1. Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) Earth Science Division - NASA Ames Research Center 2006 A concept for a sun-sky spectrometer smaller than AATS-14 and usable on manned and unmanned aircraft, both small and large. Background. Since 1985 the NASA Ames Airborne Tracking Sunphotometers (AATS-6 and -14) have made measurements on a wide variety of aircraft in missions over most of the world’s continents and oceans. AATS measurements have been analyzed to yield aerosol optical depth and extinction spectra, aerosol size distributions, water vapor columns and profiles, and ozone columns. The diverse scientific uses, described in >80 journal publications, include validations of measurements by 12 satellite instruments, by two airborne simulators of satellite instruments, and by several airborne and ground-based lidars, plus, via collaborations, studies of aerosol radiative forcing of climate, aerosol light absorption spectra, and consistency (closure) between in situ and radiometric measurements. The AATS instruments measure the direct solar beam transmission through the atmosphere at 6 or 14 discrete wavelengths (near ultraviolet through visible to near infrared) using individual photodiode detectors and filters. Sun-tracking head Optical fiber Entrance aperture Current developments. To provide additional measurement capabilities and also to broaden the types of aircraft usable, we have been exploring advanced instrument concepts and investigating their feasibility. The advanced concepts, called Spectrometers for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) are smaller than AATS-14 while extending AATS capabilities in two ways: Sky scanning: By adding the capability to measure the angular distribution of sky brightness, the new instrument concepts would enable retrievals of aerosol type (via complex refractive index and shape) and aerosol size distribution extending to larger sizes. These capabilities, currently provided on the ground by NASA’s AERONET network, would be extremely valuable in an airborne instrument. Wavelength resolution: By using a spectrometer in place of the discrete photodiodes and filters of AATS, the new concepts would improve accuracy of water vapor and ozone measurements, enable measurements of other gases (e.g., NO 2, SO 2 ) and improve accuracy of aerosol measurements via better aerosol-gas separation.

2. Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) Points of Contact: Dr. Phil Russell 650-604-5404, Philip.B.Russell@nasa.govPhilip.B.Russell@nasa.gov http://geo.arc.nasa.gov/sgg/AATS-website Dr. Beat Schmid Battelle, Pacific Northwest Division 509-375-2996, beat.schmid@pnl.gov Ground prototype. 4STAR feasibility depends on overcoming three technological hurdles: 1. Maintaining calibration to 1% stability over a period of months. 2. Demonstrating stray light rejection to permit measuring skylight within a few degrees of the sun. 3. Devising a fiber optic coupling that maintains 1% calibration stability with as many as possible of the following desirable characteristics: detachable during assembly before calibration; detachable between calibration and scientific measurements; rotatable during measurements. To investigate ways to overcome these hurdles we have developed a ground-based prototype, 4STAR- Ground. Examples of data from 4STAR-Ground are shown. 4STAR research is being conducted as a collaboration between NASA Ames and Battelle, Pacific Northwest Division. Data from 4STAR-Ground. (a) Output counts for four wavelengths during an elevation scan that combines data from diffuse (sky) and direct (sun) channels (b) Direct-sun spectrum. Photo of the ground prototype Spectrometer for Sky-Scanning, Sun Tracking Atmospheric Research (4STAR-Ground). (a) (b) Direct (sun) Diffuse (sky)