AIRBORNE AUTOTRACKING SUNPHOTOMETRY

Philip B. Russell, Robert Wrigley1, David Bergner2, Damon Ried2, Danelle Ketner2, and Rudolf F. Pueschel3

The Ames Airborne Autotracking Sunphotometer measures the attenuation of sunlight by aerosols, clouds, and certain absorbing gases (e.g. water vapor). It does this by tracking the sun and measuring the (relative) intensity of the solar beam in six spectral channels. Each channel consists of a multi-baffled entrance tube, interference filter, photodiode detector, and integral preamplifier. The entrance baffles define a detector field of view (FOV) with measured half angle YD = 2.2[[infinity]]. The six filter/detector/preamp sets are mounted in a common heat sink maintained at 45 +/-1 C. Filters are currently centered at wavelengths 382, 451, 526, 861, 940, and 1020 nm. Filter full widths at half-maximum (FWHM) are about 5 nm. Solar tracking is achieved by azimuth and elevation motors driven by error signals derived from a differential-shadowing sun sensor. Data are digitized and recorded every 2 to 10 seconds. The science data set includes the six detector signals, detector temperature, sun tracker azimuth and elevation angles, tracking errors, and time. Aerosol and cloud attenuation data are analyzed and expressed in terms of their optical depth spectrum. A laptop PC-based control and data system developed in 1994 provides a variety of scientific analysis and display products in real time in the field, as well as instrument control and data storage functions.

The instrument has flown on a variety of aircraft (e.g. the NASA DC-8, C-130, and CV-990, the University of Washington C-131, and the Department of Energy Twin Otter) and has been used to study background and volcanic stratospheric aerosols, smokes from oil and forest fires, and the impact of tropospheric aerosols on remote imaging of the earth's surface.

The sunphotometer was particularly active in 1992-94. In January-March 1992 it flew on the NASA DC-8 in the second Airborne Arctic Stratospheric Expedition (AASE II), measuring the impact of the Pinatubo volcanic aerosol on stratospheric processes, including its potential to affect ozone chemistry and temperatures both in the stratosphere and at the Earth's surface. In May 1992 it flew again on the DC-8 as part of a northern-to-southern hemisphere experiment to validate the SAGE II satellite sensor. In September-October 1992 it flew on the NASA C-130 in the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) to measure the impact that desert haze has on the satellite imagery used to study hydrology in Africa. In May-September 1993 it flew on the NASA DC-8 over Alaska, Central and South America, Australia, and the Pacific, to measure the Pinatubo volcanic decay in conjunction with the SAGE II satellite sensors and lidars. In April 1994 it participated in an instrument intercomparison at the University of Arizona's Mt. Lemmon Observatory as part of preparations for the Boreal Ecosystem-Atmosphere Study (BOREAS). In May-September 1994 it flew on the NASA C-130 in three BOREAS Intensive Field Campaigns (IFCs) in Canada. And in September 1994 it made measurements under Space Shuttle flight STS-64 to provide correlative data for the Lidar In-space Technology Experiment (LITE). The latter measurements were made both on the C-130 in Canada and on the ground in Southern and Central California. The results of these expeditions have been published in a series of journal articles and conference presentations, and further publications and presentations are in progress.

Future plans include participation in the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and the first and second Aerosol Characterization Experiments (ACE -1 and -2), field studies planned for 1995-1997. Options include flights on a variety of occupied and unoccupied aerial vehicles (UAVs). Preparations have included several designs of sunphotometers optimized for flight on UAVs.

Project participants include John M. Livingston of SRI International, Ronald Guzman of Sverdrup Tewchnologies, and Jill Baumann Hughes of San Jose State University..

Ames-Moffett contact: Dr. Philip B. Russell

prussell@mail.arc.nasa.gov

or tel: (415) 604-5404

Headquarters program office: YS

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