Satellite instrument helps tackle mysteries of
ozone-eating clouds
10 April 2006 European
Space Agency.
Polar stratospheric clouds have become the focus of many research projects in
recent years due to the discovery of their role in ozone depletion, but
essential aspects of these clouds remain a mystery. MIPAS, an instrument onboard
ESA’s Envisat, is allowing scientists to gain information about these clouds
necessary for modelling ozone loss.
"The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is
unique in its possibilities to detect polar stratospheric clouds (PSCs) since it
is the first instrument with the ability to observe these clouds continuously
over the polar regions especially during the polar night," Michael Höpfner
of Germany’s Forschungszentrum Karlsruhe GmbH said.
Using data collected by MIPAS, a German-designed instrument that observes the
atmosphere in middle infrared range, Höpfner and other scientists discovered a
belt of nitric acid trihydrate (NAT) PSCs developing in the polar night over
Antarctica in 2003 about one month after the first PSCs, which were composed of
water crystals, were detected.
There are two classifications of PSCs – Type I clouds contain hydrated
droplets of nitric acid and sulphuric acid, while Type II clouds consist of
relatively pure water ice crystals.
The presence of NAT was detected because of MIPAS’ ability to map the
atmospheric concentrations of more than 20 trace gases, including ozone as well
as the pollutants that attack it.
This has been the first evidence for the existence of NAT PSCs on a large
scale," Höpfner said. NAT particles, which contain three molecules of
water and one molecule of nitric acid, enhance the potential for ozone
destruction in polar regions.
The thinning of the ozone is caused by the presence of man-made pollutants in
the atmosphere such as chlorine, originating from man-made pollutants like
chlorofluorocarbons (CFCs). During the southern hemisphere winter, temperatures
drop to very low levels causing the chemicals in the stratosphere, which is in
complete darkness during the winter, to freeze and form PSCs that contain
chlorine.
Now banned under the Montreal Protocol, CFCs were once widely used in aerosol
cans and refrigerators – and have not vanished from the air. 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.
As the polar spring arrives, sunlight returns and creates chemical reactions
in PSCs responsible for converting benign forms of chlorine into highly
ozone-reactive radicals that spur ozone depletion. A single molecule of chlorine
has the potential to break down thousands of molecules of ozone.
NAT PSCs enhance the potential for chlorine activation and can also sediment
and irreversibly remove nitrogen from the lower stratosphere, causing a process
known as denitrification, which slows the return of chlorine to its inactive
form and allows for ozone destruction to continue.
Höpfner and fellow scientists were able to explain
the sudden NAT formation of PSCs in 2003 by temperature disturbances in waves
over the Antarctic Peninsula and the Ellsworth Mountains, suggesting a more
significant role for mountain waves in the formation of Antarctic’s PSCs than
previously thought.
According to Höpfner, the presence of PSCs could intensify in the future due
to a globally changing climate where the Earth’s surface gets warmer due to
trapped greenhouse gases but the stratosphere gets colder, providing an
environment in which the clouds can form. An increase in PSCs could counteract
the recovery of the ozone layer.
Although scientific efforts have focused on determining PSC composition and
their formation mechanisms, the process causing the ozone depletion is far from
understood. In order to gain a better understanding of ozone depletion,
scientists must continue obtaining data which allows them to measure the key
species involved in the process.
|
