SO₂ data and alert service

Introduction

Slant column density vs. vertical column density

Geographic regions

Data presentation and delivery

Solar Zenith Angle

What is the Dobson Unit?

Slant column retrieval

Background correction

Reference spectrum

Cloud cover fraction

Near-real time service

Criteria for exceptional SO2 concentrations

Air-mass factor using look-up tables

Air-mass factor using a chemistry transport model

SO2 column from OMI

Time period of available data

Data format specification

Data and Service version history

Validation of the data products

South Atlantic Anomaly

Downloading data & image files

Documentation

References

Acronyms

Acknowledgments

Criteria for exceptional SO2 concentrations

The criteria for exceptional SO2 concentrations for the SO2 data have been set up by examining the results of the retrieval and analysis processes. The results of alert events found by the processing are the source for the continuous (qualitative) validation of the alert service.

Since the measurement method and data structure of different satellite instruments is different, the criteria for an alert necessarily have to be different too.

Though the GOME-1 instrument is still operational, it has limited coverage due to permanent failure of some satellite systems, which makes it unsuitable for use in an NRT alert service. Therefore no selection criteria are set up for GOME-1 data.

Criteria for SCIAMACHY data

For the analysis only the ground pixels part of each forward scan are considered; these are also the pixels plotted in daily data at orbit coordinates, an example of which is given in this graph of the SO2 slant column.

Since the vertical column density (VCD) depends on quite a few assumptions and is only available when cloud cover information is available, the VCD is not a suitable quantity to use for setting up selection criteria for alerts. Instead the slant column density (SCD) is used, after the background correction has been applied.

The following graph shows an geographic overview of all the forward ground pixels of the nadir state just below the centre of the above SO2 slant column plot. A coloured circle in each ground pixels indicates the value of the SO2 slant column. Note that if the SO2 concentration is very low, then the DOAS retrieval may results in a negative slant column, with an error that is of the same magnitude.
 

Geographic outline of the SCIAMACHY nadir state just below the centre in the first graph. The blank rectangles show the outlines of the forward pixels of the nadir state. Colour circles indicate the level of the SO2 slant column (SCD); empty ground pixels have a negative SCD, which indicates that there is very little to no SO2.

On the basis of this schematic outline the following procedure is followed to see whether the nadir state should trigger an alert to be issued. For each ground pixel in the nadir state that has an SCD of more than 3 DU, the pixels around it are scanned. Each of these eight pixels that have an SCD greater than 3 DU counts as +1 point. And each of these pixels that have an SCD less than zero counts as -1 point, as sharp gradients in the SO2 (i.e. from +3 to less than zero) usually indicate that the signal may not be real.

The maximum number of points awarded to any given ground pixel in the nadir state is thus 8. If the number of points is 5 or more, then an alert is issued for this nadir state.

For ground pixels along the edges of a nadir state things are a little different as part of the information needed is missing: it lies outside the state. For that reason, the two points one further along the edge are also considered in the awarding of points, while one point is substracted because the pixel is at the edge. This means that ground pixels along an edge of the state can be awarded at maximum 6 points and can therefore also trigger an alert. (For ground pixels in the corners there really is too little information available to let these trigger an alert.)

To see which predefined geographic regions are to mentioned in the alert for the current state, it is checked within which of these regions lie the centre coordinates of the individual ground pixels of the nadir state. For a geographic region to be marked the centre coordinate should lie inside the region by at least 2 degrees in latitude and longitude.

Criteria for OMI data

Criteria for GOME-2 data