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

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

• In the near-real time service, the alert 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 alert 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 alert 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.

 
[more to come]