INTRODUCTION

Since 1985, the NASA Ames Airborne Tracking Sunphotometers (AATS-6 and -14) have made extensive measurements of atmospheric constituents via their effect on the Suns direct-beam transmission through the atmosphere. These AATS measurements are used extensively to validate and supplement satellite retrievals of stratospheric and tropospheric constituents, to validate airborne and ground-based LIDAR data products, to characterize horizontal and vertical distributions of gas and aerosol properties, to study closure (consistency) with in situ samplers aboard many aircraft, to test chemical-transport models, and to study the radiative effects of atmospheric constituents and Earth surfaces that are important to both climate and remote measurements.A collaboration between NASA Ames Research Center and Pacific Northwest National Laboratory is underway to develop a prototype airborne spectrometer capable of sky radiance measurements in addition to observations of the direct solar beam. To date this collaboration has yielded a ground based instrument called the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR-G).4STAR The Future Based on AATS

- AATS-like sun tracking and motion control - Direct-beam sun irradiance and sky radiance vs. angular distance from the sun.- Spectrometers allow many more wavelength channels, and with finer resolution.- Improved measurements of H2O, O3, and aerosols in the stratosphere and troposphere, while enabling measurements of additional gases such as NO2. - AERONET-like measurements from aircraft. Past, Present and Future of Airborne Sunphotometry Using the NASA Ames Airborne Tracking Sunphotometers Roy Johnson1, Philip Russell1, Jens Redemann2, John Livingston3, Stephanie Ramirez2, Beat Schmid4, Connor Flynn4, Stephen Dunagan11NASA Ames Research Center, 2Bay Area Environmental Research Institute, 3SRI International, 4Pacific Northwest National Laboratory