Stratospheric aerosol layer monitoring after volcanic and wildfire injections
Volcanic eruptions and intense wildfires can inject sulphate and smoke above the tropopause, where aerosols persist for months and measurably depress surface temperatures. Limb-scatter and solar-occultation instruments track the layer's evolution, but each technique carries hard geometric and physical limits that matter for any serious monitoring programme.
Sensors
- SAGE III / ISS: Solar and lunar occultation spectrometer (290–1040 nm) retrieving aerosol extinction profiles at roughly 0.5 km vertical resolution. Occultation geometry limits coverage to ~30 events per day, each at high latitudes, so global maps take weeks to assemble. Archive on ISS began 2017.
- OSIRIS / Odin: Limb-scatter spectrometer (280–800 nm) providing aerosol extinction profiles at ~2 km vertical resolution from roughly 10–40 km altitude. Odin is a shared science mission with limited duty cycle; spatial sampling is sparser than a dedicated aerosol satellite. Operational since 2001.
- CALIOP / CALIPSO: Dual-wavelength (532 nm, 1064 nm) polarisation lidar with ~30 m vertical and ~333 m along-track resolution. Directly measures backscatter and depolarisation, enabling sulphate-smoke discrimination. Narrow 70 m swath means a 16-day exact repeat; CALIPSO ended in 2023, with CATS and future lidars as partial successors.
- OMPS-LP / Suomi NPP and NOAA-20: Limb Profiler measuring UV-visible scattered radiance to retrieve aerosol extinction from roughly 10–40 km at ~1–2 km vertical resolution. Near-daily near-global coverage makes it the most timely of the limb instruments for tracking fresh injections, though absolute calibration uncertainty is roughly 10–20% in the lower stratosphere.
Why the stratosphere is a different problem from the troposphere
In the troposphere, aerosols wash out within days. Cross the tropopause and the rules change: no rain, very slow sedimentation, and meridional transport that spreads a single injection across a hemisphere within weeks. The 1991 Pinatubo eruption deposited an aerosol optical depth anomaly of roughly 0.1–0.15 at 550 nm globally, measurably cooling the Northern Hemisphere surface by approximately 0.5°C over the following two years. That is the physical stake.
Wildfire smoke from pyroconvective events has more recently been shown to reach the stratosphere in quantities large enough to matter. The 2019–2020 Australian fires injected absorbing smoke that was tracked by CALIOP and OMPS-LP to altitudes above 30 km, persisting for months. Smoke and sulphate behave differently: sulphate scatters efficiently and cools; black carbon absorbs and locally heats the stratosphere while still reducing surface insolation. Getting the chemistry right matters for any forecast of surface impact.
What occultation geometry gives you, and what it withholds
Solar occultation is the oldest and most precise stratospheric aerosol measurement technique. As the Sun rises or sets through the limb from a satellite's perspective, its spectrum is attenuated by the intervening atmosphere. Ratio that to the exoatmospheric spectrum and you recover extinction as a function of altitude with self-calibrating accuracy. SAGE III on the ISS achieves roughly 0.5 km vertical resolution this way, with extinction uncertainties of a few percent in the middle stratosphere under clean conditions.
The catch is geometric. Each occultation event samples a single tangent point, and the ISS orbit produces only about 30 sunrise or sunset events per day, clustered at the latitudes where the orbital plane crosses the terminator. A fresh volcanic plume injected at a mid-latitude volcano may go unsampled for several days. After a major eruption, when the aerosol loading is high, the retrieval can also saturate in the UV, forcing reliance on longer wavelengths where aerosol-gas separation is harder. Occultation is the gold standard for climatological trend work; it is not a rapid-response tool.
Limb scatter fills the gap, but introduces the extinction-to-backscatter ambiguity
Limb-scatter instruments such as OSIRIS and OMPS-LP observe sunlight scattered by the atmosphere at the limb across multiple wavelengths. By comparing the wavelength dependence of the scattered signal, they retrieve vertical extinction profiles with near-daily global coverage. That revisit advantage is real and significant for tracking a fast-evolving injection.
The ambiguity is physical. Scattered radiance depends on both the extinction of the aerosol and its phase function, which in turn depends on particle size and composition. To convert radiance to extinction, you must assume or separately constrain the single-scattering albedo and phase function. For sulphate, the assumption is well-established. For smoke, particularly fresh absorbing smoke, the phase function can differ substantially, and an incorrect assumption propagates directly into the extinction profile. Studies comparing OMPS-LP and CALIOP after the 2017 Canadian pyroconvective events found discrepancies of 30–50% in the lower stratosphere during the first weeks, narrowing as the smoke aged and its optical properties evolved toward a more scattering regime.
CALIOP's direct backscatter measurement sidesteps part of this problem but introduces its own: you still need a lidar ratio (extinction-to-backscatter ratio) to convert backscatter to extinction, and that ratio varies with aerosol type. For stratospheric sulphate the community uses roughly 50 sr at 532 nm; for fresh smoke it can exceed 70–80 sr. Misassigning the lidar ratio by 20 sr shifts the derived aerosol optical depth by a comparable fractional error.
Reading the layer: what the data actually tells a buyer
The primary product from these instruments is a vertical extinction profile, typically expressed at one or more wavelengths from the UV to the near-infrared. Integrating that profile over altitude gives stratospheric aerosol optical depth (SAOD), the quantity most directly linked to radiative forcing. A post-Pinatubo SAOD of 0.1 at 550 nm corresponds to a top-of-atmosphere forcing of roughly minus 3 W/m², a figure that climate modellers need with better than 10% accuracy to constrain forcing estimates.
Secondary products include effective radius (particle size), which governs how quickly the layer sediments and how the forcing evolves over time, and the altitude of the aerosol centroid, which affects how long the material stays aloft. Distinguishing a fresh injection at 20 km from one at 28 km matters: the higher layer can persist for two to three years rather than one. None of these secondary retrievals are as well-constrained as the primary extinction; treat them as indicative rather than definitive unless multiple instruments agree.
Sparse sampling, cloud contamination and what cannot be fixed in post-processing
All limb and occultation techniques share a fundamental limitation: they cannot see through clouds. Tropical volcanic injections often occur in regions of persistent deep convection. In the days immediately after an eruption, when the plume is still partly in the upper troposphere, cloud contamination can mask the signal entirely. The community typically relies on the volcanic SO₂ plume tracked by nadir UV instruments (a separate topic) to infer injection height and mass in the first 24–72 hours, then transitions to limb and occultation data as the aerosol settles into the stratosphere proper.
Spatial sampling gaps are not fixable by interpolation for a fresh, heterogeneous plume. A limb instrument with a 500 km horizontal averaging kernel cannot resolve a narrow filament of aerosol, and occultation's point samples cannot be gridded into a global map without weeks of accumulation. Buyers who need daily global coverage at high spatial resolution for a fresh event will be disappointed; what these instruments provide is unmatched vertical resolution and long-term consistency for trend monitoring. Satellize incorporates OMPS-LP and CALIOP heritage products into stratospheric aerosol monitoring workflows, combining them with TROPOMI SO₂ columns during the injection phase to build a more complete picture across the event timeline.
Archive depth is a genuine asset. SAGE II operated from 1984 to 2005, OSIRIS from 2001, and SAGE III/ISS from 2017, providing a multi-decade record against which any new injection can be contextualised. That historical baseline is often the most valuable thing a government science agency or reinsurance analyst can have.
Typical figures
| Vertical resolution (occultation, SAGE III) | ~0.5 km |
| Vertical resolution (limb scatter, OMPS-LP / OSIRIS) | 1–2 km |
| Vertical resolution (lidar, CALIOP) | ~30 m (stratosphere mode) |
| Horizontal / along-track sampling | Occultation: ~30 events/day globally; OMPS-LP: near-daily near-global; CALIOP: 70 m swath, 16-day repeat |
| Spectral range | SAGE III: 290–1040 nm; OSIRIS: 280–800 nm; CALIOP: 532 nm and 1064 nm; OMPS-LP: 290–1000 nm |
| Minimum detectable SAOD anomaly | ~0.001–0.005 at 550 nm under background conditions (instrument- and altitude-dependent) |
| Extinction uncertainty (middle stratosphere, sulphate) | ~5–10% for occultation; 10–20% for limb scatter; higher for fresh smoke |
| Latency (near-real-time products) | OMPS-LP near-real-time: 1–3 days; SAGE III science products: weeks to months |
| Archive depth | SAGE II from 1984; OSIRIS from 2001; SAGE III/ISS from 2017; CALIOP 2006–2023 |
| Delivery formats | NetCDF-4 HDF5 profiles; derived SAOD grids as GeoTIFF or CSV time series |
Analytics Satellize can run
| Post-injection SAOD time series | Integration of limb/occultation extinction profiles over 15–40 km altitude; multi-instrument merging weighted by uncertainty estimates | Monthly gridded SAOD anomaly maps (NetCDF or GeoTIFF) with uncertainty bands, updated as new retrievals are released |
| Aerosol centroid altitude and effective radius trend | Wavelength-dependent extinction ratio analysis (Ångström exponent approach) applied to multi-wavelength SAGE III or OMPS-LP profiles | Time-altitude curtain plots and summary statistics report, delivered as PDF and CSV within agreed latency of new data release |
| Sulphate vs. smoke classification | CALIOP depolarisation ratio thresholding combined with colour ratio (1064/532 nm backscatter); cross-checked against OMPS-LP spectral slope | Classified aerosol-type layer mask as GIS polygon layer, updated per CALIOP overpass cycle |
| Radiative forcing anomaly estimate | SAOD-to-forcing conversion using published single-scattering albedo assumptions for sulphate and smoke; benchmarked against CERES broadband flux anomalies where available | Forcing anomaly report with stated uncertainty range, suitable for input to climate-impact assessments |
| Event contextualisation against historical record | Comparison of derived SAOD against SAGE II / OSIRIS climatological baseline; percentile ranking within the post-1984 record | Historical context brief: where the current event sits relative to Pinatubo (1991), El Chichón (1982) and recent pyroconvective events |
| Multi-source injection timeline (SO₂ to aerosol) | Fusion of TROPOMI SO₂ columns (injection phase) with OMPS-LP extinction profiles (aerosol maturation phase) to reconstruct the full event arc | Annotated event timeline document and data package covering injection through peak loading to decay |
Who does the work
We can get this done for you. Satellize runs its own analyst desk and a strong science team. You do not buy a data feed and work out what it means; our people source the imagery, run the analysis described on this page, and hand you the answer with its confidence limits stated. Discuss this requirement.