Solar-Induced Chlorophyll Fluorescence (SIF)
The radiation that chlorophyll emits when exposed to sunlight, known as solar-induced chlorophyll fluorescence (SIF), is closely related to plant photosynthesis and has been used extensively to measure vegetation Gross Primary Productivity (GPP), track GPP phenology, and identify plant diseases and stress-related effects.
Satellites Used to Extract Solar Induced Fluorescence Data
With strong matches to GPP aggregated across vast areas and at monthly to annual time steps, data from many atmospheric chemistry satellites, including TanSat, TROPOMI, GOME-2, OCO-3, OCO-2 and GOSAT, have been utilized to extract SIF products at coarse geographical and temporal scales. However, several issues about what SIF reflects and whether it offers a consistent measure of photosynthesis across geographical scales and vegetation types remain unanswered due to the narrow temporal and spatial dimensions of these satellite SIF outputs.
It is not always evident how environmental factors and geographical and temporal dimensions impact the link between the GPP and SIF at the canopy level. Similar concerns exist about the proportion of the SIF signal regulated by structure and radiation absorption as opposed to physiology through photosynthetic downregulation.
Detecting SIF at Leaf or Canopy Level
The PhotoSpec, FLoX, FluoSpec 2, and FluoSpec are in-situ instruments based on high-resolution "ultraspectral" spectrometers created to detect SIF at leaf level or canopy level. Tower-based SIF sampling has recently been extended to wider regions by drone-based devices that carry high spectral resolution spectrometers without image capabilities.
However, SIF values obtained from various sensors, platforms, and methodologies might vary significantly from one another. Therefore, it is challenging to understand the causes for this variation and to harmonize the data so that it is possible to compare the results appropriately.
Several aerial technologies have been created to collect airborne SIF in the last ten years. Fluorescence Imaging of Red and Far-red Light Yield, Chlorophyll Fluorescence Imaging Spectrometer, and HyPlant are a few examples of imaging spectrometers with very high spectral resolution.
Comparing Aerial with Contemporaneous Ground SIF Measurements
In this work, researchers used fixed and mobile platforms to compare aerial with contemporaneous ground SIF measurements in a soybean field in Nebraska, USA. They compared aerial and ground-based observations over time using fixed continuous measurements and across space using a mobile platform.
Research objectives were to investigate potential error sources when comparing SIF from the ground and airborne platforms, evaluate the effects of decisions made in the atmospheric correction on airborne SIF retrievals using empirical data and develop a method for creating a harmonized SIF data set across instruments and platforms.
Ibis Imaging Fluorometer and Kestrel Imaging Spectrometer
The researchers employed the Nebraska Earth Observatory, which has an imaging spectrometer(Kestrel) and an imaging fluorometer (Ibis), to monitor SIF from the air.
The Ibis imaging fluorometer, an imaging spectrometer with an ultraspectral resolution, has a spectral resolution of 0.245 nm at a sample interval of 0.11 nm and covers the wavelength range of 670–780 nm.
The Kestrel imaging spectrometer has a hyperspectral resolution, covering the wavelength range of 400–1000 nm with a 2.4 nm spectral resolution. Ibis and Kestrel have respective fields of vision of 32.3 and 40 degrees.
SIF Retrieval Techniques Used in this Study
The researchers used the ground data to get SIF using four different SIF retrieval techniques: SpecFit, SFM, improved Fraunhofer Line Discrimination (iFLD), and Fraunhofer Line Discrimination (FLD).
SIF was calculated using SFM, iFLD, and FLD in the O2A and O2B bands, and the SpecFit technique was used to get the SIF spectrum for the whole 670–780 nm spectral range.
Significant Findings of the Study
To achieve harmonized SIF products across platforms, researchers examined the impact of equipment, retrieval techniques, and atmospheric adjustment on SIF. Because of the low reflected brightness of vegetation at the red wavelengths and the confounding effects of the neighboring water bands, all four SIF retrieval techniques were able to acquire more precise SIF findings at the O2A band than the O2B band.
In addition, the SpecFit method's agreement between airborne and ground SIF products was enhanced by removing the water bands.
Airborne SIF retrievals at the O2A band were accurately estimated by integrating ground measurements over a calibration target, and the at-sensor radiance matched with the ground SIF measurements. This platform for SIF signal harmonization allowed for a more thorough comprehension of the temporal and spatial GPP patterns.
For the interpretation of SIF across platforms and scales, it is essential to understand the causes of inaccuracy on SIF retrievals. This will help build a better understanding of utilizing SIF as a photosynthetic measure at local to global sizes.
Ran Wang, John A. Gamon, Gabriel Hmimina, Sergio Cogliati, Arthur I. Zygielbaum, Timothy J. Arkebauer, Andrew Suyker (2022) Harmonizing solar induced fluorescence across spatial scales, instruments, and extraction methods using proximal and airborne remote sensing: A multi-scale study in a soybean field. Remote Sensing of Environment. https://www.sciencedirect.com/science/article/pii/S0034425722003741