Volcanic SO₂ plume tracking and emission quantification

UV backscatter spectrometry on TROPOMI and OMPS retrieves sulphur dioxide column amounts daily at sub-kilometre to tens-of-kilometres scale, enabling near-real-time aviation hazard alerts and long-run climate forcing estimates, with honest caveats on altitude ambiguity and low-altitude degassing blind spots.

Sensors

TROPOMI on Sentinel-5P: Primary operational SO₂ instrument. UV–Vis pushbroom spectrometer covering 270–500 nm, with a ground pixel of 3.5 × 5.5 km (upgraded from 7 × 3.5 km in August 2019). Daily global coverage, ~15 orbits per day. SO₂ column retrieved via DOAS in the 312–326 nm fitting window. Detection limit for volcanic SO₂ is roughly 0.5–1 Dobson Unit under favourable conditions; large explosive eruptions can register hundreds of DU. Latency to public product: typically 3 hours for near-real-time (NRTI) stream.
OMPS Nadir Mapper on Suomi-NPP: UV backscatter spectrometer, 300–380 nm range. Ground pixel approximately 50 × 50 km at nadir for the standard SO₂ product, limiting detection to moderate-to-large eruptions. Daily global coverage. Useful as a cross-check against TROPOMI and for continuity with the long TOMS/OMI heritage record stretching back to 1978.
GOME-2 on Metop-A/B/C: UV–Vis spectrometer, 240–790 nm. Ground pixel 80 × 40 km in standard mode, 40 × 40 km in narrow swath. Three Metop satellites provide multiple daily passes at high latitudes, which is useful for polar or sub-polar volcanic arcs (Aleutians, Kamchatka, Iceland). Sensitivity is lower than TROPOMI for small passive degassers.
ASTER TIR on Terra: Thermal infrared context at 90 m resolution, 14 spectral bands from 8 to 12 µm. Does not retrieve SO₂ columns directly, but identifies thermal anomalies at the vent, lava flows, and elevated plume temperatures that help constrain eruption intensity and injection altitude, which is critical for resolving the SO₂ altitude ambiguity in UV retrievals.

Why sulphur dioxide, and why UV backscatter

Sulphur dioxide is the most useful gaseous tracer of volcanic activity from orbit. It is present in the atmosphere at very low background concentrations, so even a modest eruption stands out clearly. Water vapour and CO₂, both abundant in volcanic emissions, are swamped by their atmospheric backgrounds. SO₂ is not.

The measurement principle is differential optical absorption spectroscopy, DOAS. Sunlight scattered back from the atmosphere carries absorption fingerprints of every gas it has passed through. SO₂ has structured absorption cross-sections between roughly 280 and 330 nm. A spectrometer records the backscattered radiance spectrum; the retrieval fits the SO₂ absorption structure against a reference, removes the slowly varying background using a low-order polynomial, and converts the residual to a vertical column amount in Dobson Units or mol/m². The approach is well-established: the TOMS instrument demonstrated it in the 1980s using Pinatubo and El Chichón eruptions, and modern instruments have refined both spectral resolution and spatial sampling by orders of magnitude.

What TROPOMI actually sees, and what it misses

TROPOMI's 3.5 × 5.5 km pixel is small enough to resolve individual volcanic vents within a chain and to track plume edges with meaningful spatial fidelity. The near-real-time product reaches public servers within roughly three hours of overpass, which is operationally useful for aviation hazard coordinators. The Volcanic Ash Advisory Centres that guide aircraft routing around eruption plumes now routinely ingest TROPOMI SO₂ fields alongside trajectory models.

The limits are real and matter. First, TROPOMI requires daylight and cloud-free conditions. A plume obscured by convective cloud, common during explosive eruptions in tropical settings, will be partially or entirely missed. The retrieval assumes SO₂ is located at a specified altitude; if the true plume altitude differs from the assumed profile, the retrieved column is biased. This altitude ambiguity is the most significant systematic error for aviation applications, where knowing whether SO₂ is at 5 km or 15 km changes the hazard calculus entirely. Analysts resolve this by combining TROPOMI columns with trajectory back-calculations and, where available, ASTER or Himawari-9 thermal data to constrain injection height.

Second, passive degassing from open-vent systems at low altitude, below roughly 2 km, sits inside or below the planetary boundary layer. UV sensitivity drops sharply there because the solar path through the lower atmosphere shortens and scattering geometry becomes unfavourable. A persistently degassing volcano emitting a few hundred tonnes of SO₂ per day at low altitude may be barely detectable. Ground-based DOAS traverses remain necessary for those cases.

From column amounts to mass flux

A column map gives total SO₂ overhead at each pixel. Converting that to a mass flux requires knowing how fast the plume is moving. The standard approach integrates SO₂ columns across a transect perpendicular to the plume transport direction and multiplies by the wind speed at plume altitude, derived from meteorological reanalysis (ERA5 is the most widely used). The result is SO₂ flux in kilograms per second or tonnes per day.

Uncertainty in the wind field is often the dominant error term, not the satellite retrieval itself. ERA5 wind uncertainty at plume altitude can be 10–20% in data-sparse regions, which propagates directly into the flux estimate. For large explosive eruptions injecting SO₂ into the stratosphere, where wind fields are better constrained and the plume is above the boundary layer confusion, flux estimates are more reliable. For small, low-altitude degassers, the combined uncertainty can exceed 50%.

Climate forcing calculations care about the stratospheric fraction. Tropospheric SO₂ oxidises to sulphate aerosol within days to weeks and has limited radiative effect. Stratospheric SO₂ persists for months, converts to a global sulphate veil, and measurably reduces incoming solar radiation. The 1991 Pinatubo eruption injected an estimated 18–20 Tg of SO₂ into the stratosphere; the resulting aerosol caused roughly 0.5°C of global surface cooling over the following two years. Quantifying that stratospheric loading accurately is why SO₂ retrievals feed directly into climate model initialisation.

Distinguishing passive degassing from explosive signals

Not all volcanic SO₂ is equal in operational terms. A persistently degassing open-vent system, such as Ambrym in Vanuatu or Kilauea during effusive phases, produces a relatively steady, low-altitude plume. An explosive Plinian or sub-Plinian eruption injects SO₂ rapidly to high altitude, where it disperses across hundreds or thousands of kilometres within hours.

The satellite signature differs in three ways. Explosive events produce a sudden, spatially compact high-column-density feature that then advects downwind as a coherent cloud. The column amounts can be two to three orders of magnitude higher than passive degassing baselines. Altitude indicators, including the SO₂ centre-of-mass height retrievable from multi-layer TROPOMI products and thermal brightness temperature from geostationary imagers, shift sharply upward. Tracking this transition in near-real-time is the core of aviation hazard monitoring: the International Airways Volcano Watch depends on exactly this distinction to issue Volcanic Ash Advisories and SIGMETs.

Operational products and where Satellize fits

Several public systems already provide daily SO₂ maps: the Copernicus Atmosphere Monitoring Service disseminates TROPOMI SO₂ fields; NASA's Goddard Space Flight Center maintains the SO₂ Group archive; and NOAA's NESDIS provides near-real-time volcanic cloud products. The value of a dedicated analytics layer lies in combining those fields with trajectory modelling, historical degassing baselines per volcano, and contextual thermal data to produce decision-ready outputs rather than raw column maps.

Satellize runs analytics on open constellation data including TROPOMI and can add commercial tasking for thermal context on client licence. For governments managing aviation corridors over active volcanic arcs, or for reinsurance clients pricing eruption exposure, the relevant deliverable is not a column map but a time-stamped flux estimate with an uncertainty bound and a plume-extent polygon ready for ingestion into flight-planning systems or catastrophe models. Satellize's Overhead column has covered volcanic SO₂ events as they develop, using the same TROPOMI data pipeline.

Archive depth and what history reveals

The continuous UV SO₂ record from space begins with TOMS on Nimbus-7 in 1978, runs through Earth Probe TOMS, OMI on Aura (2004 to present), and now TROPOMI. That 45-year archive contains every major eruption of the satellite era. It is the primary source for volcanic SO₂ emission inventories used in atmospheric chemistry models and for estimating the natural sulphur contribution to stratospheric aerosol loading between large eruptions, sometimes called the background volcanic flux.

TROPOMI's archive begins in May 2018. At 3.5 × 5.5 km resolution it is the highest-resolution global SO₂ dataset ever produced from orbit. For volcanoes that were active before 2018, the OMI record at roughly 13 × 24 km provides continuity. Combining both is standard practice for trend analysis. The honest caveat: the improved sensitivity of TROPOMI means apparent emission increases at some volcanoes since 2018 are partly an artefact of better detection, not genuine intensification.

Typical figures

Spatial resolution (TROPOMI SO₂)	3.5 × 5.5 km per pixel (since August 2019)
Spatial resolution (OMPS)	~50 × 50 km (standard nadir product)
Revisit (TROPOMI)	Daily global coverage, ~15 orbits per day
Near-real-time latency (TROPOMI NRTI)	Typically 3 hours after overpass
Spectral fitting window	312–326 nm (primary SO₂ DOAS window)
Minimum detectable SO₂ column (TROPOMI)	~0.5–1 Dobson Unit under favourable conditions; higher near the boundary layer
Column unit	Dobson Units (DU) or mol/m²; 1 DU ≈ 2.69 × 10²⁰ molecules/cm²
Archive depth (continuous UV SO₂ record)	1978 to present (TOMS heritage); TROPOMI from May 2018
Coverage constraint	Daylight only; cloud cover causes partial to total retrieval failure
Flux product uncertainty (typical)	10–20% from wind-field error alone; up to ~50% for low-altitude passive degassers

Analytics Satellize can run

Near-real-time SO₂ plume alert	TROPOMI NRTI column product ingestion with threshold exceedance detection per volcano	Automated alert with plume centroid, peak column amount, and estimated onset time; delivered as JSON feed or email within 4 hours of overpass
Plume-extent polygon	Column thresholding and connected-component labelling on TROPOMI SO₂ grid	GeoJSON polygon updated daily, compatible with aviation flight-planning systems and GIS ingestion
SO₂ mass flux time series	Transect integration of TROPOMI columns multiplied by ERA5 wind speed at plume altitude	Daily flux estimate in tonnes/day with uncertainty bounds; delivered as CSV or time-series chart per volcano
Plume injection-height estimate	Multi-layer SO₂ retrieval from TROPOMI combined with ASTER or geostationary thermal brightness temperature	Altitude range (lower–upper bound in km) appended to each flux report
Passive degassing baseline profile	Statistical analysis of TROPOMI and OMI archive SO₂ columns per volcano, percentile distribution by month	Per-volcano degassing climatology report with anomaly detection thresholds
Stratospheric SO₂ loading estimate	Separation of tropospheric and stratospheric SO₂ fractions using altitude-resolved TROPOMI retrieval and trajectory analysis	Tg SO₂ estimate for stratospheric injection events, formatted for climate-model input or reinsurance catastrophe modelling

Sources

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.