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Criteria for exceptional SO2 concentrations

The near-real-time processing issues a notification of exceptional SO2 concentrations by email to users who have subscribed to that service; see this page for an introduction and description of what is provided by the notification service.

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 notification events found by the processing are the source for the continuous (qualitative) validation of the notification service.

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


Criteria for SCIAMACHY data

SCIAMACHY measurements alternate between a nadir and a limb viewing mode. For the SO2 retrieval only the data of the nadir viewing mode can be used, hence the gaps in the plots of the orbits. With this manner of measuring in mind, the natural entity of SCIAMACHY data is a nadir state, which consists of 13 forward and backward scans of 960 km wide (in the normal viewing modes). In the forward direction a nadir state measures about 500 km.

SO2 over Pacaya on 30 May 2010

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 vertical column.

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 vertical column plot. A coloured circle in each ground pixels indicates the value of the SO2 vertical column. Note that if the SO2 concentration is very low, then the DOAS retrieval may results in a negative vertical column, with an error that is of the same magnitude.

outline of a SCIAMACHY nadir state
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 Vertical Column Density (VCD); grey ground pixels have a negative VCD, 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 a notification to be issued. For each ground pixel in the nadir state that has an VCD of more than 2 DU, the pixels around it are scanned. Each of these eight pixels that have an VCD greater than 2 DU counts as +1 point. And each of these pixels that have an VCD 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 a notification 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 a notification. (For ground pixels in the corners there really is too little information available to let these trigger a notification.)

To see which predefined geographic regions are to mentioned in the notification 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.

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