Atmospheric Correction for Tower-Based Solar-Induced Chlorophyll Fluorescence Observations at O2-A Band

Abstract

Solar-induced chlorophyll fluorescence (SIF) has been proven to be an efficient indicator of vegetation photosynthesis. To investigate the relationship between SIF and Gross Primary Productivity (GPP), tower-based continuous spectral observations coordinated with eddy covariance (EC) measurements are needed. As the strong absorption effect at the O 2 -A absorption bands has an obvious influence on SIF retrieval based on the Fraunhofer Line Discrimination (FLD) principle, atmospheric correction is required even for tower-based SIF observations made with a sensor tens of meters above the canopy. In this study, an operational and simple solution for atmospheric correction of tower-based SIF observations at the O 2 -A band is proposed. The aerosol optical depth (AOD) and radiative transfer path length (RTPL) are found to be the dominant factors influencing the upward and downward transmittances at the oxygen absorption band. Look-up tables (LUTs) are established to estimate the atmosphere transmittance using AOD and RTPL based on the MODerate resolution atmospheric TRANsmission 5 (MODTRAN 5) model simulations, and the AOD is estimated using the ratio of the downwelling irradiance at 790 nm to that at 660 nm (E 790 /E 660 ). The influences of the temperature and pressure on the atmospheric transmittance are also compensated for using a corrector factor of RTPL based on an empirical equation. A series of field measurements were carried out to evaluate the performance of the atmospheric correction method for tower-based SIF observations. The difference between the SIF retrieved from tower-based and from ground-based observations decreased obviously after the atmospheric correction. The results indicate that the atmospheric correction method based on a LUT is efficient and also necessary for more accurate tower-based SIF retrieval, especially at the O 2 -A band.

SIFSpec: Measuring Solar-Induced Chlorophyll Fluorescence Observations for Remote Sensing of Photosynthesis

“…In order to obtain accurate SIF retrievals at the top of the canopy, the up-welling radiance and down-welling irradiance collected at the flux tower should be corrected to those at the top of canopy, as shown in Figure 5. According to the analysis made by previous studies [39,40], for tower-based measurements, if the height of the flux tower is greater than 10 m, an SIF retrieval bias of about 0.1 mW m −2 sr −1 nm −1 at the O 2 –A band will be introduced as a result of the radiative absorption and scattering between the sensor and the canopy. Compared with the absolute intensity of the SIF signal (<2 mW m −2 sr −1 nm −1 ), this bias is not negligible.…”

“…According to the method proposed in the previous work [39], a simple and operational method based on look-up-tables (LUTs) can be used to estimate the atmosphere transmittance between the tower-based instrument and the top of the vegetation canopy for both the downward irradiance and upward radiance. Atmospheric transmittances at the O 2 –A and O 2 –B bands are mainly related to the aerosol optical depth (AOD), radiative transfer path length (RTPL), atmospheric pressure, and temperature [42].…”

“…It shows that the SIF results are obviously higher after atmospheric correction than the uncorrected SIF results, especially for noon time. For details, please refer to Liu et al (2019) [39].…”

A Spectral Fitting Algorithm to Retrieve the Fluorescence Spectrum from Canopy Radiance

“…The typical measurement set-up follows the measurement protocol described in previous publications [12,[53][54][55][56]. In short, it consists of canopy measurements collected at few meters above the canopy (1-3 m) to limit potential issues with the atmospheric correction [57,58] (Figure 6), but on the other hand allowing us to sample a reasonably significative area of the canopy (about 1-2 m circular spot depending on the canopy height). The data were systematically processed, applying spectral and radiometric calibration procedures, to convert raw digital counts to at-sensor calibrated radiances.…”

Systematic Assessment of Retrieval Methods for Canopy Far‐Red Solar‐Induced Chlorophyll Fluorescence Using High‐Frequency Automated Field Spectroscopy

“…Additionally, while reference measurements from sky or reference panels can be used to constrain ground‐based SIF retrievals, no such reference is available for satellite measurements. When the sensor is close to the ground, the interference of the atmospheric column between the vegetation and the sensor is greatly reduced (Liu et al, 2019; Sabater et al, 2018). Therefore O 2 A‐based methods are usually preferred for ground‐based measurements (Meroni et al, 2009); as the O 2 A band is deeper and wider than the Fraunhofer lines, a stronger SIF signal can be retrieved from this region by lower‐resolution spectrometers (e.g., >0.5 nm FWHM), which are not capable of resolving single Fraunhofer lines.…”