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

Xinjie Liu,Liangyun Liu,Jian Guo,Jiaochan Hu

doi:10.3390/rs11030355

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 O2-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 O2-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 (E790/E660). 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 O2-A band.

Highlights

Solar-induced chlorophyll fluorescence (SIF) has been proven to be an efficient indicator of vegetation photosynthesis [1]
Operational and simple methods for estimating the atmosphere transmittance are needed for accurate tower-based SIF observations. Both the O2-A and O2-B bands have potential for tower-based SIF retrieval, but in this study, we focused on the atmospheric correction at the O2-A band because, (1) the absorption effect at the O2-B band is much weaker than that at the O2-A band, so the atmospheric correction for tower-based SIF observation at the O2-B band is not so important as at the O2-A band; (2) in practice, the SIF retrieved at the O2-A band is more frequently used than that at the O2-B band due to the robustness of retrieval
Different from Sabater et al [30] which mainly focused on theoretical analysis, this study aims to simplify the problem and to provide an operational solution for the atmospheric correction of tower-based observations of both the downwelling irradiance and upwelling radiance to support long-term, continuous spectral measurements

Summary

Introduction

Solar-induced chlorophyll fluorescence (SIF) has been proven to be an efficient indicator of vegetation photosynthesis [1]. A series of algorithms has been developed for SIF retrieval from ground-based [2–8], airborne [9–11] and satellite [12–15] observations. The satellite-based SIF products have shown great potential for global monitoring of vegetation gross primary productivity (GPP) [16–19]. Many uncertainties regarding the relationship between the SIF and GPP remain. The tower-based eddy covariance (EC) technique is widely used for the measurement of the carbon flux at the ecosystem scale [20]. To investigate the relationship between the SIF and GPP, tower-based, continuous spectral observations in conjunction with EC measurements are needed

Objectives

Methods

Results