Satellite measurement

SO2 Anomalies: Sulfur Dioxide Measured Around a Volcano

A volcanic ash advisory says ash was observed or is forecast. It is a notice for aviation, not a measurement of the volcano. Off-Nadir Delta reads the Sentinel-5P sulfur dioxide measurement around each volcano that has a current advisory, and records the days on which the gas is clearly above the local background: a second line of evidence, from a source that does not depend on the advisory.

See them on the Watchfloor API reference

Quick answer — what an SO2 anomaly is here

An SO2 anomaly is a record that Sentinel-5P measured sulfur dioxide clearly above the local background around a volcano. Only volcanoes with a volcanic ash advisory in the previous 3 days are checked. Pixels within 0.5° of the volcano (about 56 km) are compared with the surrounding ring out to 2°: a record is made when at least 10 nearby pixels are more than 5 times the ring’s spread above its median and above 1 Dobson Unit, and that count is at least 3 times what the ring itself would predict. There is one record per volcano per day. It states the observation time, the largest column, the background and how many pixels stood out. It does not give the size of an eruption, the height of a plume or whether ash was present. Records are events on the Watchfloor map under the category SO2 Anomalies.

What is an SO2 anomaly?

Sentinel-5P carries the TROPOMI instrument, which the Copernicus Data Space describes as providing “daily global observations” of atmospheric constituents including the total column of sulfur dioxide (Copernicus Data Space: Sentinel-5P). An erupting or degassing volcano puts sulfur dioxide into the air, so the gas around it rises above what the surrounding region shows.

A fixed amount is a poor test, because the noise in the measurement differs from place to place. In our checks, a fixed threshold was also met at many places with no volcano. So the comparison here is local: the pixels next to the volcano against the ring of pixels around them, in the same overpass.

An SO2 anomaly is therefore narrow: one volcano, one day, sulfur dioxide clearly above its own surroundings. It supports the reading that the volcano was emitting gas at that overpass. It is not a detection of an eruption and it is not an ash measurement.

How an overpass is judged

Only volcanoes named in a volcanic ash advisory in the previous 3 days are checked. A volcano with no advisory is not looked at.
Pixels with a quality value of at least 0.5 are read: those within 0.5° of the volcano (about 56 km), and the ring around them out to 2°.
The background is the median of the ring, and its spread is estimated from the ring as well.
A nearby pixel stands out when it is more than 5 times that spread above the background and above 1 Dobson Unit.
A record is made when at least 10 nearby pixels stand out, and that is at least 3 times the number the ring’s own rate would predict.
An overpass with too few usable pixels, for example under heavy cloud, is not judged.
There is one record per volcano per day (UTC). A stronger observation on the same day replaces the earlier one.

Data: near-real-time sulfur dioxide from Sentinel-5P, processed by Off-Nadir Delta. Credit: Contains modified Copernicus Sentinel-5P data, the wording the legal notice on the use of Copernicus Sentinel data asks for when the data has been modified.

What a record states

Which volcano, and the time of the satellite observation (UTC).
How many of the nearby pixels stood out.
The largest column measured near the volcano, in Dobson Units.
The background level of the surrounding ring.
That the volcano has a current volcanic ash advisory.

What it does not state

The size of an eruption. The record says the gas was elevated. It does not rank or size what the volcano did.
Plume height or ash. Sulfur dioxide and ash are different things. The record does not say whether ash was present or how high anything reached.
That the volcano is the only source. Sulfur dioxide also comes from smelters and power plants.
That a quiet day was quiet. No record on a given day does not mean the volcano was quiet. Cloud, viewing geometry and weak emission hide the gas.
Certainty. In our checks on three days (2026-10-04 to 2026-10-06), the same test was met at about 1% of random places with no volcano, so an isolated record can be noise.
A severity. These records carry no severity. An empty severity means not assessed, not minor.

Measurement, advisory and reporting are kept apart

A volcano can appear on the map three ways: as a volcanic ash advisory, as reporting, and as an SO2 anomaly. They are three different kinds of evidence from three sources.

The SO2 anomaly is its own event, placed at the volcano, and is never merged into the advisory or the reporting about the same volcano. Its title and summary are written by a fixed rule from the measured values, not by a model.

SO2 anomalies over the API

SO2 anomalies are events, so they come back from the events endpoint with their category. A page costs 3 tokens.

curl "https://offnadir-delta.com/api/v1/signals?categories=so2_anomaly" \
  -H "Authorization: Bearer ond_..."

Each one carries measured: true and measured_kind: so2. The same query works through the MCP tool query_signals. A reader for this kind alone is GET /api/v1/so2-anomalies, and client.so2_anomalies in the Python SDK. Parameters and response fields are in the API reference.

What each plan sees

On every plan, the Watchfloor map shows that an SO2 anomaly was recorded, when, and where. The marks are there without paying.

Which volcano it was, the column amounts and how many pixels stood out are part of the full account of each event, in the app, through the Delta Agent and over the API. Pricing shows which plan opens it.

How It Works

Open the Watchfloor

SO2 anomalies are on the map next to reported events. To see only these, pick “SO2 Anomalies” under Category in the filters.

Find the volcano marks

Each mark, a mountain with a plume, is one volcano where sulfur dioxide stood out from its surroundings that day.

Open one

Read what was measured: the observation time, the largest column, the background and the number of pixels that stood out.

Read it with the advisory

The volcanic ash advisory and any reporting about the same volcano are separate events on the same map.

Frequently Asked Questions

What is an SO2 anomaly?▼

A record that Sentinel-5P measured sulfur dioxide clearly above the local background around a volcano that has a volcanic ash advisory in the previous 3 days. Pixels within 0.5° of the volcano are compared with the surrounding ring out to 2° in the same overpass.

Does an SO2 anomaly mean the volcano erupted?▼

It means sulfur dioxide was elevated around the volcano at one satellite overpass, which is consistent with the volcano emitting gas. It does not give the size of an eruption, the height of a plume or whether ash was present, and sulfur dioxide also comes from smelters and power plants.

Why only volcanoes with an ash advisory?▼

The measurement is used to corroborate. An advisory is a notice that ash was observed or forecast; the SO2 measurement is an independent look at the same volcano. Volcanoes without a current advisory are not checked, so no record there says nothing about them.

Which satellite is used?▼

Sentinel-5P, whose TROPOMI instrument provides daily global observations of gases including sulfur dioxide. Off-Nadir Delta reads the near-real-time sulfur dioxide product. Credit: Contains modified Copernicus Sentinel-5P data.

How reliable is one record?▼

It is a statistical test, and it can be met by noise. In our checks on three days (2026-10-04 to 2026-10-06), the same test was met at about 1% of random places with no volcano. Those three days do not show how the rate changes with season or latitude.

Why is there no record on a day the volcano was active?▼

Cloud, viewing geometry and weak emission can hide the gas, an overpass with too few usable pixels is not judged, and a volcano without a current ash advisory is not checked at all.

Is there an SO2 anomaly API?▼

SO2 anomalies are events: GET /api/v1/signals with categories=so2_anomaly returns them, each with measured: true and measured_kind: so2, at 3 tokens a page. A reader for this kind alone is GET /api/v1/so2-anomalies, and client.so2_anomalies in the Python SDK; the API reference has the parameters.

What does each plan see?▼

Every plan sees that an SO2 anomaly was recorded, when, and where on the map. Which volcano and the measured values are part of the full account of each event, which not every plan includes. The Pricing page shows which plan opens it.