The ozone
hole typically persists until November or December, when increasing regional
temperatures cause the winds surrounding the South Pole to weaken, and
ozone-poor air inside the vortex is mixed with ozone-rich air outside it.
The ozone hole of 2002 was an
exception to this general pattern, when a late September slowdown of the polar
vortex caused the ozone hole to split in two and dissipate early. Envisat's
predecessor mission, ERS-2, monitored the process.
"Envisat carries an instrument
called the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography
(SCIAMACHY), based on a previous instrument flown aboard ERS-2, called the
Global Ozone Monitoring Experiment (GOME)," said Henk Eskes of the Royal
Netherlands Meteorological Institute (KNMI). "The two instruments give us a
combined data set that stretches over ten years, one that Envisat adds to every
day with fresh observations.
"This data set presents a very good means of eventually identifying long-term
trends in ozone. Whether or not the ozone layer is starting to recover is a
hotly debated topic at the moment."
The stratospheric ozone layer
protects life on Earth from harmful ultraviolet (UV) radiation. The ozone
thinning represented here is ultimately caused by the presence of man-made
pollutants in the atmosphere such as chlorine, originating from man-made
pollutants like chlorofluorocarbons (CFCs).
Now banned under the Montreal
Protocol, CFCs were once widely used in aerosol cans and refrigerators. CFCs
themselves are inert, but ultraviolet radiation high in the atmosphere breaks
them down into their constituent parts, which can be highly reactive with ozone.
Just because they were
banned does not mean these long-lived chemicals have vanished from the air, so
scientists expect the annual South Polar ozone hole to continue to appear for
many years to come.
During the southern hemisphere
winter, the atmospheric mass above the Antarctic continent is kept cut off from
exchanges with mid-latitude air by prevailing winds known as the polar vortex.
This leads to very low temperatures, and in the cold and continuous darkness of
this season, polar stratospheric clouds are formed that contain chlorine.
As the polar spring arrives, the
combination of returning sunlight and the presence of polar stratospheric clouds
leads to splitting of chlorine into highly ozone-reactive radicals that break
ozone down into individual oxygen molecules. A single molecule of chlorine has
the potential to break down thousands of molecules of ozone.
ESA's ten-instrument Envisat spacecraft carries three instruments to measure the
atmosphere; the results here come from SCIAMACHY, which provides global coverage
of the distribution of ozone and other trace gases, as well as aerosols and
clouds.
KNMI processes SCIAMACHY data in
near-real time as the basis of an operational ozone forecasting service. This is
part of a suite of atmospheric information services provided by a project called
TEMIS (Tropospheric Emission Monitoring Internet Service) that also includes UV
radiation monitoring and forecasting.
TEMIS is backed by ESA as part of the Agency's Data User Programme, intended to
establish viable Earth Observation-based services for communities of users.
The TEMIS atmospheric ozone
forecast seen here has atmospheric ozone measured in Dobson Units (DUs), which
stands for the total thickness of ozone in a given vertical column if it were
concentrated into a single slab at standard temperature and atmospheric pressure
– 400 DUs is equivalent to a thickness of four millimetres, for example.
Envisat results to be revealed
Launched in March 2002, ESA's Envisat satellite is an extremely powerful means
of monitoring the state of our world and the impact of human activities upon it.
Envisat carries ten sophisticated optical and radar instruments to observe and
monitor the Earth's atmosphere, land, oceans and ice caps, maintaining
continuity with the Agency's ERS missions started in 1991.
After two and a half years in
orbit, more than 700 scientists from 50 countries are about to meet at a special
symposium in Salzburg in Austria to review and discuss early results from the
satellites, and present their own research activities based on Envisat data.
Starting next Monday, the Envisat
Symposium will address almost all fields of Earth science, including atmospheric
chemistry, coastal studies, radar and interferometry, winds and waves,
vegetation and agriculture, landslides, natural risks, air pollution, ocean
colour, oil spills and ice.
There are over 650 being
presented at the Symposium, selected by peer review. Presentations will include
results on the Prestige oil spill, last year's forest fires in Portugal, the
Elbe flooding in 2002, the evolution of the Antarctic ozone hole, the Bam
earthquake and pollution in Europe.
Numerous demonstrations are
planned during the week in the ESA Exhibit area. An industrial consortium
exhibit on the joint ESA-European Commission Global Monitoring for Environment
and Security (GMES) initiative is also planned.
http://www.esa.int/esaCP/SEM3B90XDYD_index_0.html
