Research Staff: Philip B. Russell Beat Schmid, Jens Redemann, John Livingston, Robert Bergstrom, James Eilers, Richard Kolyer, Duane Allen, Stephanie Ramirez, Dawn McIntosh

Sunphotometry is the measurement of solar beam transmission through the atmosphere. Such measurements, made for narrow bands of ultraviolet, visible, and infrared radiation, provide valuable information on the properties of aerosols and such trace gases as water vapor and ozone. Atmospheric aerosols (suspensions of particles comprising hazes, smokes, and thin clouds in the troposphere and stratosphere) play important roles in influencing regional and global climates, in determining the chemical composition of the atmosphere, and in modifying transport processes. In all these roles aerosols interact with trace gases through processes such as evaporation and condensation, photochemical reactions, and mutual interactions with the radiation field.

Using a single technique, sunphotometry, to study both aerosols and trace gases is often an advantage in understanding their properties and these interactions. A major objective of the Ames airborne sunphotometry team is to make unique measurements of aerosols, water vapor, and ozone that address current scientific questions by taking advantage of the three-dimensional mobility of aircraft and other platforms (Figure 1). Another equally important objective is to use those and related measurements together with models in studies that clarify the roles of aerosols and trace gases in radiative transfer, climate change, and the processes that determine atmospheric composition and transport.

Figure 1. Illustration of how the 6- and 14-channel Ames Airborne Tracking Sunphotometers (AATS-6 and AATS-14) have been used in a variety of experiments. They have flown on many different aircraft to study the globe’s major aerosol types and the associated water vapor and/or ozone. To help validate and extend satellite retrievals, AATS measurements are often coordinated with satellite overflights. Such coordinated measurements–with both nadir-viewers like the Earth Observing System (EOS) Terra platform and limb-viewers like the Stratospheric Aerosol and Gas Experiments (SAGE)–also help place the AATS measurements in a larger spatial context. As shown, AATS-6 and -14 are autotracking instruments that use motors to keep detectors pointed at the sun, independent of aircraft motion. Each instrument’s tracking head mounts external to the aircraft skin, both to increase data-taking opportunities relative to in-cabin sunphotometers and to avoid data contamination by cabin-window effects.

In 2001, efforts of the Ames airborne sunphotometry team focused on integrating AATS-6 and AATS-14 on three aircraft, making many flights in two major field campaigns, and analyzing data from these and previous campaigns. For the Spring 2001 campaign of the Asia-Pacific Aerosol Characterization Experiment (ACE-Asia), AATS-6 was integrated on the National Science Foundation’s C-130 aircraft, and AATS-14 was integrated on the Twin Otter of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS). For the Summer 2001 Chesapeake Light Atmospheric Measurements for Satellites (CLAMS) campaign, AATS-14 was re-integrated on the University of Washington CV-580. AATS-6 and AATS-14 made 15 and 19 flights, respectively, in ACE-Asia, and AATS-14 made another 10 flights in CLAMS. Analysis of AATS data from ACE-Asia and CLAMS is now underway.

Also in 2001, AATS-6 and AATS-14 results from previous experiments were analyzed and published. Results from the July 2000 Puerto Rico Dust Experiment (PRIDE) were published. Results from PRIDE and the August-September 2000 dry season campaign of the Southern African Regional Science Initiative (SAFARI 2000) were presented at several conferences.

The AATS-14 data sets are also being used in continuing studies of several techniques for separating aerosol and ozone contributions to solar-beam attenuation. The goal of these studies is to provide insights into aerosol-ozone separation for the Stratospheric Aerosol and Gas Experiment (SAGE II and SAGE III) spaceborne sensors, particularly when their measurements extend downward from the stratosphere into the troposphere. Related studies by team members combined SAGE II measurements with those by the Cryogenic Limb Array Etalon Spectrometer (CLAES) to develop maps and histories of stratospheric aerosol properties before and after the Pinatubo volcanic injection to the stratosphere.

Members of the Airborne Sunphotometer-Satellite Group are also active in studies of aerosol and trace gas properties and effects that do not necessarily use AATS data. In 2001, these other studies produced journal papers on light absorption by carbon and other materials in aerosols, plus comparisons of water vapor measurements by AATS-6 and many other techniques in a 1997 experiment at the Department of Energy’s Southern Great Plains Site.

Collaborators: NASA Goddard Space Flight Center; Langley Research Center; Jet Propulsion Laboratory; NOAA; Caltech; Georgia Tech; San Jose State University; SUNY; UC San Diego; University of Illinois; University of Washington; Bremen University; Stockholm University; Tokyo University of Mercantile Marine; UK Meteorological Research Flight, UK Meteorological Office; University of Sao Paulo, University of Tokyo