Detailed descriptions of the EarthCARE mission and the ATLID instrument are provided by and ; here we summarise only the characteristics relevant to the present study. EarthCARE orbits at approximately 393 km altitude in a sun-synchronous orbit with a 25 d repeat cycle. ATLID provides vertically resolved profiles of aerosol scattering ratio, backscatter, extinction, and linear depolarisation ratio with a vertical resolution of approximately 100 m below 20 km and 500 m above, and a nominal along-track horizontal resolution of approximately 285 m for single shots. For all ATLID products used in this study, we relied on the EarthCARE processing baselines BA, BB and BC, i.e. successive versions of the ESA ground-processing chain. Baselines are identified by two letters: the first denotes a major processing/reprocessing generation, and changes in the second indicate incremental updates. BA corresponds to the first homogeneous reprocessing of the EarthCARE data record with the B-generation processors, while BB and BC contain subsequent updates .

The official ATLID Level-2 (L2) product provides aerosol extinction profiles derived from the HSRL . However, as suggested by (hereafter K26) and confirmed in this study, the L2 processor frequently returns zero extinction under low-aerosol background conditions, in the absence of a sufficiently strong aerosol signal. For optically thicker layers such as volcanic plumes, the L2 algorithm does return non-zero extinction, but it overestimates the peak extinction (Sect. ), and the resulting sAOD is only weakly correlated with the reference measurements (Sect. ). To overcome this limitation and enable a systematic comparison across all aerosol regimes, we derive ATLID aerosol extinction directly from the Level-1 scattering ratio (SR) using the K26 method. This method retrieves the unattenuated particulate backscatter coefficient βaer from the molecular reference profile in three steps. First, the HSRL design of ATLID separates the particulate (Mie) and molecular (Rayleigh) contributions to the backscattered signal; since both are attenuated by the same two-way transmission, the ratio of the particulate attenuated backscatter (sum of the co-polar and cross-polar Mie signals of the L1 product) to the Rayleigh attenuated backscatter is free of attenuation and equals the ratio βaer/βmol of the unattenuated particulate and molecular backscatter coefficients. Second, the molecular backscatter coefficient βmol is computed for Rayleigh scattering at 355 nm from the ECMWF pressure and temperature profiles provided with the Level-2 data, interpolated onto the ATLID vertical grid. Third, βaer is obtained by multiplying the attenuation-free ratio by βmol, i.e. βaer = (βMie′/βRay′)βmol. The aerosol extinction coefficient αaer is then obtained as: 1αaer(z)=Saer⋅βaer(z), where Saer is the lidar ratio. Because this derivation relies entirely on non-attenuated inputs derived from the molecular signal, the resulting extinction profiles are free from cumulative attenuation errors inherent to elastic-backscatter lidar inversions, though they remain sensitive to the lidar ratio.

The aerosol extinction coefficient is derived from the ATLID L1 particulate backscatter coefficient using Eq. (). Wherever the ATLID L2 lidar ratio is available and falls within physically realistic bounds (30–90 sr), it is used directly in the retrieval. In all other cases, including levels where the L2 processor returns zero extinction (no aerosol detected) or unphysical lidar ratios, a constant fallback value of 50 sr is applied, consistent with values reported in the literature for aged stratospheric sulfate aerosols (see Sect.  for the 355 nm case). The L2 lidar ratio therefore enters the L1-derived extinction only at the levels where it is available and lies between 30 and 90 sr, so issues affecting the L2 retrieval can affect the L1-derived product only at these levels, and only through the lidar ratio, not through the L2 extinction itself. Since the extinction is proportional to the lidar ratio, a relative error on the lidar ratio gives the same relative error on the extinction; with the 30–90 sr bounds, an erroneous L2 value can change the extinction by -40 % to +80 % relative to the 50 sr value at the levels concerned. The backscatter comparison with the ground-based lidars (Sect. ) does not involve any lidar ratio and shows the same negative bias just above the tropopause, which indicates that the main features of the comparison are not driven by the L2 lidar ratio. The sensitivity of the retrieved extinction to the lidar ratio is illustrated for the case studies of Fig.  and discussed in Sect. .

Individual ATLID Level-1 profiles within each collocation window (see Sect. ) are averaged to reduce random noise, and a 3-point boxcar smoothing is applied in the vertical dimension to the resulting mean profile. This smoothing improves the signal-to-noise ratio at stratospheric altitudes without significantly degrading the vertical resolution relevant to the features of interest.

The Stratospheric Aerosol and Gas Experiment III aboard the International Space Station (SAGE III/ISS) has been operational since June 2017 . It measures solar and lunar occultation spectra to retrieve vertically resolved profiles of aerosol extinction, ozone, water vapour, and other trace gases . The solar occultation technique provides self-calibrating, high-accuracy measurements with a vertical resolution of approximately 0.5 km. SAGE III/ISS delivers approximately 30–40 solar occultation profiles per day, distributed across two latitude bands that precess over time due to the ISS orbital inclination.

In this study, we use SAGE III/ISS Version 6 aerosol extinction profiles at 355 nm, derived from the 384 nm channel and converted to 355 nm using the Ångström exponent computed from the 384–521 nm wavelength pair, for the period August 2024–April 2026. Prior to use, profiles were screened following the quality-control (QC) recommendations provided with the Level-2 product. Occultations affected by event-level anomalies, including pointing, blockage, timing, thermal-control, or ephemeris issues, were excluded, as were altitude bins flagged for signal disturbance or interpolation artefacts. For the aerosol comparisons, only measurements classified as background, perturbed, or enhanced aerosol by the SAGE aerosol classification flag were retained; profiles flagged as transmission anomalies, aerosol–cloud mixtures, or polar stratospheric clouds were discarded. Finally, only positive extinction values with a relative uncertainty below 50 % were retained in the point-by-point comparison with ATLID. SAGE III/ISS constitutes the primary reference instrument for this validation study owing to its high measurement accuracy, self-calibrating nature, and the large number of collocations achievable with ATLID over the study period.

ATLID and SAGE III/ISS profiles are collocated using the following criteria: a maximum spatial separation of 50 km, computed using the haversine formula between the SAGE III occultation tangent point and each individual ATLID footprint, and a maximum temporal difference of 12 h between the SAGE III occultation time and the ATLID overpass. For each SAGE III occultation satisfying these criteria, all ATLID Level-1 profiles falling within the collocation window are averaged together, and the resulting mean profile is interpolated onto the SAGE III vertical grid with 500 m step. In the individual comparison figures, the distance (Dist) is the great-circle distance between the SAGE III/ISS tangent point and the closest of the averaged ATLID profiles, all of which lie within 50 km of the tangent point, and the time difference (Δt) is computed between the SAGE III/ISS event and each collocated ATLID overpass.

For ground-based lidar collocations, the same temporal criterion of 12 h is applied, with the spatial criterion adapted to the fixed station geometry: all ATLID overpasses occurring within a great-circle distance of 100 km from each lidar station are considered.

The agreement between ATLID and reference instruments is quantified using the metrics in Table , computed for the full collocation dataset.

Over the study period from August 2024 to April 2026, the ATLID-SAGE III/ISS collocation procedure described in Sect.  produced a total of 895 co-located profile pairs, providing a statistically robust sample for the evaluation of ATLID performance across diverse atmospheric conditions. The number of co-located overpasses with the ground-based lidar stations is considerably smaller, reflecting the fixed-station geometry and the 25 d repeat cycle of EarthCARE: 55 events at TMF and 26 at OHP.

The comparison between ATLID L1 and SAGE III/ISS extinction coefficients is presented in Fig. . Figure a displays all collocated altitude-level pairs as a 2D density histogram, in which the colour scale encodes the local sample count on a logarithmic scale. The bulk of the distribution is concentrated in the range 0–2 Mm⁻¹, consistent with the dominance of near-background aerosol conditions in the dataset (Sect. ). A linear regression through the full sample yields a slope of 0.97 and a correlation coefficient of R = 0.79, indicating a reasonably strong point-to-point correlation with a slight systematic underestimation of SAGE III/ISS extinction by ATLID L1 (∼ 3 %). The scatter is substantial: a significant population of points lies well above the 1:1 line at low extinction (SAGE III/ISS < 2 Mm⁻¹). This is expected from the random noise of the ATLID L1 extinction, which comes from the detection noise of the ATLID channels (in particular the Mie channels, whose signal is weak in the stratosphere) and whose absolute magnitude does not depend on the aerosol loading: a random error of roughly constant absolute magnitude produces a relative scatter that increases as the extinction decreases.

Figure b presents the distribution of the relative difference (αL1-αSAGE)/αSAGE as a function of altitude relative to the local tropopause, z-ztrop (ATLID tropopause height; only levels at least 1 km above the tropopause are retained, consistent with the sAOD integration), for the full set of N = 895 collocated profiles (see Sect. ). The overall median relative difference is -2.2 %, confirming the near-zero systematic bias suggested by the regression slope in Fig. a and consistent with the near-unity sAOD regression reported in Sect. . The spread is large at all altitudes, with the distribution spanning approximately ±25 % between the 10th and 90th percentiles, reflecting the dominant contribution of retrieval noise to individual altitude-level comparisons under background conditions.

The profile of the median bias (solid black line in Fig. b) reveals a distinct vertical structure. In the first ∼ 3 km above the tropopause, the median is negative (about -10 % to -15 %). Between about 5 and 15 km above the tropopause, it is close to zero, and it becomes negative again further up. Expressing the altitude relative to the local tropopause avoids mixing profiles with different tropopause heights; in geometric altitude, a small number of profiles with a low tropopause produced an apparent positive bias at 10–13 km. The small overall median of -2.2 % therefore hides an altitude-dependent structure, discussed further in Sect. , which is more physically meaningful than the single-number summary statistic.

Figure  compares the ATLID L1 sAOD with the reference sAOD from SAGE III/ISS and the available ground-based lidar stations (TMF, OHP), integrated from the ATLID tropopause +1 to 30 km. For SAGE III/ISS, the sAOD is computed by integrating the extinction profiles over the same altitude range; the stratospheric_aerosol_optical_depth parameter of the SAGE III/ISS Level-2 product is not used. Reference sAOD values span approximately 0–0.030 for the SAGE III/ISS matchups, covering background conditions, the volcanic perturbation associated with the Ruang eruption (April–May 2024), and the wildfire smoke events of boreal summer 2025.

A linear regression applied to the SAGE III/ISS matchups yields a slope of 1.04 ± 0.07, an intercept of (-0.8±0.9)×10-3 (R = 0.67), indicating that the ATLID L1 product is, on average, essentially unbiased with respect to SAGE III/ISS integrated sAOD. However, the scatter around the regression line is not uniform across the dynamic range and reveals contrasting behaviour between the low- and high-aerosol-loading regimes. While the dense core of the data remains broadly consistent with the 1:1 line, the high-loading regime (reference sAOD > 0.010) exhibits an asymmetric dispersion. Specifically, there is a prominent subset of ATLID L1 retrievals that yield significantly higher values than the SAGE III/ISS reference, creating a pronounced positive tail in the distribution.

The class medians of Fig.  (white circles) show how the agreement varies with the aerosol loading. Above about 0.02, few matchups remain and the interquartile range widens.

Two factors may contribute to the positive tail at high aerosol loading. (i) Coverage of the lowermost stratosphere: both sAODs are integrated from the tropopause +1 to 30 km, but in dense layers the lowest SAGE III/ISS levels are more likely to be removed by the quality screening (e.g. aerosol-cloud mixture flag) or not retrieved at all, as the transmission along the line of sight decreases, whereas the ATLID L1 profile retains these levels. When a dense layer lies just above the tropopause, as in Fig. c, where ATLID shows large extinction values between the tropopause and 12.5 km that are absent from the SAGE III/ISS profile, the ATLID sAOD therefore includes a contribution that is missing from the SAGE III/ISS sAOD. (ii) Sampling geometry: SAGE III/ISS integrates along a limb path of a few hundred kilometres, whereas the ATLID extinction is an average of nadir profiles within 50 km. For compact plumes, ATLID can sample the core of the plume while SAGE III/ISS sees it diluted along its line of sight, which produces large positive differences; in the opposite case, the negative difference is limited by this dilution. This favours a positive tail at high aerosol loading.

Among the ground-based reference instruments, the TMF matchups (orange triangles) scatter on both sides of the 1:1 line without a clear directional offset, broadly mirroring the noise-dominated behaviour of the SAGE III/ISS ensemble at comparable sAOD levels. The OHP matchups (green squares) follow the same qualitative low-/high-loading pattern, with a tendency for points to fall below the 1:1 line as the reference sAOD increases; more matchups lie below than above the regression line overall, foreshadowing the systematic negative offset identified independently for this station from the point-to-point backscatter comparison in Sect. . For these sAOD comparisons, the ground-based backscatter profiles are converted to extinction with a lidar ratio of 50 sr (Sect. ), while the ATLID L2 lidar ratio is used only where it lies within 30–90 sr and 50 sr is used elsewhere, so the two conversions differ only at these levels. The backscatter comparison of Sect.  is independent of the lidar ratio.

Figure  presents the relative sAOD difference (sAODL1-sAODref)/sAODref as a function of latitude (panel a) and time (panel b) for the SAGE III/ISS, TMF, and OHP matchups. Across all matchups, the relative sAOD bias has a global mean of -3.2 % with a standard deviation of 65.7 %, indicating a near-zero systematic offset combined with a large point-to-point scatter.

The latitudinal distribution (Fig. a) reveals a clear meridional gradient in the SAGE III/ISS-based comparisons, with ATLID L1 exhibiting a systematic negative bias in the Northern Hemisphere extratropics that increases with latitude. The grey shaded envelope around the dashed line represents the ±2 standard-error (±2 SE) interval of the binned median, i.e. an approximate 95 % confidence range in each bin; it is narrowest where collocations are dense (mid-latitudes) and broadest where they are sparse, in particular in the Northern Hemisphere tropics and subtropics (10–30° N), providing a visual guide to which median excursions are statistically robust rather than an artefact of limited sampling. The scatter is large at all latitudes, with individual relative differences frequently exceeding ±50 %, consistent with the standard deviation reported above. Among the ground-based stations, the TMF matchups (triangles, 34° N) span a wide range of both positive and negative relative biases, while the OHP matchups (squares, 44° N), though limited in number, are predominantly negative, consistent with the tendency noted in Sect.  and confirmed independently in Sect. .

The temporal distribution (Fig. b) shows no obvious secular drift over the 20-month record, indicating broad stability of the ATLID L1 calibration throughout the study period. The apparent see-saw pattern of the 30 d binned median likely reflects the changing latitudinal sampling of SAGE III/ISS, with intervals dominated by Northern Hemisphere extratropical matchups showing a more negative bias, consistent with the low bias identified for NH mid- to high-latitude comparisons in Fig. a, rather than any systematic temporal drift of the ATLID L1 product. The ±2 SE envelope on the median widens during intervals with fewer collocations, and no monotonic trend emerges beyond this envelope, reinforcing the absence of a significant temporal drift and the broad stability of the ATLID L1 calibration.

The comparisons presented above rely, to varying degrees, on the conversion from ATLID L1 backscatter to extinction via a lidar ratio (Eq. ). This lidar ratio is taken from the ATLID L2 product when it is available and lies within 30–90 sr, and set to a fixed value of 50 sr otherwise (see Sect. ). To isolate the contribution of this conversion step from that of the underlying scattering-ratio retrieval, we additionally compare ATLID L1 particulate backscatter coefficients directly against collocated ground-based lidar backscatter profiles at OHP and TMF, applying the same point-to-point relative-difference metric defined in Sect.  but evaluated on βaer rather than on extinction.

Figure  shows the resulting distributions of the relative backscatter difference as a function of altitude for the N = 26 OHP overpasses (panel a) and the N = 55 TMF overpasses (panel b). At OHP, the overall median relative difference is -10.6 %, while at TMF it amounts to -5.5 %. Both values are negative, confirming a systematic tendency for ATLID L1 to underestimate ground-based backscatter, consistent with the underestimation identified in the SAGE III/ISS extinction comparison (Sect. ). The difference in magnitude between the two stations (OHP exhibiting a roughly twice as large negative offset as TMF) may reflect the smaller OHP sample size, differences in the altitude ranges effectively sampled by each lidar, or a genuine site-dependent component.

Interestingly, both lidar stations confirm a slight negative bias in the lower stratosphere just above the tropopause, consistent with the behaviour found in the SAGE III/ISS comparison in tropopause-relative altitude (Fig. b). The scatter of the differences is also similar to that observed for the collocations with nighttime ATLID overpasses (Sect. ). The OHP profile displays a similar structure, with the negative bias concentrated between 11 and 23 km and a clearer positive excursion near 23–27 km. Because both comparisons are, by construction, independent of the lidar ratio, this three-instrument consistency strongly indicates that the altitude-dependent bias structure identified throughout Sect.  originates predominantly in the Level-1 scattering-ratio retrieval itself, rather than in the extinction-conversion step (Sect. ).

Having established the robustness of the Level-1-based (K26) extinction product across diverse stratospheric conditions, we now evaluate the added value of this approach relative to the native ATLID Level-2 (L2) aerosol retrieval. Figure a compares the L2 stratospheric AOD with the reference sAOD from SAGE III/ISS, TMF, and OHP. A linear regression to the SAGE III/ISS matchups yields a slope of only 0.64 and an intercept of 0.1 × 10-3, well below the near-unity slope obtained for the L1 product (slope = 1.04 ± 0.07, intercept = (-0.8±0.9)×10-3; Sect. , Fig. ). In about half of the matchups (321 of 652), the L2 extinction is zero at all levels of the integration range (sAOD_L2 = 0, bottom row of Fig. a), reflecting the frequent absence of aerosol detection by the L2 processor noted in Sect. . These zeros are reported by the L2 product itself and are used as such in the sAOD integration, while true fill values are discarded. The L2 comparison also includes fewer matchups than the L1 comparison (652 versus 895) because the daily L2 files contain fewer profiles than the L1 files (about 6400 d⁻¹ versus 22 750 d⁻¹).

Restricting the comparison to profiles for which the L2 processor returns an actual stratospheric retrieval (sAOD_L2 > 0; Fig. b) partially compensates for the systematic underestimation seen in panel (a): the SAGE III/ISS regression slope rises from 0.64 ± 0.13 to 0.94 ± 0.19, formally consistent with unity. This improvement is, however, largely artificial. It results from the removal of the numerous null retrievals (sAOD_L2 = 0) that pull the full-sample slope downward, rather than from any genuine skill of the L2 product. Indeed, the correlation remains very weak (R = 0.26) and the fit acquires a significantly positive intercept of (4.1 ± 2.6) ×10-3, indicating that the retained L2 retrievals scatter almost independently of the true aerosol loading and, on average, overestimate the low-sAOD background. Even when the L2 processor does converge to a non-zero solution, the resulting sAOD is therefore only loosely tied to the reference value, and the apparent near-unity slope in panel (b) should not be interpreted as evidence of reliable quantitative agreement. The class medians confirm this picture: because about half of the matchups have sAOD_L2 = 0, they stay close to zero over most of the sAOD range in Fig. a, and when these cases are excluded (Fig. b), they fall on both sides of the 1:1 line with wide interquartile ranges, so that no sAOD range of consistent agreement emerges.

At low reference sAOD (≲ 0.02), where the bulk of the dataset lies, L2 retrievals show essentially no relationship with the reference values: some matchups return spuriously high sAOD (up to 0.05–0.06) while others return values close to zero, regardless of the true aerosol loading. This behaviour is illustrated at the profile level by the selected case studies shown in Fig. , spanning the three aerosol regimes sampled in this study. Taken together, the aggregate sAOD comparison and the case-level profiles demonstrate that the skill of the native L2 retrieval degrades sharply as the aerosol loading decreases, and justify the systematic use of the L1-based K26 product as the primary ATLID extinction estimate throughout the remainder of this study.

The altitude-dependent bias identified independently in the SAGE III/ISS extinction comparison (Sect. ) and in the OHP and TMF backscatter comparisons (Sect. ) is consistent across the three instruments near the tropopause: when expressed as a function of altitude above the local tropopause, the SAGE III/ISS comparison shows a negative bias just above the tropopause, as do the two ground-based lidars.

This consistency between the lidar-ratio-dependent extinction comparison and the lidar-ratio-independent backscatter comparisons argues against the conversion step being the dominant driver and instead points to an origin within the Level-1 scattering-ratio retrieval itself; the exact physical mechanisms driving this vertical bias structure require further investigation.

This behaviour is broadly consistent with the lessons learned from the CALIOP stratospheric aerosol product. showed that the improved Version 4 calibration of CALIOP enabled a reliable retrieval of stratospheric aerosol extinction, but also that agreement with SAGE III/ISS remains strongly latitude- and altitude-dependent: comparatively good in the tropical stratosphere between about 20 and 30 km, and substantially poorer at higher latitudes and near the tropopause, where differences can exceed 100 %. Those results are particularly relevant here because they show that, even for a mature elastic lidar product, the stratospheric regime remains challenging when the aerosol loading is weak and the signal-to-noise ratio is low.

ATLID itself exhibits an analogous latitudinal signature: the sAOD bias distribution (Sect. , Fig. a) shows a systematic negative bias in the Northern Hemisphere extratropics that increases with latitude. Part of this gradient may be linked to aerosol type rather than to latitude per se: the boreal wildfire smoke events sampled during this study are concentrated at Northern Hemisphere mid-to-high latitudes, and the lidar ratio of smoke particles may differ from the 50 sr fallback used in the absence of a valid L2 value (Sect. ). The case studies of Fig.  show, however, that this effect alone cannot explain the negative bias: in the smoke case of Fig. c, the L2 provides a lidar ratio of about 60 sr within the layer and the L1 sAOD exceeds the SAGE III/ISS value, while in Fig. d the L1 sAOD obtained with the 50 sr fallback is close to the SAGE III/ISS value. The lidar ratio of smoke is therefore only one possible contribution, and a dedicated analysis comparing the summer 2025 smoke period with the rest of the record is left for future work.

To further contextualize the degree of agreement observed between ATLID L1 and SAGE III/ISS, it is instructive to compare our results with other satellite intercomparison studies. For instance, evaluated the consistency between SAGE III/ISS extinction and CALIOP backscatter-derived extinction (see their Fig. 13). They demonstrated that direct comparisons using a constant lidar ratio for CALIOP yielded regional biases frequently exceeding 50 % in the lower stratosphere and at higher latitudes. It was only after applying an empirical, altitude- and latitude-dependent scale factor to the CALIOP data that the differences relative to SAGE III/ISS were reduced to within ±20 %. In this context, the agreement found in our study without relying on any empirical post-scaling highlights the robust performance of the ATLID L1-based retrieval and the distinct advantage of its HSRL measurement capabilities in the stratosphere.

The clear contrast between the regression slopes obtained for the L1 (1.04) and native L2 (0.64) sAOD products against SAGE III/ISS (Sect. ) has direct practical implications. The native L2 stratospheric aerosol product, in its current processing baseline, should not be relied upon for quantitative assessments of background or weakly perturbed stratospheric aerosol, the regime that dominates the EarthCARE record outside of the Ruang eruption and the 2025 boreal wildfire season, since it exhibits essentially no skill in this regime and frequently defaults to either spuriously elevated or null retrievals. The L1-based K26 product evaluated throughout this study provides a substantially more faithful representation of the true aerosol loading across the full dynamic range sampled here, and we recommend its use, at least in the interim, for stratospheric aerosol monitoring and trend studies based on ATLID, pending future refinement of the operational L2 algorithm.

The quality of the SAGE III/ISS reference data is also important in interpreting these comparisons. The extinction profiles used here are derived from the 384 nm channel and converted to 355 nm using the 384–521 nm Ångström exponent, with profiles affected by transmission anomalies, cloud contamination, or large fractional uncertainty excluded following established quality-control recommendations . Using the 384/449 nm pair instead (mean Ångström exponent of 0.704 over the ATLID observation period, compared with 0.866 for the 384/521 nm pair) would change the 384-to-355 nm conversion factor from 1.070 to 1.057, i.e. the converted 355 nm extinction by only about 1.3 %, which is small compared with the variability and uncertainties of the ATLID-SAGE III/ISS comparison. SAGE III/ISS is therefore a robust and well-characterized reference, but not a noise-free one; some of the residual scatter in the ATLID comparison likely reflects the combined uncertainties of both instruments.

Beyond the reference dataset itself, the extinction retrieval relies on a lidar ratio that is either taken from the ATLID L2 product, where it lies within 30–90 sr, or fixed at 50 sr otherwise (Sect. ). For aged plumes and near-background stratospheric conditions, where the L2 product reports zero extinction, the 50 sr value is an assumption. Typical lidar ratios of stratospheric sulfate are about 45–50 sr at 532 nm , but direct estimates at 355 nm are scarce: used 40–50 sr at 355 nm for the 2019 Raikoke and Ulawun sulfate layer, checked against Raman measurements, and a constant 50 sr is used for sulfate aerosol with the 355 nm LiO₃S lidar at Maïdo (K26). The 50 sr value thus lies at the upper end of the values used at 355 nm. Since the L1-derived extinction scales linearly with the lidar ratio, a ±10 sr departure from 50 sr changes the extinction by ±20 %; this sensitivity is shown as an envelope in Fig.  at the levels where the 50 sr fallback is used. A targeted test of the smoke-related, high-latitude hypothesis raised in Sect.  is left for future work. We note, however, that the qualitative consistency between the lidar-ratio-dependent SAGE III/ISS extinction comparison and the lidar-ratio-independent OHP and TMF backscatter comparisons (Sect. ) suggests that the dominant altitude-dependent bias pattern identified in this study is not primarily a lidar-ratio artefact.