Stratospheric Aerosol Measurement II (SAM II) Langley DAAC Project Guide
The Stratospheric Aerosol Measurement II (SAM II) experiment flew aboard the
Nimbus-7 spacecraft and provided vertical profiles of aerosol extinction in
both the Arctic and Antarctic polar regions. The SAM II data coverage began on
October 29, 1978 and extended through December 18, 1993 until SAM II was no
longer able to acquire the Sun. The data coverage for the Arctic region extends
through January 7, 1991, and contains data gaps beginning in 1988 that increase
in size each year. The data coverage for the Antarctic region is continuous
through December 18, 1993 except for a time period from mid-January 1993
through October 1993. The data gaps for both the Arctic and Antarctic regions
are due to an orbit degradation associated with the Nimbus-7 spacecraft.
The SAM II instrument, aboard the Earth-orbiting Nimbus-7 spacecraft,
was designed to measure solar irradiance attenuated by aerosol particles in
the Arctic and Antarctic stratosphere. The scientific objective of the SAM
II experiment was to develop a stratospheric aerosol data base for the polar
regions by measuring and mapping vertical profiles of the atmospheric extinction
due to aerosols. This data base allows for studies of aerosol changes due
to seasonal and short-term meteorological variations, atmospheric chemistry,
cloud microphysics, volcanic activity and other perturbations. The results
obtained are useful in a number of applications, particularly the evaluation
of any potential climatic effect caused by stratospheric aerosols.
The measurement technique is solar occultation. The spectrometer is
activated to take solar irradiance measurements during the periods when the
line-of-sight from the SAM II instrument to the Sun has tangent altitudes
between sea level and 150 km. It is self-calibrating in that an exoatmospheric
measurement is made before a sunset or after a sunrise measurement. Attenuation
of sunlight by a species is measured and processed to produce profile data.
All the points obtained during one day in a given region will be at very
nearly the same latitude, but as time progresses, the latitude of the
measurements will slowly change with the season by one to two degrees each
week. The latitude range for SAM II varies with season. The latitude of the
measurements for years 1978 through 1987 gradually moves from the lowest
latitude, 64 degrees, at the solstices to the highest latitude, 83 degrees, at
the equinoxes. After 1987, the Antarctic coverage gradually moved equatorward
and by 1992, the latitude of measurements moved from the lowest latitude, 53.1
degrees at the solstices to the highest latitude, 69.2 degrees at the
equinoxes. In the Arctic region, the latitude of measurements by 1991
gradually moved from the lowest latitude, 64 degrees a the solstices to the
highest latitude, 86.2 degrees at the equinoxes. The longitude interval for
consecutive sunrises or consecutive sunsets is about 26 degrees.
The SAM II instrument is a single-channel Sun photometer employing a
cassegrainian telescope and interference filter to define the spectral
passband. Solar radiation is reflected off a scan mirror into the telescope
with an image of the Sun formed at the slot plate. The instrument's
instantaneous field of view, defined by the aperture on the slot plate, is a
30-arc-second circle which produces a vertical resolution on the horizon of
approximately 0.5 km. Radiation passing through the aperture is collected with
a field lens, passes through an interference filter, and is measured by a
silicon photodiode detector. The spectral passband, defined by the interference
filter, has a 0.038 micron bandwidth centered at a wavelength of 1.0 micron.
The entire optical and detector system is contained in the azimuth gimbal to
allow the instrument to be pointed at the Sun. Prior to spacecraft sunrise or
sunset, the instrument is moved (i.e., pointed to the predicted solar
acquisition angle). When the Sun enters the instrument field of view, the
instrument locks onto the radiometric center of the Sun within +- arc minute in
azimuth and then acquires the Sun in elevation by rotating the scan mirror.
As the Sun sets or rises relative to the Earth's horizon, the elevation
mirror scans vertically across the solar disk at a nominal rate of 15 arc
minutes per second. The radiometric data are then sampled at a rate of 50
samples per second, digitized to 10-bit resolution, and recorded for later
transmission back to Earth for data reduction.
The SAM II instrument uses the Sun as a constant irradiance source (thus is
self-calibrating before each sunset or after each sunrise) and measures the
radiation that passes through the Earth's atmosphere during a sunrise or sunset.
The photometer assembly scans the Sun at a nominal rate of 15 arc
minutes per second. The photometric data are sampled at a rate of 50
samples-per-second and digitized to 10-bit resolution.
The SAM II instrument, along with a number of other sensors, is mounted on
the Earth-orbiting spacecraft. The orbital characteristics of this spacecraft
determine the frequency and geographic locations of the SAM II measurements.
The mode of operation of the SAM II instrument is such that it takes data
during each sunrise and sunset encountered. The spacecraft has an orbital
period of 104 minutes, which means that it circles the Earth nearly 14 times
per day. There is a measurement opportunity for the SAM II each time that the
spacecraft enters into or emerges from the Earth's shadow. Consequently, the
instrument takes data during approximately 14 sunrises and 14 sunsets each
Earth day. The spacecraft was placed in a high-noon, Sun-synchronous orbit;
that is, the spacecraft crossed the Equator during each orbit at local noon. In
general terms, this means that the orbital plane of the spacecraft was fixed
with respect to the Sun, and thus all sunsets occur in the Arctic region and
all sunrises occur in the Antarctic region.
In the course of a single day, measurements of the stratospheric aerosol
are obtained at 14 points spaced 26 degrees apart in longitude in the Arctic
region and similarly for the Antarctic region. All the point obtained during 1
day in a given region are at very nearly the same latitude, but as time
progresses, the latitudes of the measurements slowly change with the season by
1 to 2 degrees each week, gradually sweeping out the area from approximately
64.0 to 83.0 degrees. The lowest latitude coverage occurs at the solstices
whereas the highest latitudes are measured at the equinoxes.
In the course of 1 week, therefore, the instrument makes about 98
measurements in each region, all in a band of latitude of approximately 1.0
degree. Theses measurements give a fairly spatially dense set of data points.
When the locations of all the measurements obtained in one week are plotted on
a geographic set of axes, one finds that the separation between points is only
about 4.0 degrees in longitude. In a 6-month period of time, the total number
of observations is on the order of 5000.
However, due to an orbit degradation associated with the spacecraft, there
has been a change and disruption in the collection of SAM II data beginning in
1987. During the period of time from 1987 through 1993, orbital precession
caused the spacecraft to cross the equator earlier than the planned high-noon
crossing. This gradually moved the Antarctic coverage equatorward. Initially
the Antarctic latitudinal coverage extended from the lowest latitude, 64.5
degrees at the solstices, to the highest latitude, 81.0 degrees at the
equinoxes. By 1992 the Antarctic coverage gradually shifted to extend from 53.1
degrees at the solstices, to 69.2 degrees at the equinoxes. In the Arctic
region the initial latitudinal coverage extended from the lowest latitude, 64.1
degrees at the solstices, to the highest latitude, 83.0 degrees at the
equinoxes. Gradually by 1991 the highest Arctic latitudinal coverage extended
to 86.2 at the equinoxes.
The orbital precession also affected the spacecraft orientation and
prevented the SAM II instrument from acquiring the Sun for certain periods of
time. In the Arctic region many sunset events were lost because an S-band
antenna blocked SAM II's view to the Sun. Sunset events were lost for the
following periods of time: mid-June through mid-August 1988; mid-March through
mid-September 1989; mid-January through September 1990; and from January 7,
1991, through December 1993. In the Antacritc region the SAM II instrument was
not able to acquire the Sun for the period of time from mid-January through
October 1993. The final 2 months of SAM II data for the Antarctic region were
collected during November and December 1993.
Atmospheric transmittance data at one wavelength are obtained
during solar occultation at each satellite sunrise and sunset. Solar irradiance
measurements are made at 1.0 micron. Each record contains eight parameters:
vertical profiles of extinction km-1, extinction km-1 uncertainty, extinction
ratio, extinction ratio uncertainty, NMC temperature, temperature uncertainty,
and pressure as a function of altitude.
Langley DAAC User and Data Services Office
NASA Langley Research Center
Mail Stop 157D
Hampton, Virginia 23681-2199
Telephone: (757) 864-8656
FAX: (757) 864-8807
Kathleen A. Powell
Aerosol Research Branch
Mail Stop 475
Atmospheric Sciences Division
NASA Langley Research Center
Hampton, Virginia 23681
Telephone: (757) 864-2688
FAX: (757) 864-2671
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Res., 89, no. D4, 5303-5314, June 30, 1984.
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in the Stratospheric Aerosol Associated With the North Cyclonic Polar Vortex as
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Measurements of 1.0 micron Aerosol Extinction in the Free Troposphere,J. Appl. Meteorol., 27, 269-279, March 1988.
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Altitude Stratospheric Aerosols Measured by the SAM II Satellite System in 1978
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McCormick, M. P., H. M. Steele, P. Hamill, W. P. Chu, and T. J. Swissler,
Polar Stratospheric Cloud Sightings by SAM II,J. Atmos. Sci.,
39, no. 6, 1387-1397, June 1982.
McCormick, M. P., C. R. Trepte, and G. S. Kent, Spatial Changes in the
Stratospheric Aerosol Associated With the North Polar Vortex,Geophys.
Res. Lett., 10, no. 10, 941-944, October 1983.
McCormick, M. P., P. Hamill, and U. O. Farrukh, Characteristics of Polar
Stratospheric Clouds as Observed by SAM II, SAGE, and Lidar,J.
Meteorol. Soc. Japan, 63, no. 2, 267-276, April 1985.
McCormick, M. P. and J. C. Larsen, Antarctic Springtime Measurements of
Ozone, Nitrogen Dioxide, and Aerosol Extinction by SAM II, SAGE, and SAGE
II,Geophys. Res. Lett., 13, no. 12, Nov. Suppl., 1280-1283,
McCormick, M. P. and C. R. Trepte, SAM II Measurements of Antarctic PSC's
and Aerosols,Geophys. Res. Lett., 13, no. 12, Nov. Suppl.,
McCormick, M. P. and C. R. Trepte Polar Stratospheric Optical Depth
Observed Between 1978 and 1985,J. Geophys. Res., 92, no. D4,
4297-4306, April 30, 1987.
McCormick, M. P., C. R. Trepte, and M. C. Pitts, Persistence of Polar
Stratospheric Clouds in the Southern Polar Region,J. Geophys. Res.,
94, no. D9, 11241-11251, August 20, 1989.
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Space Research, 13, no. 1, 7-29, 1993.
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Processing,NASA CR-178244, February 1987.
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Aerosol Measurements During the 1989 AASE,Geophys. Res. Lett., 17,
Osborn, M. T., L. R. Poole, and P. -H. Wang, SAM II and Lidar Aerosol
Profile Comparisons During AASE,Geophys. Res. Lett., 17, 401-404,
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Experiment MA-007, Apollo-Soyuz Test Project - Preliminary Science Report,NASA TMX-58173, 1976.
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Stratospheric Clouds Determined from SAM II Observations,Digest of
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(Optical Society of America, Washington D. C., 1990), 4, 206-209.
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Pinatubo and Cerro Hudson on Antarctic Aerosol Levels During the 1991 Austral
Spring,Geophys. Res. Lett., 20, no. 22, 2451-2454, November 19,
Poole, L. R. and M. P. McCormick, Polar Stratospheric Clouds and the
Antarctic Ozone Hole,J. Geophys. Res., 93, no. D7, 8423-8430, July
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Interannual Variability of Polar Stratospheric Clouds and Related Parameters in
Antarctica During September and October,Geophys. Res. Lett., 16,
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on SAM II Observations from 1978-1989, in press, J. Geophys. Res.,
Pueschel, R. F., K. G. Snetsinger, P. Hamill, J. K. Goodman, and M. P.
McCormick, Nitric Acid in Polar Stratospheric Clouds: Similar Temperature of
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17, 429-432, 1990.
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and an Airborne Lidar,J. Atmos. Sci., 38, no. 6, 1295-1312, June
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Livingston, and T. J. Pepin, Satellite and Correlative Measurements of the
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and L. R. McMaster, Satellite Correlative Measurements of the Stratospheric
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SAM II Measurements of the Polar Stratospheric Aerosol:
Vol. I - October 1978 to April 1979, NASA RP-1081.
Vol. II - April 1979 to October 1979, NASA RP-1088.
Vol. III - October 1979 to April 1980, NASA RP-1106.
Vol. IV - April 1980 to October 1980, NASA RP-1107.
Vol. V - October 1980 to April 1981, NASA RP-1140.
Vol. VI - April 1981 to October 1981, NASA RP-1141.
Vol. VII - October 1981 to April 1982, NASA RP-1164.
Vol. VIII - April 1982 to October 1982, NASA RP-1165.
Vol. IX - October 1982 to April 1983, NASA RP-1244.
CDC - Control Data Corporation DAAC - Distributed Active Archive Center DBMS - Database Management System EOSDIS - Earth Observing System Data and Information System ERB - Earth Radiation Budget GSFC - Goddard Space Flight Center GUI - Graphical User Interface IMS - Information Management System LaRC - Langley Research Center MET - Meteorological NASA - National Aeronautics and Space Administration NMC - National Meteorological Center NOAA - National Oceanic and Atmospheric Administration PSC - Polar Stratospheric Cloud SAGE - Stratospheric Aerosol and Gas Experiment SAGE I - Stratospheric Aerosol and Gas Experiment I SAGE II - Stratospheric Aerosol and Gas Experiment II SAM II - Stratospheric Aerosol Measurement II UDS - User and Data Service URL - Uniform Resource Locator