is accurately determined. In addition to
the 16 ESRL Cooperative Dobson Network stations,
total column ozone is measured from about 80 sites around the
world. Most of the Dobson instruments in use today have calibrations
that are traceable to Dobson No. 83 that is maintained as the
World Standard Dobson by ESRL as part of the World Meteorological
Organization (WMO) designated World Dobson Calibration Facility.
The earliest record of continuing total column
ozone observations dates back to 1925 at Arosa, Switzerland.
This record and a number of other long-term measurements dating
back 40 years have provided a valuable context for understanding
ozone changes of the past two decades. They have also provided
important continuity for a number of satellite-borne total ozone
instruments that have been deployed in recent decades. Most
of the ground-based total ozone instruments are part of the
WMO coordinated Global Atmosphere Watch program.
For additional information on the Dobson instrument
as well as data from the ESRL sites visit the ESRL