Hyperspectral Remote Sensing: Ground-based Calibration/Validation for Air Quality and Atmospheric Composition Satellite Missions

Hyperspectral remote sensing is the next step up from multispectral, especially in the UV- to visible and near-infrared spectral range, as the need to measure atmospheric composition grows with increasing issues, such as worsening air quality posing a hazard to human health. Currently, the newly launched NASA Tropospheric Emissions: Monitoring of Pollutants (TEMPO) instrument in the GEO orbit will measure air quality hourly over North America. The GeoXO satellite system will have a hyperspectral spectrometer called the Atmospheric Composition Instrument (ACX) to study air quality by measuring trace gases, and aerosols that interact with sunlight at a variety of wavelengths. The ACX will continue observations over North America after TEMPO. Calibration and validation of the satellite instruments can be performed by taking in-situ ground measurements using a spectrometer. The goal of this internship was to show how calibration and validation of satellite measurements is done, using the TROPOMI instrument aboard the Sentinel 5-Precursor by the ESA. We took in-situ measurements with an Ocean SR Miniature Spectrometer, a ASD FieldSpec spectroradiometer, and a MicroTOPS II Sunphotometer. The spectrometer has a spectral range from 190 nm to 1050 nm, and the spectroradiometer has a spectral range of 350 nm to 2500 nm. The sunphotometer has five channels at 440 nm, 500 nm, 675 nm, 870 nm and 1020 nm. We collected measurements with the spectrometer using a Spectralon solar diffuser and optical fiber (spectral range limited to 350 to 1050 nm), and then calculated the radiance. Irradiance measurements were also done with a cosine corrector by aiming the fiber straight up. Measurements were taken with the spectrometer at the same time on the same clear sky day that TROPOMI passed closest to our location, on July 17th and July 28th. The radiance measurements on July 17th at three different times were plotted. Two groups of measurements taken with both the spectrometer and the spectroradiometer were averaged on July 28th. Solar irradiance from each instrument was compared to the top-of-atmosphere irradiance from the standard Thuillier 2002 and TSIS datasets. Gas absorption features are identified from the spectral curves. Challenges in this process are identified, and further improvements are recommended. These techniques will be used in the calibration and validation of measurements of the future GeoXO mission.