheric CO at altitudes above the 380 K isentrope (p14 ll11-20). 380 K are certainly stratospheric, but as Fig. 2b in this study shows, 9 km is clearly tropospheric and CO has its highest concentration in the troposphere. So, about 90 % of the CO signal shown in Fig. 4b comes from the troposphere, especially at lower latitudes. This comparison is not meaningful. It would be better to either use the same altitude criterion or calculate the column above the (lapse rate) tropopause for each data point individually.
The second part of this manuscript (pages 16 to 22) employs backward trajectories to study the origin of the air masses in the column above Reunion. Unfortunately, it is not fully clear where and when the backward trajectories were started. However, the results (Figs. 7 & 9) clearly indicate that the air masses at different altitudes (stratosphere and upper troposphere) come from different directions. But this altitude dependency is not explicitly stated in the conclusions. For the stratospheric part (~400 K isentrope) the transport pathway was investigated, but for the tropospheric part no such analysis was performed.
The discussion section (Section 5) does not contain a discussion comparing the results of this study with literature, but it rather presents an analysis of the tropospheric part of the enhanced extinction over Reunion without explicitly stating it. Also, the altitude confusion becomes evident again. The tropopause over Reunion is at about 17 km, the lidar extinction is shown between 15-25 km (Fig. 6), but the analysed CO partial column goes down to 9 km (Figs 10, 12) and the stratospheric optical depth is shown for 15-30 km (Figs. 5, 13). There is no consistency between the data sets, which makes comparisons meaningless.
Moreover, the manuscript is hard to read and assess, as it is very long, wordy, repetitive, and does not follow ACP manuscript preparation guidelines. It contains one page long paragraphs (e.g. p3-4, p19-21), many small errors, and a large amount of references is missing in the reference list.
In my opinion, this study contains a lot of material that deserves publishing. But, I strongly recommend revising the manuscript. I'd suggest to focus on one scientific question and write down the interpretation of the material, instead of just presenting the material in a descriptive manner and leaving the interpretation to the readers. The focus could be on the observed biomass burning plume, aerosol and CO, over Reunion that has most likely two origins that are altitude dependent.
Specific comments and examples
Please note, I only provide examples, but like to ask the authors to fix all other occurrences in their manuscript also.
p1l5: Please make sure to have the correct and current affiliations of all Co-authors.
Please add the required author contributions section.
Please introduce all abbreviations first, e.g
p2l15: "BB" -> biomass burning (BB),
p11l11: GFAS
p19l26: "RMSC" (Regional Specialized Meteorological Center) -> Regional Specialized Meteorological Center (RSMC)
p22l17: BC and OCPlease revise the wording of “aerosols smoke layers” (p2l18). I'd suggest to use “smoke” or “biomass burning aerosol” and stick to it throughout the manuscript. “aerosols smoke layers” does not make sense, as smoke is a carbon containing aerosol. Please also check the use of the term “aerosols”. It is often wrong, i.e. plural instead of singular.
Many references are missing in the reference list, e.g.
p2 l29 (Bencherif et al., 2020; Garstang et al., 1996; Holanda et al., 2020),
l31 (Andreae and Merlet, 2001; Duflot et al., 2010,
l33 Dowdy and Pepler, 2018
p9 l22 De Mazière et al., 2017
There are no references to the data sets introduced in Section 2. Please provide references for the data sets including a DOI in the reference list, instead of a link to the data in the text. E.g. for CALIPSO data a DOI is available 10.5067/CALIOP/CALIPSO/CAL_LID_L1-Standard-V4-51 (https://asdc.larc.nasa.gov/project/CALIPSO/CAL_LID_L1-Standard-V4-51_V4-51).It is common standard to use past tense when describing what was done in this study, e.g.
p9 l10 “... are downloaded from the NDACC website ...” -> were downloaded
p16 l17: are -> were
p16 l18: is -> wasp9 ll5-20: Are the Lauder and Reunion lidars the same? Then one description for both instruments would be sufficient. Are the retrievals different for two sites in the same network, or are they the same? If they differ, please explain why and how both data sets are comparable then.
p10 ll26-32: The model and model setup descriptions are misleading and incorrect. Backward trajectories certainly do not consider dry and wet deposition as well as convection parameterization. Please describe only the processes that were used for the backward simulation. Also, the description states that backward trajectories were started over Reunion between 15 and 19 km every 0.5 km, but in the manuscript only the results for 18 and 16 km are shown and discussed. Did you release the trajectories at one latitude/longitude combination, if yes which, or did you distribute the 20000 particles over a certain area? Did you release the particles all at the same height level every 0.5 km, or did you distribute them around the height levels? Did the trajectories stop if they hit the ground, or did they bounce back?
p11 l31: Why did you drive the MIMOSA model with ECMWF meteorological analyses (which?) and not ERA5 as used with FLEXPART? Using the same meteorological input data would ensure consistency between different model simulations.
p12 l8-p13l2: Please split the nearly one page long paragraph.
p13 l1-2, Fig1, Fig2: The heights of the plume observed by CALIOP (15-18 km) and by the Lauder lidar and FTIR (5-12 km) do not agree. Please comment on that.
p13 l14: Why did you set the tropopause to 12 km? Figure 2 shows that there is variability in the tropopause height of ±1 km. For both the sAOD and the stratospheric CO column, this makes a significant difference.
p13 ll28-31, Fig. 4: Both are not comparable, because the columns range from 16 to 30 km for the aerosol and 9-30 km for the CO. CO is mostly confined to the troposphere and decreases significantly in the stratosphere. This means that Fig. 4 shows mainly tropospheric CO and stratospheric AOD. Hence, there is no correlation to be expected in the first place and many differences can be found. The difference in altitude also explains the differences described later on p14 ll15-18.
p14 ll15-18: Of course, you do not see the same plume as Khaykin et al. (2020). Khaykin et al. (2020) looked at heights above the 380K isentrope (~17 km) and the CO signal in Fig. 4b is from ~9-17 km.
p15 l27: Please explain what the different Ångström exponents mean.
p16 l3: What is the difference between fresh and aged smoke? Please describe what you mean and how you can tell from your data.
p16 ll8-14: That is quite a lot of speculation here. You could easily check if transport occurred at the Reunion lidar site by looking at the wind speeds (e.g. in ERA5) at the different altitudes.
p17 ll15-21: “Given the Australian BB aerosol are mainly located in the mid-latitudes (Fig. 4a), we can reasonably conclude that the filament reaching the SWIO basin contains aerosol from Australian BB event.” and “These results demonstrate that the unusual aerosol load observed in the UT-LS above SWIO is to be linked to the Australian fires.” I think both sentences are kind of redundant and this part should be rephrased to be more concise.
p18 l32-p19 l3: This is speculation. Either prove or remove. The fires are clearly north of the observed enhanced CO columns and the referenced studies (p19ll 3-5, which by the way also are not in the reference list) do not suggest latitude crossing diagonal convection.
p19 ll18-22: Instead of lengthy explanation, you should add the position of the ITCZ to Fig 10.
p20 l3: Which month did you use to calculate the “monthly background”? January 2020, or January 2019, or some multiannual mean?
p20 l9-19: This is an unnecessary lengthy and incomplete description of Fig. 11 (l 14 a value is missing “15 & to %”). It would be better to present the results and implications this figure has.
p33 l2: It says “FIGURES AND TABLE”, but there is no table. Also the axis labels are quite small in Fig. 1b.
p34: Why are there more days of measurement available in the FTIR measurements than in the lidar measurements? Was there a stricter cloud filter for the lidar data?
p35: Is there a typo in the y-axis label in Fig. 2a? 500 nm vs 532 nm
p39 and 41: The colour bar labels are too small to read.
Citation: https://doi.org/10.5194/egusphere-2023-1946-RC2 -
AC2: 'Reply on RC2', Nelson Bègue, 16 Feb 2024
Dear all,
First of all, the authors acknowledge referee 2 and the editor for the time spent to review this manuscript and also for their constructive comments. Our responses are in the attached file (supplement). The modifications are indicated by red bold fonts in the revised manuscript.
Best regards,
Nelson BEGUE
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AC2: 'Reply on RC2', Nelson Bègue, 16 Feb 2024
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