Measurement and Correction of Atmospheric Effects at Different Altitudes for Remote Sensing of Sun-Induced Fluorescence in Oxygen Absorption Bands

Under sunlight illumination, the shape of the atmospheric oxygen bands [O₂-B (687 nm) and O₂-A (760 nm)] of the vegetation radiance is modified by chlorophyll fluorescence as a result of the filling-in process. However, for far-range measurements, atmospheric effects also modify this shape. In this paper, measurements of the depth of O₂-A and O₂-B absorption bands have been performed at different altitudes, from 324 to 3123 m, over different targets, including bare soil and wheat fields. It is observed that bands depth increases significantly with altitude. In the O₂-B band, the total magnitude of variation is on the same order of magnitude as the change induced by vegetation, while it is much greater in the O₂-A band. Three main factors affect the measurements of band depth: 1) the transmittance of the air column between ground and sensor, 2) the path radiance, and 3) the additional contribution of the ground surface outside the field of view scattered by the atmosphere (adjacency effect). We used the MODerate spectral resolution atmospheric TRANsmittance algorithm (MODTRAN) 4 to compute separately these atmospheric and environmental terms, and we corrected airborne measurements to retrieve ground-level band depths. A study was conducted to assess the sensitivity of the absorption band depth to several atmospheric parameters. The magnitudes of these effects were compared to the band depth variation induced by fluorescence. We identified temperature, air pressure, and aerosols as the most critical parameters. After correction of airborne measurements from atmospheric effects, it is found that the variation of band depth with altitude is greatly reduced. The residual variation, particularly in the case of O₂-B band over vegetation, is attributed to an inaccurate assessment of the environment contribution that is scattered by the atmosphere to the sensor. These results show that the MODTRAN - is able to simulate oxygen band depth variations at moderate spectral resolution (i.e., 0.5-1 nm) and can be used to compute atmospheric and environmental factors for corrections of airborne measurements of oxygen band profiles. We also showed that an accurate evaluation of the contribution of environment is needed to correct the band depth from atmospheric effects in the O₂-B band.