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SAGE II Introduction
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The SAGE II (Stratospheric Aerosol and Gas Experiment II) sensor was launched into a 57 degree inclination orbit aboard
the Earth Radiation Budget Satellite (ERBS) in October 1984. During each sunrise and sunset encountered by the orbiting
spacecraft, the instrument used the solar occultation technique to measure attenuated solar radiation through the Earth's
limb in seven channels centered at wavelengths ranging from 0.385 to 1.02 µm. The exo-atmospheric solar irradiance
is also measured in each channel during each event for use as a reference in determining limb transmittances.
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Earth Radiation Budget Satellite
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Solar Occultation Technique
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The transmittance measurements are inverted using the "onion-peeling" approach to yield 1-km vertical resolution
profiles of aerosol extinction (at 0.385, 0.453, 0.525, and 1.02 µm), ozone, nitrogen dioxide, and water vapor.
The focus of the measurements is on the lower and middle stratosphere, although retrieved aerosol, water vapor, and ozone
profiles often extend well into the troposphere under non-volcanic and cloud-free conditions. SAGE II was preceded into orbit
by sister instruments SAM II (Stratospheric Aerosol Measurement II), which has been measuring 1.0 µm aerosol extinction
in the polar regions since 1978, and SAGE I, which provided near global measurements of aerosol extinction
(at 0.45 and 1.0 µm), ozone, and nitrogen dioxide from 1979-1981. Since the solar occultation technique is inherently
self-calibrating, accurate estimates can be made of long-term trends in the retrieved atmospheric constituents to aid in
assessing their role in global change. This is a summary of major results which have emanated from the SAGE II
(augmented by SAM II and SAGE I) measurements.
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Stratospheric Aerosols
Stratospheric aerosols affect the atmospheric energy balance by scattering and absorbing
solar and terrestrial radiation. They can also alter stratospheric chemical cycles by catalyzing
heterogeneous reactions which markedly perturb odd nitrogen, chlorine and ozone levels. Evidence suggests
that these aerosols are small (0.1µm radius) sulfuric acid solution droplets produced primarily through
oxidation of sulfur dioxide injected into the stratosphere by volcanic eruptions.
Figure 1 presents the long-term, weekly-averaged 1 µm optical depth record derived from SAM II
measurements in the Arctic and Antarctic. The figure clearly shows the perturbations caused by the eruptions
of El Chichon in 1982 and Mount Pinatubo in 1991 and the lag time for recovery to pre-eruption conditions.
It is thought that Pinatubo has had the greatest impact on the stratosphere of any volcanic eruption of the century,
producing some three times the 12 megatons of aerosol material produced by El Chichon.
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Figure 1. Long-term, weekly-averaged Optical Depth Record
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Figure 2. Mount Pinatubo Eruption Optical Depth Comparison
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Figure 2 compares the 1.02 µm stratospheric optical depth observed by SAGE II just after
the Pinatubo eruption (June-July 1991) with that observed a year later. Although the volcanic material was
initially concentrated in the Tropics, it spread across virtually the entire globe in the ensuing year and
increased global stratospheric optical depth by a factor of 10 to 100 over pre-eruption levels.
The Pinatubo aerosol layer had warmed the local subtropical stratosphere by about 2.5-3 degrees Celsius
within three months after the eruption, and a statistically significant global average surface cooling was
predicted by the end of 1992.
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Stratospheric Ozone
Beyond the well-known Antarctic ozone hole, significant downward trends in lower stratospheric ozone
at mid-latitudes have been inferred from ground and space-based instruments (see Figure 3).
The stability of measurement systems on time scales of decades is an important element in the validity
of the inferred trends.
SAGE measurements of stratospheric ozone extend from 1979-1981 and 1984-present.
This long-term, stable data set has proven invaluable in determining the decadal trend
in ozone particularly in the lower stratosphere. SAGE ozone measurements are a key element in on-going
assessments of ozone trends by SPARC (Stratospheric Processes and their Role in Climate)
and UNEP (the United Nations Environmental Programme). SAGE III will extend this data set through
much of the next decade.
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Figure 3. Comparative Ozone Trends between SAGE II and other Ground and Space-borne Instruments
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Nitrogen Dioxide
Nitrogen dioxide (NO2) is a key constituent in a catalytic cycle that destroys ozone between altitudes of
30 and 40 km and is also involved in sequestering other chemically active stratospheric species (e.g., chlorine) which
are important to the overall ozone budget. Robust NO2 measurements are, therefore, necessary for the correct
interpretation of observed changes in stratospheric ozone and for predicting its future concentration as well.
Increased industrialization has led to increasing levels of atmospheric N2O, the source gas for stratospheric NO2.
Hence there is a need for long-term global measurements of NO2 in the stratosphere such as those provided by SAGE I and
SAGE II.
Figure 4. Nitrogen Dioxide Number Density Ratios
Of special recent interest has been the effect of the Pinatubo volcanic eruption on NO2 levels. As mentioned earlier,
the aerosols produced by the eruption began to spread to middle and high latitudes within several months afterward.
Concomitantly, SAGE II began to show anomalously low NO2 values in the 25 to 30 km region of the Southern Hemisphere.
Since these altitudes are near the density peak of the normal NO2 vertical profile, anomalously low NO2 column values
would be expected as well. Ground-based twilight NO2 column measurements during the same period from Lauder, New Zealand,
do in fact show a large column decrease relative to the 11-year historical record. In February and March of 1992, SAGE II
began to show large decreases in NO2 in the Northern Hemisphere. Figure 4 shows as a function of altitude and latitude
the ratio of NO2 number densities observed in March 1992 to the average of NO2 number densities observed during
March 1985- 1990. It can be seen that 1992 NO2 levels in the 25 to 30 km region are significantly lower at virtually all
latitudes than the 1985-1990 average, with reductions well in excess of the typical 30 percent interannual variability.
The reductions are thought to be attributable to aerosol-catalyzed heterogeneous chemical reactions mentioned previously.
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Water Vapor
Figure 5. SAGE II Water Vapor Measurements
There is a great need for accurately determining the water vapor distribution in the upper troposphere and lower stratosphere.
Since water vapor plays an important role in tropospheric and stratospheric chemical cycles, photochemical models need an
accurate account of water in all its phases to represent properly natural chemical interactions and their sensitivity to
anthropogenic perturbations. Water vapor is also an important tracer for tropospheric-stratospheric exchange and, as such,
can help in delineating and quantifying the mechanisms for such exchange (e.g., penetrating convection).
With respect to climate models, parameterization of cloudiness depends to a large extent on the accuracy of the relative
humidity in the middle and upper troposphere. There are additional questions concerning the water vapor response if convection
should increase as the Earth's surface warms due to increasing CO2. Most global climate models are currently estimating a
global warming of about 4 degrees Celsius for an atmosphere with doubled CO2, of which 1.6 degrees Celsius is due to the
positive feedback from additional tropospheric water vapor. It has been suggested that increased convection might dry rather
than moisten the upper troposphere, a hypothesis which can be tested by comparing tropospheric water vapor profiles in
convective and non-convective regions. SAGE II water vapor measurements clearly show that increased convection in the summer
hemisphere leads to moistening of the middle and upper troposphere (Figure 5), a finding which contradict the previous
suggestion and qualitatively verifies the positive water vapor feedback in general circulation model studies of climate
sensitivity.
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Summary
The spaceborne SAGE II instrument provided self-calibrating, near global measurements of atmospheric aerosols,
ozone, NO2, and water vapor. These data, in conjunction with data from sister instruments SAM II and SAGE I,
can be used to estimate long-term constituent trends and identify responses to episodic events such as volcanic eruptions.
Major results of these programs include illustration of the stratospheric impact of the 1991 Mount Pinatubo eruption,
identification of a negative global trend in lower stratospheric ozone during the 1980s, and quantitative verification
of the positive water vapor feedback in current climate models. The constituent record provided by SAGE II will be continued
and improved by its successor SAGE III,
which successfully launched onboard a Meteor-3M spacecraft on December 10, 2001.
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