Sentinel-5P TROPOMI: trace gases

TROPOMI does not measure a concentration at ground level. It measures how much of a trace gas stands in the air column above a location: for nitrogen dioxide the tropospheric column, for sulphur dioxide the total column. The sensor analyses sunlight backscattered by the atmosphere, each gas removes particular wavelengths, and the amount follows from that signature.

A high value may come from a chimney, but equally from a plume that has drifted across the area at two kilometres altitude. For the question “how much is somebody in Kakanj breathing in” this is the wrong quantity. For the question “where is today's plume” it is the right one.

A TROPOMI pixel measures 3.5 by 5.5 kilometres. The satellite passes over the working area once a day, practically simultaneously with NOAA-20 around 11:30 to 12:45 UTC; the processed tile becomes available roughly two to three hours later. In the morning the current day is therefore largely absent, the map then shows the last complete scene and states its date.

At that grid width a city such as Zenica is a few pixels across; a single plant is none.

Sulphur dioxide is the politically most important quantity in the working area: Bosnia and Herzegovina carries a European peak load from coal-fired power generation, and the pilot area contains the Kakanj power station.

This is exactly where satellite measurement reaches its limit. A total column at 3.5 by 5.5 kilometres, measured once a day, is designed for large sources, volcanic eruptions, major fires, entire industrial regions. Whether a single power station is currently exceeding a limit value is not a question it answers.

For this data path samoEKO deliberately gives no numerical detection limit: none is published for the ready-made colour tiles from GIBS, and an estimated figure would be worse here than none at all. The answer to the sulphur dioxide question therefore lies not in orbit but on the ground, in electrochemical cells in the full station of the samoEKO sensor box, calibrated against the same official monitoring station the authority uses.

GIBS carries exactly two TROPOMI products as ready-made, keyless colour tiles: the tropospheric nitrogen dioxide column and the sulphur dioxide total column. Both are connected, and both reach back to 6 May 2018.

All further quantities from this satellite, carbon monoxide, methane, aerosol index, ozone, reside solely in the Copernicus Data Space Ecosystem, whose services require registration and a configured instance identifier. That would be a second data path with its own account management and is deliberately not connected.