The plot shows the location of the SO2 alerts of the selected month, or
to be more precise: it shows the location of the centre of the
segment that triggered the alert.
Coloured circles are used to symbolise different cases:
- Alerts indicated by red circles, with small (large) circles for
low (high) SO2 concentrations (VCD, given in Dobson Units), are in most
cases probably real (either related to volcanic or to anthropogenic
activity). The higher the solar zenith angle (SZA), however, the more
difficult the SO2 retrieval is, and SO2 concentrations for SZA > 70
degrees or so are suspect.
- Alerts related to measurements taken over the geographic area
known as the South Atlantic Anomaly (SAA) are indicated by yellow
circles for all SZA. These alerts are in most cases false,
although they could be related to activity of volcanoes along the
west coast of South America.
- Alerts related to measurements taken at really high SZA, i.e. at low
Sun, and outside the SAA are indicated by blue circles.
These alerts are most likely false, unless a volcanic cloud with high
SO2 concentrations is moving at high latitudes.
Alerts issued by email are limited to SZA <= 80 degrees.
Data in the plots (maps) was limited to SZA <= 85 degrees up to July 2009;
after that the plots are limited to SZA <= 80 degrees (data files have
data for SZA <= 85 degrees).
Numbers of alerts in the different categories are listed in the table
below (as of August 2009 the 'SZA > 80' column should be zero).