| 26 Feb 2026
A global view of the stratospheric background, volcanic and wildfire aerosol in the CALIOP era (2006–2023)
Abstract. This study deals with the stratospheric aerosol during the 17 years of lidar measurements with CALIOP. To obtain extinction from the backscattering measurements, we estimated the lidar ratios of the main aerosol injections into the stratosphere. The stratospheric background is estimated by making a subdivision of the stratosphere into nine parts, spanned by three latitude and altitude intervals, reaching background conditions individually at different times. The extracted background shows excellent agreement with solar occultation measurements in the volcanically quiescent period 1998–2000. Our results show that 70 % of the background aerosol in the deep Brewer-Dobson (dBD) branch is formed above 19 km altitude, indicating strong influence of carbonyl sulfide on the stratospheric background aerosol. The stratosphere was clearly affected by 15 volcanic eruptions and 5 wildfires. Their combined aerosol load affected Southern extratropics, tropics and Northern extratropics almost equally, and the altitude distribution shows that the shallow Brewer-Dobson branch was most affected (43 %) followed by the dBD (31 %) and lowermost stratosphere (26 %). The most important events in order of maximum AOD were the Hunga Ha’apai eruption (2022), Australian wildfires (2019-20) and the eruptions of Raikoke (2019), Sarychev (2009) and Nabro (2011). These events induced strong variability in the stratospheric aerosol optical depth (AOD), causing highly variable climate impact in the period studied with yearly average global effective radiative forcing ranging from -0.14 W/m2 at background conditions to -0.4 W/m2. CALIOP provided important data for stratospheric aerosol climatologies during its 17 years of operation.
Review of "A global view of the stratospheric background, volcanic and wildfire aerosol in the CALIOP era (2006 – 2023)" by Martinsson et al.
This paper deals with the stratospheric aerosol loading, both the background and perturbations from the volcanoes and wildfire events observed during the mission lifetime of spaceborne lidar CALIOP on-board the CALIPSO satellite. It attempts to delineate the background aerosol in nine different parts of the stratosphere on the assumption that the stratospheric background levels can exist at different levels and time. After subtracting this background, the authors discuss the strong perturbations from several volcanic events and wildfire events, for which they derive the lidar ratio and obtain the AOD. They highlight the importance of space lidars like CALIOP to characterize the stratospheric aerosol burden, in relation to the solar occultation and limb scatter instruments. The paper is within the scope of ACP and generally well written. However, in several places the information is generally well known, in particular the overall impacts of the volcanic and wildfire events in recent years. I am a little unsure of the motivations for this work, particularly because a new level 3 CALIOP stratospheric aerosol product with several updates is currently available--perhaps the authors could better emphasize the new information from this study. In any case, the paper presents an independent assessment of CALIOP stratospheric measurements and I recommend publication of the manuscript. I have a few comments to improve the paper in no particular order.
1. The authors have used version 4.51 level 1 CALIOP backscatter measurements for their study. Why not use the latest version 5 data product, although the results presented here should not change much. Also, the version 2.0 level 3 stratospheric aerosol product as available in the CALIPSO database incorporates the schemes to address the low energy laser shots which have been impacting the measurements after 2017 during which time several of the strong volcanic and wildfire events discussed in this paper occurred. It is not clear to me if the authors addressed the low energy shots in some way. I wonder how their background component compares with the official product.
2. line 34--can the seasonally present and anthropogenically sourced ATAL be considered as background aerosol?
3. line 41--suggest adding more recent references from the works of Fromm and co-authors.
4. line 66--suggest adding SAGE II/III references. Also please spell out the acronym GloSSAC and OSIRIS in line 72.
5. line 101-- Add "respectively" after " ...300m"
6. Line 104.--Add MERRA 2 reference
7. The methods section (2.1) needs to be expanded a little bit. For instance, in line 109 the authors mention the depolarization of the signal, without first mentioning the measurements in the perpendicular channel. Please add a few sentences on the perpendicular channel at 532 nm and 1064 channel in CALIOP measurements. What was the threshold used for cloud depolarization? Since the authors do not attempt any validation, it is not clear as to how much clouds might impact their lowermost stratosphere results.
8. Line 158 and later--what is meant by "beside"--is it the top of the layer?
9. Figure 5--May be point out that the vertical scales for the different regions are different--it's a little difficult to read all 9 panels in this plot.
10. Line 380 "the order 100 days"--please rephrase
11. I think the authors should discuss the fidelity of their lidar ratio estimates presented, which impact the AOD calculations crucially. I didn't find any comparison with estimates by other authors e.g. Prata et al. (2017) gave 69 sr for PuyeHue Cordon Caulle, much more than ~55 sr shown in Figure 7a. The lidar ratios for the Australian fires of January 2020 were ~ 100 sr as retrieved by a Raman lidar in Punta Arenas (Ohneiser et al., 2020, https://doi.org/10.5194/acp-20-8003-2020), again much higher than estimated here (need to define Fi20 in Figure 2). Any clues as to why the Raikoke lidar ratio is distinctly lower (~45 sr) than others? In particular the latter might be relevant to the recent debate about presence of smoke in Raikoke plumes (Ohneiser et al., 2021, https://doi.org/10.5194/acp-21-15783-2021 etc.). Are some of these differences coming from the multiple scattering factor? Figure 7a shows the Effective Lidar Ratios whereas Figure 7d shows Lidar Ratios--may be I am missing something here.
12. Line 479: "unverified assumption of a lidar ratio of 50 sr"-- in a recent paper in JGR, Deshler and Kalnajs, 2026, https://doi.org/10.1029/2025JD045262, from decades long OPC measurements provide a single value for the stratospheric aerosol lidar ratio of 49.9 sr at 532 nm.
13. Add unit of extinction coefficient to the color bars in Figure S4-S6.