Validation of the data products

In setting up and fine-tuning the SO₂ slant column (SCD) retrieval with the DOAS method, the fitting results have been studied so as to optimise the settings for this retrieval. The slant columns have been compared to other results in an ad-hoc way for this, and in a few case studies. The SO₂ SCD values can thus be considered to be fairly accurate, but a more rigorous quantitative validation is necessary.

The SO₂ vertical column data (VCD) presented on the website are computed using an air-mass factor using look-up tables and an assumed height of the SO₂ plume. This approach provides only a rough estimate of the value of the VCD. Hence, the values given for the VCD should not be taken as hard numbers.

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.

• In the near-real time service, the notification events are listed on dedicated web pages. At the bottom of these dedicated web pages there is room for adding notes and remarks, e.g. to link different notification to one and the same SO₂ event, to link to external information, etc. This has to be done by hand and is therefore done on an an-hoc basis.

• In the archive service, the processing provides an overview of notification cases for each month in the form of a table with some information on each SO₂ peak event found. Most of this table is generated automatically, but there is room to add remarks and other info by hand to each table.