The Chlorophyll Fluorescence Imaging Spectrometer (CFIS), mapping far red fluorescence from aircraft
Introduction
In recent years, global remote sensing products of solar induced chlorophyll fluorescence (SIF) have become widely available from satellites such as GOSAT, GOME-2, SCIAMACHY and OCO-2 (Joiner et al., 2011, Frankenberg et al., 2011a, Frankenberg et al., 2011, Guanter et al., 2012, Joiner et al., 2012, Frankenberg et al., 2014, Köhler et al., 2015, Sun et al., 2017). A small fraction of light absorbed by chlorophyll is re-radiated at longer wavelengths (660–800 nm), which is termed chlorophyll a fluorescence and has been widely used in photosynthesis research for decades (e.g. Krause and Weis, 1984, Genty et al., 1989, Moya et al., 2004, Corp et al., 2006, Baker, 2008, Campbell and Middleton, 2008. To first order, steady-state SIF, as measured by satellites, places a direct constraint on the amount of excited chlorophyll a, hence on the total absorbed Photosynthetically Active Radiation (APAR) by chlorophyll only. To second order, the yield of SIF (fraction of excited energy undergoing fluorescence) is also related to the amount of non-photochemical quenching (NPQ), which can provide a constraint on environmental stress. Combined, information on SIF and NPQ provides insight into gross primary production (GPP) and thereby helps us gain a more mechanistic understanding of ecosystem carbon exchange (Porcar-Castell et al., 2014). Unlike what we might expect at the leaf scale (Porcar-Castell et al., 2014, Magney et al., 2017), a surprisingly linear relationship between SIF and GPP was observed globally (Frankenberg et al., 2011, Guanter et al., 2012, Yang et al., 2017), which lacks a thorough mechanistic understanding. Vegetation structure and chlorophyll content affect canopy illumination, scattering and absorption of light within the canopy and escape probabilities of SIF photons emitted at different parts of the canopy. At the temporal and spatial scales observed from space, we are currently unable to cleanly attribute variations in SIF to changes in light interception or environmental stress.
Accurate airborne observations of SIF are needed to I) validate satellite-based SIF retrievals, II) disentangle the contributors to changes in SIF at the canopy and landscape scale and III) testing and improving canopy radiative transfer models. The first proposed airborne instrument was the Fraunhofer line discriminator MKII (Plascyk and Gabriel, 1975). Several studies underlined the importance and possibility of observations at fine spatial scale only possible from aircraft (Rascher et al., 2009, Damm et al., 2011, Damm et al., 2015). HyPlant (Rascher et al., 2015), built in support of a the FLEX satellite mission (FLEX: FLuorescence EXplorer), was the first airborne instrument specifically designed to measure SIF from 670 through 780 nm with a spectral resolution of 0.24 nm. Most of the ground-based and airborne SIF retrievals to date rely on changes in the fractional depth of oxygen absorption features in the 760 and 685 nm bands. Here, we will not discuss these methods but refer to Meroni et al. (2009) for an overview of fitting techniques using oxygen absorptions and to Frankenberg et al. (2011a) for a discussion as to why these are more complex to use in space-borne applications.
All currently available satellite retrievals are based on changes in the fractional depth of solar Fraunhofer lines (absorption features in the sun's photosphere) due to the additive SIF signal. These lines are unaffected by elastic scattering within the Earth's atmosphere. None of the aforementioned satellite missions have been specifically designed to retrieve the SIF signal. However, they have still enabled retrievals because the spectral range of the instruments covers Fraunhofer lines and the oxygen A-band at 760 nm that is used in atmospheric remote sensing to account for scattering effects in the atmosphere. Owing to the narrow bandwidth of current satellite instruments, instruments with spectral resolutions <0.1 nm (GOSAT, OCO-2) only measure a small set of Fraunhofer lines. Others, such as GOME-2 or SCIAMACHY provide a larger spectral range with much higher signal-to-noise ratios (SNR) but at much coarser spectral resolution. Both of these trade-offs impact accuracy and precision of SIF retrievals in various ways. Based on experience from prior satellite missions, we designed and built the Chlorophyll Fluorescence Imaging Spectrometer (CFIS), a dedicated airborne instrument with the specific aim of enabling SIF retrievals using Fraunhofer lines, just as current satellites do. CFIS enables SIF retrievals at a much finer spatial resolution than satellites and features a spectral resolution of 0.07 nm and SNR close to OCO-2 but with a much larger spectral range suitable for retrieval. A simulated nadir radiance observed over a typical canopy is shown in Fig. 1, also indicating the spectral ranges of OCO-2 and CFIS, where CFIS is exploiting the extended wavelength range with isolated Fraunhofer lines around 750 nm while keeping almost the same spectral resolution as OCO-2. The primary objective of CFIS was inter-sensor comparison with OCO-2, which has been obtained using OCO-2 underpasses (Sun et al., 2017). In this manuscript, we focus on a detailed technical description of the instrument, the retrieval strategy and retrieval performance using real data. We also underline the potential of CFIS data using overflights near Mead, Nebraska but focus the discussion on more technical and retrieval-related aspects, with an in-depth scientific interpretation being beyond the scope of this work.
Section snippets
Instrument overview
The rationale for CFIS instrument characteristics is largely based on the proven experiences from GOSAT and OCO-2 retrievals using Fraunhofer lines only. The OCO-2 instrument is a 3-channel grating spectrometer launched into a sun-synchronous orbit on July 2, 2014. SIF retrievals are performed using high resolution spectra of the O2 A-band (0.757–0.775μm, FWHM=0.042 nm) recorded at 3 Hz readout rate with 8 independent along-slit focal plane array readouts. The nominal spatial resolution per
SIF retrieval strategy
In the absence of a complete ILS knowledge, we rely on data-driven methods to retrieve SIF (Guanter et al., 2012). If we consider spectral windows without atmospheric absorption features, we can model the observed spectrally resolved radiance as a function of the state vector elements xi: where I0 is the basis vector containing the average spectral shape in the absence of any fluorescence signal. Without any additive signal, any changes in this
Discussion and conclusions
We have presented CFIS, the Chlorophyll Fluorescence Imaging Spectrometer, the first high-resolution airborne spectrometer to retrieve far-red fluorescence using only Fraunhofer lines for retrieval. CFIS has been used for satellite inter-comparison (Sun et al., 2017) and here, we provide a technical overview, ranging from instrument design to calibration and retrieval theory and real fit examples. We developed a relatively simple and fast retrieval algorithm, which can be applied to all CFIS
Acknowledgments
The research described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. ©2018. Additional funding from NASA's Terrestrial Ecology program is acknowledged (16-TE16-0034).
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