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 OC

Please 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 -> was

p9 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.