the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2026
Monitoring Stratospheric Aerosols over Europe: A 16-Year CALIPSO Dataset Analysis (2007–2022)
Abstract. Stratospheric aerosols exert strong radiative and dynamical impacts due to their long residence times and efficient transport. Using CALIPSO Level 2 version 4.51 profiles, we develop a 16-year (2007–2022) climatology of stratospheric aerosol occurrence, vertical structure, subtype composition, and optical depth over Europe, analyzing nighttime overpasses across 12 subregions. The record captures the dominant influence of major volcanic eruptions (Okmok and Kasatochi 2008, Sarychev Peak 2009, Grímsvötn and Nabro 2011, Raikoke 2019) and pyrocumulonimbus-driven wildfire events (e.g., Western Canada 2007, Pacific Northwest Event (PNW) 2017, Siberia 2019–2022). These episodic injections drive strong interannual variability, with annual mean nighttime stratospheric aerosol optical depth (sAOD) ranging from 0.032 in 2007 to a peak of 0.067 in 2017. Vertical distributions broaden over time, with frequent detections reaching heights of 17–19 km and extreme cases up to ~29.8 km, especially following Raikoke’s eruption and major Siberian fire seasons. Subtype analysis reveals sulfate dominance in northern regions and smoke-driven anomalies during major wildfire years, while southern regions show larger unclassified fractions. While no monotonic trend is found, analysis reveals a change-point after 2016, marking a transition to a higher-loading regime. These results highlight the strongly event-driven nature of European stratospheric aerosol variability and CALIPSO’s key role in long term monitoring of volcanic and wildfire impacts on the lower stratosphere.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2307', Larry Thomason, 30 May 2026
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RC2: 'Comment on egusphere-2026-2307', Anonymous Referee #2, 31 May 2026
The manuscript by Papanikolau and coauthors presents a 16-year analysis of CALIOP Level 2 observations of stratospheric aerosols over Europe, focusing on aerosol occurrence, vertical distribution, subtype classification and stratospheric aerosol optical depth. The topic is relevant and timely. Stratospheric aerosols produced by volcanic eruptions and wildfire-driven pyroCb events have important radiative, chemical and dynamical impacts, and the CALIOP record provides a long-term vertically resolved dataset for documenting their evolution. Europe is also an interesting receptor region for long-range transport of volcanic and wildfire plumes and benefits from complementary ground-based lidar observations within a dense European lidar network.
However, I find that the manuscript requires major revision before publication. My main concern is that the regional subdivision of Europe into 12 boxes, especially the four longitudinal sectors within each latitude band, is not sufficiently justified physically. The main aerosol sources considered in the study — Aleutian/Kuril volcanic eruptions, Nabro, Raikoke as well as North American and Siberian wildfires — are located thousands of kilometers away from Europe. By the time these plumes reach Europe in the lower stratosphere, their transport is largely controlled by large-scale quasi-zonal circulation. One would therefore not expect physically meaningful changes in aerosol optical properties from one European longitude sector to the next within the same latitude band.
Apparent differences between these longitude sectors may instead reflect CALIPSO sampling geometry, the timing of individual overpasses, small-number statistics, or the arbitrary intersection of filamentary plumes with the satellite track. The manuscript briefly acknowledges that regional variations may be affected by the intersection of plume transport and satellite orbital geometry, but this caveat is not sufficiently reflected in the interpretation. Several statements about “regional transport pathways”, “localized variations”, “longitudinal fingerprints” or east–west differences appear overinterpreted in the absence of trajectory analysis, event-resolved plume tracking, or statistical evidence that such longitudinal differences are robust.
As far as the latitudinal subdivision is concerned, differences between northern, central and southern Europe may be physically meaningful, given the influence of high-latitude sources, the Brewer–Dobson circulation and the latitudinal structure of transport. However, the physical meaning of the longitudinal breakdown is not demonstrated. I recommend that the authors either provide a much stronger justification for this subdivision, supported by transport diagnostics or event-based analysis, or simplify the analysis by aggregating the data zonally within latitude bands.
My second concern is that many conclusions drawn from the regional analysis are overinterpreted relative to the information content of the figures. The manuscript often reads as a descriptive walkthrough of multi-panel figures rather than a physically supported analysis. In several places, apparent differences between subregions are attributed to specific volcanic or wildfire sources without sufficient evidence. The attribution of European sAOD enhancements to “geographical proximity” of remote eruptions is particularly problematic, since stratospheric source-receptor relationships depend on injection altitude, season, isentropic transport or diabatic self-lofting, mixing and the timing of satellite sampling, not simply on geographical distance.
The use of occurrence counts also needs clarification. The manuscript refers to “profiles containing stratospheric aerosols”, but background stratospheric aerosol is always present. The authors should specify whether they mean CALIOP-detected stratospheric aerosol features above the detection threshold, enhanced aerosol layers, or another quantity. This distinction is important because much of the discussion relies on counts of stratospheric aerosol detections.
Finally, the change-point and trend discussion should be toned down. The record is strongly event-driven, and the post-2016 period includes several major episodic perturbations, including the 2017 PNE wildfire event, Raikoke in 2019, and later North American and Siberian fire seasons. This clustering of events may produce an apparent step-like change without implying a new persistent background regime. The authors should clearly distinguish between temporary event-driven enhancement and a real change in the baseline state of European stratospheric aerosol.
Specific comments.
l.16 To the best of my knowledge, neither Raikoke nor Siberian plumes have been reported to rise above 29 km.
l.94–95: Please clarify how sAOD is calculated, including the vertical integration limits, the tropopause product used, and the effect of tropopause uncertainty.
l.100–105: The boxes are 10° × 10° in latitude/longitude, but their physical size varies with latitude. Please clarify whether averaging is area-weighted or sampling-weighted.
l.127–132: The limitations of CALIOP subtype classification should be discussed more fully, especially for mixed or aged aerosol layers.
l.234–237: Please define “profiles containing stratospheric aerosols”. Background stratospheric aerosol is generally present, so the detection criterion needs to be clear.
l.253–255: The claim that vertical broadening is largely driven by increasing pyroCb frequency should be supported quantitatively.
l.263–266: The statement that changes in C-4 “directly” reflect Siberian smoke transport is too strong without event-specific transport evidence.
l.295–299: The interpretation of the north–south gradient in unclassified aerosol fraction as aging or mixing is plausible but speculative. Please support it with optical-property diagnostics or phrase it more cautiously.
l.306–317: The attribution of sAOD enhancements to the geographical proximity of eruptions is oversimplified for stratospheric transport.
l.326–331: The claim that northern regions are higher because they are closer to sources and transport pathways needs stronger dynamical support.
l.346–358: The claimed “distinct longitudinal fingerprints” and eastward intensification are not convincing. Please demonstrate their robustness or avoid interpreting them as transport signatures.
l.354–358: The caveat about satellite orbital geometry is important and should be expanded, as it may explain much of the apparent regional variability.
l.440–455 and Fig. 12: The trend is not statistically significant and should be discussed more cautiously.
l.500–506: The interpretation of a post-2016 change-point as a change in aerosol baseline is too strong. It may simply reflect clustering of episodic events.
Citation: https://doi.org/10.5194/egusphere-2026-2307-RC2
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