The total ozone maps are based on ground-based measurements
available from the World
Ozone and Ultraviolet Radiation Data Centre. Preliminary near real-time data
from ground-based observations were also used for the most recent maps. Total
ozone values are given in Dobson
Units. The numbers represent observations taken from ground stations
situated at the bottom left corner of the number.
Maps of deviations represent total ozone deviations from the 1978-1988
level estimated using Total Ozone
Mapping Spectrometer (TOMS) data for all areas except the Antarctic and from
the pre-1980 level estimated using Dobson data over the Antarctic.
Over areas with poor data coverage adjustments are made
according to TOMS on Nimbus-7,
Meteor-3, ADEOS and Earth Probe satellites. Over the polar night area Dobson and
Brewer moon observations and/or NOAA's
TIROS Operational Vertical Sounder (TOVS) satellite data are used. TOVS data
are also used when the more reliable TOMS data are not available. The mapping
algorithm is similar to those used by the WMO
Ozone Mapping Centre.
TIROS Operational Vertical Sounder(TOVS)
TIROS Operational Vertical Sounder(TOVS) is a suite of three instruments:
the Microwave Sounding Unit(MSU), the High resolution Infrared Radiation
Sounder(HIRS), and the Stratospheric Sounding Unit(SSU). Each instrument
measures radiation emmitted by the Earth at several different wavelengths.
The HIRS channel 9 measures Earth's emmitted infrared radiation at 9.7
microns (10-6 meters). This is a "window channel"
meaning that the radiation measured by the HIRS instrument is emmited from
the earth's surface (as opposed to radiation being emmitted at other
levels of the earth's atmosphere). The amount of radiation reaching the
HIRS instrument is dependant upon how much ozone is in the earth's
atmosphere (less ozone = more radiation). Therefore, the TOVS Total Ozone
algorithm uses this channel (along with information from other HIRS
channels) to estimate the total amount of ozone in the earth's atmosphere.
The greatest contribution
of the emmitted radiation occurs in a region between 200 hPa and 30
hPa (13km to 27km). This "lower stratosphere" region is below
the levels where the greatest
contribution to the total ozone amount occurs(50hpa to 10hPa or 20km
to 30km). Thus the ozone amount measured by the TOVS Total Ozone algorithm
is not a true measure of the "total" amount of ozone in the
earth's atmosphere. Rather it is a better measure of the ozone amount in
the lower stratosphere. To obtain a "total" ozone amount, the
TOVS Total Ozone algorithm adjusts the lower stratosphere ozone amount by
a climatological amount that is variable with season and latitude.
is in contrast with satellite instruments which measure the amount of
backscattered radiation at various ultraviolet wavelengths. Backscattered
radiation levels at wavelengths where ozone absorbtion does and does not
take place are compared with the same wavelenghts measured directly from
the sun to derive a "total ozone" amount in the earth's
atmosphere. This methodology is used by the NASA
TOMS and the NOAA
SBUV/2 ozone monitoring programs. This methodology provides a truer
measure of the total ozone amount in the earth's atmosphere. One drawback
is that this method uses "backscattered" sunlight. Which means
that data cannot be retrieved in the earth's shadow or polar night
TOVS Total Ozone algorithm can determine ozone amounts at all times since
it is derived from the Earth's emmitted infrared radiation. There are
drawbacks to the TOVS infrared methodolgy though. When the earth's surface
is either too cold (e.g., the high Antarctic Plateau) too hot (e.g., the
Sahara desert) or too obscured (e.g., by heavy tropical cirrus clouds) the
accuracy of this methodolgy declines.