4STAR Spectrometer for Sky-scanning Sun-tracking Atmospheric Research: Airborne Concepts and Ground Prototype Measurements

The 4STAR (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research) concept combines airborne sun tracking capabilities of the Ames Airborne Tracking Sun Photometer (AATS-14) and Aeronet-like sky scanning capability with state-of-the-art monolithic spectrometry. The overall science goal for the 4STAR is to improve knowledge of atmospheric constituents and their links to climate. The high-resolution spectral capability will improve retrievals of gas constituents (e.g., H2O, O3, and NO2) and thereby improve determination of aerosol properties as residual components of the total optical depth. The sky scanning capability will enable retrievals of aerosol type (via complex refractive index and shape) and aerosol size distribution extending to larger sizes than attainable by direct-beam sun photometry alone. 4STAR also carries the potential for improved cloud microphysical retrievals when combined with spectral measurements of upwelling flux. Vertical profiles of cloud and aerosol properties may then be obtained through combination of the direct-beam and sky-scanning measurements from an airborne platform flying through different altitudes. When flown as a component of an integrated field campaign, the 4STAR would tighten the closure between satellite and ground-based remote sensing measurements while providing an exact match of layers sampled by airborne in-situ instruments. The extensive track-record of the AATS-6 and AATS-14 sun photometers with more than 80 peer-reviewed publications from more than a dozen field campaigns establishes the value of this approach. We have developed a ground-based prototype, 4STAR-Ground. The 4STAR-Ground operating performance has been extensively characterized through laboratory tests, field tests, and instrument intercomparisons. Laboratory tests include calibration of the radiance (sky) measurements with integrating sphere, and a variety of repeatability tests with the fiber optics. Field testing has included FOV scans (for both sun and sky optics), solar tracking measurements, and numerous almucantar and principal plane sky radiance scans. Intercomparisons include a 4-way intercomparison of 4STAR, AATS-14, Prede Sun Photometer, and three Cimel Sun Photometers at Mauna Loa Observatory, Hawaii.