Review of “Stratospheric impact of the anomalous 2023 Canadian wildfires: the two vertical pathways of smoke” by Khaykin et al.

The manuscript focuses on the extreme wildfire season in 2023 in Canada. It is highlighted that smoke is able to reach the upper troposphere and lower stratosphere in the absence of pyroCb convection if it is injected into a warm conveyor belt. This result is interesting and promising, however, a few general and technical comments follow here:

General comments:

Line 100: Maybe it would be good to change the order of 2.4 TROPOMI and 2.5 OMPS Limb Profiler in order to have OMPS Nadir Mapper and OMPS Limb Profiler in a row.

Line 135: How do you justify the thresholds: “0.01 for SAOD and 8 for ER”

Line 238: North America or Canada?

Line 252: Is the threshold of AAI > 15 arbitrarily chosen?

Table 1: WCB used as acronym but only defined in line 391

Fig. 1: Why starting at different years 2003, 2013, 2012 for A, B, C and not in the same year for all?

Fig. 4: legend caption: kg m³ there is a minus missing before the “3”

Fig. 4 and lines 457-463: The events 3, 4, 5, 6 all bring parts of the smoke into the lower stratosphere. You write that the lofting of smoke is meteorologically driven in the WCB and that the diabatic lofting plays a minor role. I understand that vertical transport of smoke in the WCB towards the tropopause is predominantly meteorologically driven. But how can you be sure that the smoke transport through the barrier of the tropopause would happen if it is only meteorologically driven? How would this barrier be passed even in a WCB? How can you exclude that at this point diabatic lofting might dominate to come across the barrier as diabatic lofting plays a big role in the stratosphere? And could differences in the absorptivity of the smoke compared to the Australian wildfire smoke in 2020 explain differences in diabatic lofting behavior? Please discuss this in your manuscript in more detail. Another question that just comes to my mind: Did you also find smoke transport towards the UTLS (within the tropopshere) if the smoke was not within a WCB (or pyroCb) before?

Line 688-690: Too general. In France? Or at that station? Or for Canadian smoke? How can you be sure it was a new record? At least the Australian 2020 smoke had a higher AOD for single layers in the stratosphere.

General: Maybe it would be a good idea to include a schematic figure comparing pyroCb and WCB vertical pathways, showing uplift speed, plume structure, and evolution over time.

General: It is good to see that the model could show the lofting of the aerosol in the WCB. It is good to see that the lidar profile shows an AOT of around 1 with a thick smoke plume in the stratosphere. But do you have any case where you also see the observational evidence that the smoke plume does not originate from a pyroCb but was injected at around 2km height in Canada and was later found at a significantly higher altitude? The manuscript would benefit from it.

Technical comments:

Line 37: Empty space missing after „precipitation”

Line 39: Bracket opened but not closed

Line 56: Which year of the study Peterson et al. ? (“n.d.”)

Line 60: Bracket opened but not closed

Line 62: Commas around “however” missing

Line 62: Brackets around Zhang et al need to be removed

Line 78: Brackets around Fromm et al need to be removed

Line 106: Empty space before “OMPS” missing

Line 109: Point missing at the end of the sentence

Line 134: Bracket opened but not closed

Line 168: remove brackets around Guth et al…

Line 184: remove brackets around El Amraoui et al., 2022; Sič et al., 2015

Line 197: remove brackets around (Nédélec et al., 2015)

Line 198: remove brackets around (Blot et al., 2021)

Line 220: remove brackets around Khaykin et al., 2017

Line 392: lofting air instead of lifting

Line 436: these instead of this

Line 505: remove brackets once around Yu et al.

Line 576: 23 UTC, not 23 h UTC

The manuscript provides an in-depth examination of the unprecedented wildfire season of 2023, emphasizing how smoke plumes from Canada and Siberia affected the upper troposphere and lower stratosphere. A key contribution is the distinction between two transport mechanisms: the direct, explosive injection through pyrocumulonimbus (PyroCb) events, and the less reported but significant role of warm conveyor belts (WCB), which can elevate smoke to the lower stratosphere. The authors employ a solid event-by-event classification supported by diverse observational datasets, making the analysis both comprehensive and convincing. This dual-pathway perspective is particularly relevant given the growing frequency of wildfires under climate change and their implications for atmospheric chemistry and climate forcing. The findings are very interesting although some aspects warrant further clarification. Overall, the work makes a very valuable contribution and is recommended for publication after minor revisions.

General comments:

Section 2.2: I would suggest to add in the supplementary material some illustration of a PyroCb as identified from brightness temperature.

Section 2.5: Please provide more details about OMPS-LP NASA v2 uncertainty which depends on the wavelength as explained in Taha et al. (2021). The choice of the 869 nm channel can be explained.

Line 150: I am surprised that the Shiveluch volcanic eruption in 2023 was significant enough to impact the stratospheric aerosol content on a seasonal scale. From OMPS-LP observations, my view is that this signal was unclear at the hemispheric scale and drowned by the (slight) propagation of the Hunga plume to the NH extratropical latitudes. I would suggest to tone down the statement about the impact of this eruption.

Line 176: could the authors provide a reference or website link to CAMS inventory?

Section 3.1 and figure 1: Fire cumulative energy is increasing steadily throughout the season while pyroCb cumulative counts show different evolution with for instance stable values (around 130) in August 2023. What is the reason for such difference? Is it because fire cumulative energy also accounts for fires without pyroCb formation?

Section 3.2: I do not clearly grasp how the authors find pyroCb cases organizing as clusters. I guess this is done from geostationary imaging but the reader does not have any illustration or explanation. Does it correspond to pyroCb occurring at the same time nearby or at far distances? Also, how do the authors estimate a “measurable stratospheric impact” from 7 specific events? It is not obvious to derive a number of 7 events reaching at least ~10 km in altitude in Figure 2A and 2B since, as stated in the manuscript, a given plume can survive transport more than twice around the globe and other plumes, freshly emitted, can complexify the readability of Figure 2. To clarify these points, perhaps an additional figure with brightness temperature (BT) could be helpful.

Section 3.3: this is not a major issue but from Figure 3, the model seems to mostly underestimate the altitude of the aerosol layer peak. This is a bit surprising since the timing of transport and the overall horizontal distribution of the plume is rather well reproduced by the model. Some improvement can be seen approaching 17/08. Is this due to inadequate injection height (constrained to 2 km everywhere) and/or issues regarding vertical transport of particles (e.g. resulting from WCB representation in ARPEGE). Did the authors test higher injection heights with some information taken from BT? Another possibility could be radiative lofting process of optically-absorbing smoke aerosols which is not computed in the model, although the uplift rate through this mechanism is expected to be lower.

Lines 393-397: Some of the information about MOCAGE given here is redundant with Section 2.9. Please simplify accordingly.

In section 3.4, the authors propose that differences in aerosol concentrations can be the reason for differences in plume altitudes and time evolution for Pyrocb versus WCB. Could we have any comparison with large reported events (ANYSO and PNE events) in term of ER to emphasize the lower amplitude of the 2023 episodes? Also, the idea of the authors is plausible but no mention of any role of aerosol microphysics is provided. Bigger particles would give higher ER but would not match the lower ER for WCB. In this case, coalescence could increase the sizes of the particles but would reduce concentrations. Could MOCAGE provide some indications about aerosol microphysical evolution (I see there are 6 bins in the microphysical module)? We have a few information in the literature on smoke particle sizes from Pyrocbs. From the FIREX-AQ data (see Peterson et al. BAMS, 2022), aerosol and cloud particle sizes within a pyroCb are not static which could explain the differences the Alberta and Siberia cases. They evolve vertically due to microphysical processes (freezing, coalescence) and temporally as the plume ages. The efficiency of the pyroCb as a smoke injection mechanism could be intrinsically linked to these particle sizes through the precipitation scavenging feedback loop. Larger particles promote scavenging and less efficient transport, while smaller particles, fostered by intense updrafts, facilitate massive injection of smoke into the stratosphere. I suggest these elements to be clarified (or ruled out) in the text in a few sentences.

Line 518: please provide the date of the case discussed here.

Technical errors:

Line 33: correct to Salawitch and McBride, 2022

Line 114, 116: Choose the acronym AAI instead of AI for homogeneity throughout the manuscript.

Line 56 (and in other locations in the manuscript): Peterson et al., n.d. Do you have the year of this publication? Is it in open access? I cannot find the reference.

Line 126: SALD acronym is supposed to be for Stratospheric Aerosol Layer Detection and is defined in the title of section 2.6 which is enough to me.

Line 137: add the bracket after “maximum ER”

Line 149: Add “stratosphere” after global.

Lines 174-175: correct to Bechtold et al. (2001) and Louis (1979).

Line 203: add “s” to “wavelength”.

Section 2.12 title: define acronym for LTA

Figure 2: use the term AAI instead of AI in the figure axis title of Figure 2C. Within the figure, individual pyroCb events are not marked as a small but as an open triangle conversely to what is indicated in the caption.

Line 423: correct to “analyzed”

Line 518: the date of the first event from Figure 6A is missing in the text.

Line 521: correct to “a SCV-like”

This manuscript presents a detailed analysis of the 2023 wildfire season in Canada. The author discuss and compare two different mechanism that can uplift smoke particles in the upper troposphere/lower stratosphere: pyroCb-driven injection and lofting within a warm conveyor-belt-type circulation (WCB). Observations and methods are well presented. The analysis of the results and their discussion is detailed and comprehensive. Some minor revisions would be beneficial and clarify the discussion in parts of the text.

The authors argue that during the Canada wildfire season in 2023, uplifting of smoke into the lower stratosphere was mainly caused by a WCB mechanism rather than by diabatic heating (and self-lofting) of air masses laden with smoke particles. However, is pyroCb activity still required for the WCB mechanism to effectively raise particles into the lower stratosphere? In the meteorological WCB circulation over the region, what was the lowest altitude at which smoke particles could have entered the conveyor belt? Were smoke plumes not associated with pyroCb activity high enough?

If diabatic heating is not at all necessary in the WCB mechanism, in order to breach through the upper troposphere smoke particles need to be carried by the WCB circulation. Does the WCB circulation always penetrates the lower stratosphere? If so, some discussion on this point is necessary. If not, at what altitudes were smoke particles delivered and how did they cross into the lower stratosphere?

If the WCB mechanism can efficiently lift smoke particles into the lower stratosphere, it should be able to do the same with other particulate species. Is there any evidence of non-black/brown carbon aerosols carried into the lower stratosphere by this process?

The authors argue that lifting timescales differ between diabatic self-lofting and WCB-driven injection of stratospheric smoke. Are the altitudes of injection also different? Would a stratospheric smoke layer delivered by WCB be lower than one delivered through diabatic self-lofting? If so, could this difference be used to constrain the lifting process?

Minor corrections:

A number of citations have extra parentheses. For example, on line 20 "(e.g. (Khaykin et al., 2020; Peterson et al., 2021)." Other occurrences appear throughout the text.

On line 300: "...were linked respectfully to..." ->  "...were linked respectively to..."?

On line 661: "...in terms of the emitted power" should be "...in terms of the emitted energy", since the quantity is a "TW h".