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Stratospheric
Chemistry Measurements by MLS
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The EOS MLS
instrument will provide measurements of many chemical species involved
in the destruction of stratospheric ozone. This instrument is a
greatly enhanced version of the UARS
MLS instrument,
including use of latest technology to measure important species
such as OH, BrO
and many others which could not be measured by MLS
at the time the UARS
instrument was developed, as well as more precise measurements and
measurements over a larger altitude range. The EOS
Aura orbit will allow MLS
measurements to be made to high latitudes every day on each orbit,
whereas the UARS
orbit required MLS
high-latitude coverage to switch, approximately monthly, between
the northern and southern hemispheres with critical periods being
missed.
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O3
& ClO Measurements |
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Chlorine monoxide (ClO) and ozone (O3).
ClO is the predominant
form of reactive chlorine that destroys stratospheric ozone. These
maps of ClO and ozone
were made by the Microwave Limb Sounder on NASA's
Upper Atmosphere Research Satellite (UARS). The map on 21 September
1991 is the first ever made of ClO,
and was obtained fewer than 10 days after launch of UARS.
The full extent of correlation between enhanced ClO
and depleted ozone is evident in these results. ClO
becomes enhanced over the Antarctic because of processes triggered
by polar stratospheric clouds (PSCs) which form at low temperatures.
These processes lead to the conversion of chlorine already in the
stratosphere from relatively 'safe ' forms to
the reactive forms, such as ClO,
which destroy ozone. [For more information, see J.W. Waters et al.,
Nature, Vol. 362, pp.
597-602 (1993)]
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The images above show MLS
O3 &
ClO measurements made
during development of the 1996 Antarctic ozone hole. There are no
such measurements at this time in 1997 due to degradation of the
UARS
power supply system. Timely delivery of the EOS
Aura mission will allow for continuation of measurements which are
critical for understanding ozone depletion.
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Lower
Stratospheric ClO &
HNO3
Measurements |
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The plots above show MLS
measurements from 1993 and 1992 in both the Arctic and Antarctic
regions of ClO and
HNO3.
The EOS Aura MLS
instrument will be able to take measurement of both hemispheres
every day on every orbit, something that UARS
MLS instrument
is not able to do
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Measurements
of ClO, HNO3
and O3
in the 1996 Arctic winter |
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The above plot shows ClO,
HNO3
and O3
measurements made by UARS/MLS
in February 1996 for the northern hemisphere.
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Comparison
of ClO and Temperature
measurements in 1992 and 1993 |
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Temperature (T) and chlorine monoxide (ClO) in the northern hemisphere
lower stratosphere.ClO,
the predominant form of reactive chlorine that destroys ozone, can
become enhanced when lower stratospheric temperatures are below
~195K. Polar stratospheric clouds
occur at these temperatures and trigger processes which convert
chlorine in the stratosphere to reactive forms. This figure shows
the striking difference in northern hemispheric ClO
abundance on 15 February 1993 when temperatures were slightly below
195 K, and on 15 February 1992
when temperatures were slightly above 195 K.
Increasing greenhouse gases will cool the lower stratosphere and,
while stratospheric chlorine abundance remains high, exacerbate
ozone depletion by this process. The ClO
data are from the Microwave Limb Sounder on NASA's
Upper Atmosphere Research Satellite and the temperature data are
from NOAA's
National Meteorological Center. [For more information, see J.W.
Waters et al., Nature, Vol.
362, pp. 597-602 (1993); J.W. Waters
et al., Geophysical Research Letters, Vol.
22, pp. 823-826 (1995)]
JWW:CL.T.FEB.TEX (April 27, 1995)
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Evolution
of ClO in '91 - '92
Northern Hemisphere winter and '92 Southern Hemisphere winter |
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Enhanced ClO in the
Arctic and Antarctic winter vortices. Chlorine monoxide (ClO) is
the predominant form of reactive chlorine that destroys ozone. These
maps are from the Microwave Limb Sounder experiment on NASA's
Upper Atmosphere Research Satellite and show the enhanced abundance
of ClO which occur
in both the Arctic and Antarctic. White contours indicate the approximate
edge of the polar vortices, and green contours indicate where temperatures
are low enough for the polar stratospheric clouds which trigger
ClO enhancement. The
long duration of enhanced ClO
in the Antarctic leads to the ozone hole. Arctic ClO
abundance is comparable in both magnitude and spatial extent to
those in the Antarctic, but occurs over a shorter period of time
with consequently less ozone depletion. [For more information see,
J.W. Waters et al., Nature, Vol.
362, pp. 597-602 (1993); J.W. Waters
et al., Geophysical Research Letters, Vol.
20, pp. 1219-1222 (1993)]
JWW:CLN59192.TEX (April 27, 1995)
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Evolution
of lower stratosphere ClO
& O3 in '92 Southern
Hemisphere winter |
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The plots above illustrate the dominance of downward transport
increasing ozone in early winter and chemistry depleting ozone in
mid-late winter.
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Interhemispheric
Difference in the Evolution of Polar Vortex HNO3
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Interhemispheric Differences in Stratospheric Nitric
Acid. Substantial interhemispheric differences exist in the behavior
of lower stratospheric nitric acid (HNO3),
as shown in these maps from the Microwave Limb Sounder on the Upper
Atmosphere Research Satellite. The colder Antarctic stratosphere
has a greater removal of HNO3,
through sedimentation of polar stratospheric cloud particles, than
the Arctic. The presence of HNO3
moderates chlorine depletion of ozone, but increasing abundance
of greenhouse gases will cool the stratosphere and could lead to
decreased HNO3
and more ozone depletion in the Arctic. The color bar gives HNO3
abundance in parts per billion by volume. White contours show the
approximate edge of the polar vortices. Black contours show temperatures
of 195 and 190K; polar stratospheric
clouds can form when it is colder than ~195K. [for more information,
see M.L. Santee et al., Science, Vol 267, pp 849-852 (1995)]
JJW HNOND.TEX (April 26, 1995)
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::: Return To MLS Instrument Science :::
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