Stratospheric Aerosol Measurement II

Project Description

Scientific Objectives:

The Stratospheric Aerosol Measurement (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.

Project Description: The SAM II instrument consists of a single-channel Sun photometer with a 0.04 micron passband centered at a wavelength of 1.0 micron. This is a region of the spectrum where absorption by atmospheric gases is negligible; consequently, any attenuation of sunlight is due to scattering by aerosol particles and air molecules. In operation, the instrument is activated shortly before each sunrise or sunset encountered by the satellite. A sensor with a wide field-of-view is used to indicate the Sun's presence. Two similar sensors then point the SAM II to within +-0.03 degrees in azimuth (left and right). A mirror begins a rapid vertical scan until the Sun's image is acquired by the SAM II telescope. The mirror then slowly scans vertically across the Sun at a rate of 0.25 degree per second reversing itself each time a Sun-limb crossing occurs. The entrance window to the SAM II telescope only passes sunlight of wavelengths greater than 0.9 micron. A circular aperture placed at the image plane serves to define the instantaneous field of view of the instrument to be 0.5 minute of arc. This corresponds to a vertical resolution in the atmosphere of approximately 0.5 km altitude. From the telescope, the light is directed through an interference filter, which rejects all but the 1.0 micron wavelength (+-0.02 micron) passband, to a photodiode detector. The solar intensity as a function of time is digitized, recorded, and periodically telemetered back to Earth. A description of the SAM II instrument, and of the experiment in general, is given by McCormick et al. (1979). The SAM II instrument, along with a number of other sensors, is mounted on the Nimbus 7 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 Nimbus 7 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 Nimbus 7 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 points 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. These 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 Nimbus 7 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 Nimbus 7 spacecraft to cross the equator earlier than the planned high-noon crossing. This gradually moved the Antarctic coverage equatorward and the maximum latitudinal Arctic coverage slightly poleward. 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 degrees 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 the present. In the Antarctic 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, and no further data is expected beyond 1993. Data Used and Produced: The SAM II satellite data are processed after being telemetered to the ground, with the data on solar intensity versus time being mathematically inverted to yield extinction coefficient versus altitude (extinction profile) for each sunrise or sunset event. The mathematical inversion used is described by Chu and McCormick (1979). The basic data product, therefore, is the extinction profile obtained during each measurement opportunity, which can be analyzed to determine the spatial and temporal variations in the upper tropospheric and stratospheric aerosol. These extinction data are archived at the Langley Distributed Active Archive Center (DAAC), NASA Langley Research Center, Hampton, VA, after being subjected to an extensive validation program including comparisons with correlative aerosol observations. A detailed description of the archived data products is given in the SAM II Data User's Guide (Chu et al.,1988). The SAM2_AERO_PRF_NAT data set contains over 15 years of polar Arctic and Antarctic aerosol profiles obtained with the SAM II satellite experiment. The data coverage begins October 1978 and extends through December 1993. For each measurement event, vertical profiles of extinction km-1, extinction km-1 uncertainty, extinction ratio, extinction ratio uncertainty, NMC temperature, temperature uncertainty, and pressure are provided as a function of altitude. Questionable profiles identified using the procedures described in the SAM II Data User's Guide have been removed from this data set. In addition, a small number of events displaying incorrect measurement locations were deleted.

Project Archive Contact: Langley DAAC User Services Office
Mail Stop 157D
NASA Langley Research Center
Hampton, VA 23681-0001
Phone: (757) 864-8656
FAX: (757) 864-8807
Home Page:

Project Manager Contact: M. Patrick McCormick

The following list of references is provided as a starting point for
someone wishing to learn more about the SAM II instrument, inversion
method, validation studies and recent scientific studies.
Albritton, D. L., et al., Scientific Assessment of Stratospheric
Ozone: 1989, WMO Global Ozone Research and Monitoring Project Report
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Chu, W. P. and M. P. McCormick, Inversion of Stratospheric Aerosol and
Gaseous Constituents From Spacecraft Solar Extinction Data in the
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Chu, W. P., M. T. Osborn, and L. R. McMaster, SAM II Data Users'
Guide, NASA RP-1200, July 1988.
Hamill, P. and L. R. McMaster, Polar Stratospheric Clouds - Their Role
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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