Segments of an orbit used for the notification

SCIAMACHY and OMI data are received and processed as orbits, whereas the GOME-2, IASI and AIRS data arrives in the 3-minute segments.

SCIAMACHY measures alternatively in nadir and in limb mode along its orbit. The SO₂ service can only use nadir measurements. This in a naturaly way implies that nadir states of SCIAMACHY -- which constitute about one minute of measurements -- are used as segments of orbits for the notification service.

For GOME-2, IASI, AIRS the data arrives in so-called PDU ("Product Dissemination Units") which consitute 3 minutes of observations. The processing evaluates each PDU separately, i.e. each PDU serves as a segment of an orbit for the notification service.

OMI measures in nadir mode along its orbit. Our SO₂ service uses these measurements as segments of orbits for the notification service.
 

The notification process

The near-real time process system evaluates the satellite measurements in the above described separate segments of an orbit. If the SO₂ concentrations in such a segment satifies certain criteria then this segment is marked as showing an exceptional SO₂ concentration, i.e. an "SO₂ event".

The criteria for the notifications are based on the total (vertical) column SO₂ density of a set of ground pixels around a ground pixel that shows a peak value. Details of the criteria are given in the product information.

A single process run can analyse one or more orbits of measurements, each with several segments. Per process run there can thus be one or more segments that merit a notification. A given segment can lie (partly) in one or more of the predefined geographic regions. And more than one segment can issue a notification in a given region. It is at this point not possible to automatically group together different segments that belong to the same SO₂ emission event.

For a given process run, the segments that merit a notification are grouped together in one email message to the users. For each segment the message provided brief information on the location of the event and the total (vertical) column SO₂ value, and it gives a link to a dedicated web page that shows an image of the SO₂, and gives some more details of the event. Click here for an example of a notification message and some additional information on the data given in the notification.
 

Dedicated web page of a notification

The dedicated web page of the notification message points to starts with some info on the segment that triggered the notification, such as the average location and the SO₂ peak value -- see the page on the example of a notification for some details on this.

Then follow images of SO₂ and cloud cover in a 30 by 30 degree region around the segment that issued the notification, 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. That part of the page is generated automatically by the near-real time process.

The SO₂ and cloud cover maps show only the data from the orbit (for SCIAMACHY and OMI) or segment (GOME-2) that triggered the notification, not the data of adjacent orbits -- details to be confirmed

Below the images there is room for additional information, which is added by hand(!) and can, for example, combine different notifications of the same SO₂ emission event (e.g. a volcanic eruption), provide other relevant images, and provide links to external web pages with more information regarding the event.
 

Archive of notifications

The menu on the left, both here and on the dedicated webpage showing the SO₂ event images, gives access to an archive of the notifications sent out by the processing. The events can be accessed in the following ways:

• Archive of notifications per day
  This shows a list of notifications issued per day for a selected month and it comes in two variants:
  • Sorted by the date at which the data was processed
    For each time the process that issued one or more notifications, this gives access to the dedicated web pages with the images of the SO₂ events, as well as the full text of the notification message. The most recent process is listed first. The process time given is the time of the start of the process.

  • Sorted by the date at which the measurements where taken
    For the different processes that issued a notification, this lists the date and time of the processing and gives access to the individual web pages with the images of the SO₂ events.

• Archive of notifications per region
  This shows a list of notifications issued per region for a selected month, along with the date and time of processing and measurement.

These pages also provide links to the near-real time data pages for the regions within which the notification falls.
 

Some further notes and remarks

• Notifications based on the data of the different satellite instruments can results from the same "SO₂ event". But since the data of the different instruments comes along different data streams, they are analysed in separate processes, and they can therefore not be linked automatically to the same event.

• No attempt is made to automatically identify the source of the SO₂, as this would be very tricky -- it could easily lead to false identifications.

• The timeliness of the notifications depends mainly on the delay with which the data is received by the processing system. In most cases the notifications should be available around 3 hours after the satellite took the measurements.
  For part of the SCIAMACHY orbits, however, the delay is usually much more, depending on the ground station that receives the satellite data. As it happens, data from orbits early in the day (orbits starting earlier than, say, 06:00 UTC) may arrive after data from later orbits. And two or three orbits may arrive (and be processes) together.