SO2 data and alert service
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Notifications of SO2 events Introduction Archive of notifications per day Archive of notifications per region Subscription to email list

Example of a notification message The near-real time process system evaluates the satellite measurements in separate segments of an orbit to determine whether criteria for an exceptional SO2 concentration are met and a notification of an "SO2 event" have to be send. This page gives some information the content of a notification: Example of a notification email Info on the dedicated web page Some notes on cloud data usage The "full text" of an alert in the archive

 

Example of a notification email

The following is an example of a notification email

SACS notification of exceptional SO2 concentration ================================================== Process date : 2009 05 28 Process time : 19:00:01 CEST Instrument : SCIAMACHY No. notices : 3 Alert notice : 1 -------------------- http://sacs.aeronomie.be/alert/?alert=20090528_190001_001 Start date : 2009 05 27 Start time : 14:28:46.303 UTC Aver. long. : -70.2 deg. Aver. latit. : -19.5 deg. Aver. sza : 51.6 deg. Max. SO2 vcd : 11.3 DU Alert notice : 2 -------------------- http://sacs.aeronomie.be/alert/?alert=20090528_190001_002 Start date : 2009 05 27 Start time : 14:30:50.736 UTC Aver. long. : -72.0 deg. ... [end of file]

At the start of the message, date and time of the analysis process are given, as well as the instrument that made the observations, and the number of notices (alerts) that are in the message -- since several segments are analysis in one process, there can be more than one alert per process.

For each alert the message gives the URL of a dedicated web page about the alert (with info and maps), followed by a few lines of info on the orbit segment that triggered the alert:

• The date and time of the first measurement of the segment.
• The average longitude and latitude of the segment, averaged over all measurements in the segment; this more or less corresponds to the centre of the state.
• The average solar zenith angle ("sza"), averaged over all measurements in the segment.
• The peak value of the SO2 total (vertical) column density ("vcd") somewhere within the segment, in Dobson Units.

If the average solar zenith angle ("sza"), averaged over all measurements in the segment, is very high (SZA > 75 degrees), a 'Remark' line is added:

Remark : measurement taken at SZA > 75 deg.

It is relevant to know this, because measurements of SO2 at high SZA are notoriously difficult and this leads to artifacts in the data and "false alerts" to be issued. To avoid most of these "false alerts", no email message is send for events with an average SZA > 80 degrees. These messages are made, though, and available via the web site. (Note that only data with SZA < 85 degrees is processed.)

Measurements taken over the South Atlantic Anomaly (SAA; see the product information for details) can give rise to artifacts in the SO2 data and therefore to false alerts. If the orbit segment that triggers an alert lies partly in a region defined for the SAA, this remark is added:

Remark : segment lies (partly) in SAA region

Note:   Below the line with "[end of file]" follows a code given between angle brackets; this code is for internal use to check the email service.
 

Info on the dedicated web page

The dedicated web page of a certain alert starts with giving the same info on the location of the segment that triggered the alert (see above), with two additional lines:

• The peak value of the SO2 slant column density ("scd") somewhere within the segment, in Dobson Units.
• Information on the cloud data used for the SO2 total column -- see note below.

Then follow images of SO2 and cloud cover in a 30 by 30 degree region around the segment that issued the alert, plus a plot of the location of the volcanoes in that region (if any). The web page further provides links to the near-real time data pages for the pre-defined geographic regions the segment (partly) lies in.

If no cloud cover information was available, the map of the SO2 shows the values assuming clear-sky conditions and the cloud cover map is empty.
 

Some notes on cloud data usage

The product information (see the menu at the top of the page) provided background information on the retrieval of the SO2 data and how the total (vertical) column density (VCD) is derived. For that derivation it is necessary to have information on the (effective) cloud cover fraction for that pixel.

In can happen that the cloud cover information is not available in the near-real time processing, e.g. because it is retrieved from an external data source. In case cloud info is absent one can assume that the pixel is cloud-free and compute a VCD on the bases of that -- this is probably not accurate but gives at least some information.

For this reason the alert info on the web pages mention briefly something about the cloud data. The following messages can appear:

• used for vcd
  This means that valid cloud cover data was available and used to computed the VCD.
• assuming clear-sky
  This means that valid cloud cover data was not available and that the VCD is computed assuming the pixel is free of clouds.

Assuming that pixels are cloud-free in the cases where no cloud cover information is available, is done only in the near-real time processing, not in the off-line archive processing.
 

The "full text" of an alert in the archive

In the overview per month of the notifications sent each day, sorted by the date of the processing, there is a link called "full text" for each process. The link lead to a text page with the alert notifications of that process. For most users of the notification service this "full text" is not really relevant.

For each alert the page provides the same info as the email alert and dedicated web page -- as described above -- plus some process details: the numbers of the pre-defined regions involved in the alert, the name of the data file with the segment that triggered the alert, and the process version. This additional information is only important for expert users.

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