Solar angle matters: Diurnal pattern of solar-induced chlorophyll fluorescence from OCO-3 and TROPOMI

Abstract

Solar induced fluorescence efficiency (SIF) provides a unique chance to capture the dynamics of photosynthesis or gross primary production (GPP). However, the relationship between GPP and SIF observed by sensors (SIFobs) is affected by the solar-target-viewing geometry. Although the effects of viewing geometry on SIFobs and its relationship with GPP have been recognized, solar geometry effects on SIFobs have been ignored and less studied in recent decades. This study used SIFobs from the Orbiting Carbon Observatory-3 (OCO-3) and the TROPOspheric Monitoring Instrument (TROPOMI) to investigate solar geometry effects on GPP-SIFobs relationships at the diurnal scale. Furthermore, we derived total canopy SIF emission (SIFtotal) that should be insensitive to solar-target-viewing geometry in theory. Our results indicated that SIFtotal can effectively mitigate solar geometry effects on SIFobs (including SIF observed at nadir, SIFnadir) and its relationship with GPP from six flux towers. After mitigating the solar angle effects in OCO-3 SIFnadir, the correlation (R2) with GPP increased from 0.73 to 0.87 for SIFnadir to 0.84–0.95 for SIFtotal. The correlations between TROPOMI SIFobs and GPP at three high-latitude sites were also clearly improved by TROPOMI SIFtotal. In addition, the diurnal pattern of SIFtotal was more consistent with that of GPP compared to SIFnadir or SIFobs. This study aims to reveal solar geometry effects on SIF measurements, which should be considered when studying the SIF-GPP relationship especially at the diurnal scale with large variations in solar geometry.