Solar-induced Fluorescence Research Articles

Estimation of solar-induced fluorescence using the canopy reflectance index

This article presents a methodology to quantitatively extract the solar-induced fluorescence (SIF) using the canopy reflectance index. The sensitivity analysis was conducted with a spectral vegetation Fluorescence Model (FluorMOD), and the results demonstrate that Sun zenith angle (θ), fluorescence quantum efficiency (Fi), leaf inclination distribution function (LIDF), leaf temperature (T), leaf area index, and leaf chlorophyll a + b content (chl-a+b) had large effects on the fluorescence radiance at 761 nm (LF,761). Based on the results of the sensitivity analysis, the input parameters θ, Fi, LIDF, T, and chl-a+b varied within a certain range during the generation of the simulated data. Based on the simulated data, R740/R630, R685/R850, and R750/R710 were thought to be the best candidates to extract the fluorescence radiation. The quantitative relationships between the fluorescence retrieved by R740/R630, R685/R850, and R750/R710 and LF,761 were analysed and expressed as functions of θ, Fi, T, and reflectance index. The correlation coefficients (r) between the fluorescence retrieved using R685/R850, R740/R630, and R750/R710 and LF,761 are 0.94, 0.95, and 0.95, respectively, and the root mean square errors (RMSEs) were 0.32, 0.29, and 0.30 W m−2 μm−1 sr−1, respectively. Through comparison with FLD and 3FLD, the method presented in this article yielded better results, and could be used to estimate the fluorescence. This methodology provides new insights into the quantitative retrieval of SIF from the reflectance spectrum.

Effects of spectral resolution and SNR on the vegetation solar-induced fluorescence retrieval using FLD-based methods at canopy level

The accuracy of solar-induced chlorophyll fluorescence (SIF) retrieval is dependent on the specifications of the sensor used. Here, we present a quantitative accuracy assessment of the SIF retrieval for different spectral resolution (SR) and signal-to-noise ratio (SNR) at five potential bands (Hα, O2-B, Fe, O2-A, and KI). Firstly, the sensitivities of the SIF retrieval to the correction factors in the FLD algorithm or the radiance noise increased rapidly when the SR decreased, especially for the O2-B band. Then, the dominant factor influencing the SIF retrieval was the SNR for the narrow bands, while was the SR for the broad band. Finally. the accuracy assessment of the SIF retrieval for the potential space-based hyperspectral sensors, including GOSAT TANSO-FTS, SCIAMACHY, OCO-2, Metop GOME-2, and FLEX FLORIS, showed a large error (>35%) for GOSAT, and relative small errors from 5% to 20% for other sensors.

Solar-induced fluorescence of terrestrial chlorophyll derived from the O2-A band of Hyperion hyperspectral images

Satellite-based detection of solar-induced chlorophyll fluorescence, an important remote sensing tool for the assessment of plant physiology and environmental monitoring, is growing popular nowadays. The present work proposes a methodology for the estimation of chlorophyll fluorescence from the existing hyperspectral images acquired by the Hyperion sensor on Earth Observing-1 satellite over the last decade. The principle is to compare the radiation ratio of the O2-A band to that of its adjacent bands for vegetated and non-vegetated regions of the same image. The process is demonstrated with three different images of different years and seasons, and the results are compared with earlier reported values. A simulation with leaf-level data affirms the capability of an instrument with Hyperion’s characteristics to detect chlorophyll fluorescence from O2-A band absorption.

Development of a canopy Solar-induced chlorophyll fluorescence measurement instrument

A portable solar-induced chlorophyll fluorescence detecting instrument based on Fraunhofer line principle was designed and tested. The instrument has a valid survey area of 1.3 × 1.3 meter when the height was fixed to 1.3 meter. The instrument uses sunlight as its light source. The instrument is quipped with two sets of special photoelectrical detectors with the centre wavelength at 760 nm and 771 nm respectively and bandwidth less than 1nm. Both sets of detectors are composed of an upper detector which are used for detecting incidence sunlight and a bottom detector which are used for detecting reflex light from the canopy of crop. This instrument includes photoelectric detector module, signal process module, A/D convert module, the data storage and upload module and human-machine interface module. The microprocessor calculates solar-induced fluorescence value based on the A/D values get from detectors. And the value can be displayed on the instrument's LCD, stored in the flash memory of instrument and can also be uploaded to PC through the PC's serial interface. The prototype was tested in the crop field and the results demonstrate that the instrument can measure the solar-induced chlorophyll value exactly with the correlation coefficients was 0.9 compared to the values got from Analytical Spectral Devices FieldSpec Pro spectrometer. This instrument can diagnose the plant growth status by the acquired spectral response.

Remote Sensing of Solar-Induced Chlorophyll Fluorescence at Atmospheric Oxygen Absorption Band Around 760 nm and Simulation of That Absorption in Laboratory

The present work establishes that the usual method of remote measurement of vegetation reflectance can also detect steady-state solar-induced fluorescence of vegetation. It identifies the hyperspectral signature of chlorophyll fluorescence as a small spike on the reflectance spectral curve of vegetation at the oxygen absorption (O2-A) band around 760 nm. It justifies the origin of the fluorescence signature with both leaf- and pigment-level studies and quantifies it with the following: (1) presence/absence of chlorophyll in leaves; (2) varying concentration of chlorophyll in solution; (3) time of exposure under illumination; and (4) correctness of the calibration of leaf reflectance with respect to the standard white surface. The O2-A band is detectable in solar radiation measured on Earth because of light absorption by the atmospheric oxygen column. This paper suggests an experimental technique to simulate that absorption in laboratory also. Oxygen gas compressed at high pressure can produce the oxygen absorption band artificially at indoor condition.

Passive remote sensing of solar-induced fluorescence spectra of crude oil

The solar-induced fluorescence (SIF) of a simulated oil slick is remotely retrieved from the infilling of the solar Fraunhofer lines of its radiance spectrum acquired by means of a sub-nanometric spectral resolution fluorescence lidar operated in passive mode (i.e. as a spectroradiometer). SIF data, retrieved from the infilling of several Fraunhofer lines selected in the 390–660 nm range, were exploited to reconstruct the oil fluorescence spectrum. This spectrum was finally compared with laser-induced fluorescence spectra acquired on the same oil slick by using the fluorescence lidar. The results open new prospects for the remote sensing of oils and their classification.

Estimating energy balance fluxes above a boreal forest from radiometric temperature observations

The great areal extent of boreal forests confers these ecosystems potential to impact on the global surface-atmosphere energy exchange. A modelling approach, based on a simplified two-source energy balance model, was proposed to estimate energy balance fluxes above boreal forests using thermal infrared measurements. Half-hourly data from the Solar-Induced Fluorescence Experiment, carried out in a Finnish boreal forest, was used to evaluate the performance of the model. Energy balance closure, determined by linear regression, found all fluxes to underestimate available energy by 9% ( r 2 = 0.94). Significance in the energy balance of the heat storage in the air and in the soil terms was also analyzed. Canopy temperatures, measured by a CIMEL Electronique CE 312 radiometer, together with ancillary meteorological variables and vegetation characteristics, were used to run the model. Comparison with ground measurements showed errors lower than ±15 W m −2 for the retrieval of net radiation, soil heat flux and storage heat flux, and about ±50 W m −2 for the sensible and latent heat fluxes. A sensitivity analysis of the approach to typical operational uncertainties in the required inputs was conducted showing the necessity of accurate measurements of the target radiometric surface temperature.

Evaluation of the B‐method for determining actual evapotranspiration in a boreal forest from MODIS data

Boreal forests occupy about 11% of the terrestrial surface and represent an important contribution to global energy balance. The ground measurement of daily evapotranspiration (LEd) is very difficult due to the limitations on experiments. The objective of this paper is to present and explore the applicability of the B‐method for monitoring actual LEd in these ecosystems. The method shown in this paper allows us to determine the surface fluxes over boreal forests on a daily basis from instantaneous information registered in a conventional meteorological tower, as well as the canopy temperature (T c) retrieved by satellite. Images collected by the MODIS (moderate resolution imaging spectroradiometer) on board EOS‐Terra have been used for this study. The parameters of the model were calibrated from the SIFLEX‐2002 (Solar Induced Fluorescence Experiment 2002) campaign dataset in a northern boreal forest in Finland. A study of these parameters was made on an hourly basis in order to make the method applicable, not only at midday but within an interval of 7 h around it. This is an important advance with respect to the original formulation of this approach since the overpass time of satellites can be very variable. The comparison between T c ground measured with a thermal infrared radiometer, and T c retrieved from land surface temperature (LST) MODIS data, showed an estimation error of ±1.4°C for viewing angles from 5 to 60°. A complete sensitivity analysis was carried out and an estimation error of about ±35%, corresponding to the interval 10.00–11.00 h UTC, was shown as the lowest in LEd retrieval. Finally, the method was validated over the study site using 21 MODIS images for 2002 and 2003. The results were compared with eddy‐correlation ground measurements. An accuracy of ±1.0 mm/day and an overestimation of 0.3 mm/day were shown in the LEd retrieval.

Column‐integrated aerosol optical properties in Sodankylä (Finland) during the Solar Induced Fluorescence Experiment (SIFLEX‐2002)

A study has been made of the column aerosols using solar irradiance extinction measurements at ground level in a boreal region (Sodankylä, Finland) during spring 2002. The aerosol properties have been related to air mass origin. In general, the aerosol levels were observed to be very low, independent of the air mass origin, with an aerosol optical depth (AOD) value at 500 nm of less than 0.09 ± 0.03. Two characteristic patterns were observed depending on whether the air masses originated in the north and west or from the south and east. In the first case (north and west origins) the aerosol load was very small, with very low optical depths in the range 0.03 ± 0.02 to 0.09 ± 0.03 for 500 nm wavelengths. The size distribution usually showed two modes, with a strong contribution from the large‐particle mode, probably a consequence of the presence of maritime‐type particles originating in the ocean. When the air masses originated from a south or east direction, the behavior was the opposite to that noted before. In these cases the AOD was rather larger, above all for air masses originating in central Europe and Russia with an average value at 500 nm of 0.14 ± 0.02.

In situ surface temperature retrieval in a boreal forest under variable cloudiness conditions

Canopy temperature retrieval was one of the purposes during the Solar Induced FLuorescence EXperiment (SIFLEX‐2002) of the European Space Agency, carried out in a Finnish boreal forest. In this work, we describe the strategy used to determine this temperature from ground thermal infrared (TIR) data under skies with variable cloud cover. TIR radiance was measured by a CIMEL Electronique CE 312 radiometer. An analysis of the radiative transfer equation showed which terms were necessary to obtain accurate surface temperatures during the campaign. Atmospheric correction was considered negligible due to the small atmospheric path, but hemispheric downwelling sky radiance determination was needed for the emissivity correction. Since most days during the campaign the sky showed partial cloudiness, a methodology to estimate this last term was proposed, using continuous information of cloudiness amount and cloud height given by a weather station. These thermal data were used to analyse some correlations between canopy and air temperatures and plant‐activity‐related variables in the context of the SIFLEX‐2002 campaign.

Detecting solar-induced chlorophyll fluorescence from field radiance spectra based on the Fraunhofer line principle

It is difficult to quantify the amount of chlorophyll fluorescence emitted by a leaf or canopy under natural sunlight because the reflected light obscures the fluorescence signal. In this study, two diurnal experiments were conducted on winter wheat (Triticum aestivum L.) and Japan Creeper (Parthenocissus tricuspidata) to detect the solar-induced chlorophyll fluorescence from field radiance spectra. In the separation of the fluorescence emissive signal from canopy radiance spectrum based on Fraunhofer lines, two Fraunhofer lines of the terrestrial oxygen absorption at 688 and 760 nm were observed in the radiance spectra by an Analytical Spectral Devices FieldSpec Pro NIR spectrometer, which largely overlaps the chlorophyll fluorescence emission spectrum of leaves. Therefore, Fraunhofer lines at 688 and 760 nm were selected to detect the emissive fluorescence. The diurnal changes of chlorophyll fluorescence in the two experiments were primarily affected by the diurnal changes of photosynthetically available radiation (PAR). The correlation coefficients (R/sup 2/) were greater than 0.9 for all the relationships between PAR and the solar-induced fluorescence of winter wheat and Japan Creeper at 688 and 760 nm based on Fraunhofer line-depth (FLD), suggesting that the solar-induced fluorescence could closely track the changes of PAR and chlorophyll fluorescence. The relative solar-induced fluorescence based on FLD was negatively related to Fv/Fm measured by an OS1-FL modulated chlorophyll fluorometer. The correlation coefficients (R/sup 2/) were 0.97 at 688 nm and 0.99 at 760 nm for winter wheat, and 0.79 at 688 nm and 0.78 at 760 nm for Japan Creeper. These results demonstrate that the solar-induced fluorescence from plant canopies can be detected from field radiance spectra based on the Fraunhofer line principle.

Molecular Beams in Space: Sources of OH(A→X) Emission in the Space Shuttle Environment

OH(A→X) emission bands have been observed in the molecular beam jets produced by Space Shuttle engine exhaust using the GLO imager spectrograph located in the payload bay. Spectra were collected at a resolution of 4 Å for both daytime and nighttime solar illumination conditions, all at an altitude of ∼390 km. A spectral analysis is presented that identifies and quantifies four separate OH(A) excitation processes. These include (i) solar-induced fluorescence of the OH(X) in the exhaust flow, (ii) solar-induced photodissociation of H2O in the exhaust at the strong Lyman-α solar emission line (1216 Å), (iii) solar-induced photodissociation of H2O in the far UV, at shorter wavelengths than Lyman-α, and (iv) luminescent collisions between atmospheric species and exhaust constituents, most probably the reaction O + H2O → OH(A) + OH(X). Process (i) produces a very rotationally cold and spectrally narrow component due to the rapid cooling of the OH(X) in the supersonic expansion of the exhaust flow. Processes (ii) and (iii) produce extremely excited OH(A), not well characterized by thermal vibrational or rotational distributions. The O + H2O chemiluminescent reaction has a substantial activation energy, 4.79 eV, and is only slightly above threshold for the ram geometry, where the engine exhaust is directed into the atmospheric wind. Evidence for process (iv) is observed in the night ram but not the night perpendicular exhaust atmospheric interaction, consistent with the threshold energy. Through the use of a nonequilibrium spectral emission model for OH, the integrated intensity, spectral distribution, and OH(A) internal state characterization for each of the above processes was deduced. Additional confirmation of the analysis is provided through the use of a model simulation of the space experiment to predict the total integrated intensities for processes (i) and (ii), for which the underlying spectroscopy, absorption cross sections, and solar excitation intensities are well established. Analysis of process (iii) has established, for the first time, a value for the far-UV conversion efficiency of absorbed photons to OH(A) photons of 0.26, which is twice the established value for Lyman-α. Under the assumption that O + H2O collisions are the source of process (iv), the analysis has established a chemiluminescence cross section at ram conditions of 1.7 × 10-2 Å2. Evidence of OH(A) emission bands from predissociated vibrational levels suggests that the total reaction cross section for process (iv) may be significantly higher. While this cross section assumes a single-step reaction of O with H2O, the possibility of a two-step process of O with other plume species has yet to be explored.

Contribution of fluorescence to the spectral signature and perceived color of corals

Solar‐induced fluorescence from pigments in the host tissues of reef corals can make a significant contribution to their spectral signature and can affect their apparent color as perceived by a human observer. The relative magnitudes of the reflectance and fluorescence contributions to the spectrum can vary as a function of illumination conditions. We have combined measured coral reflectance and fluorescence spectra with modeled downwelling spectral irradiances to investigate the contribution of fluorescence to coral spectra and color. Variations in depth, fluorescence efficiency, and solar zenith angle were modeled. Fluorescence enhancement is greatest when the coral absorbs light at wavelengths that are transmitted well by seawater and emits light efficiently at wavelengths that are moderately attenuated. The methodology provides a means of predicting the combined reflectance and fluorescence spectral signatures and perceived colors of fluorescent corals under arbitrary illumination conditions.

Solar-induced fluorescence (SIF) of C 2 radical

C 2 Swan band emission ( d 3 π g → a 3 π u ) near 517 nm is observed in a solar reactor for fullerene synthesis. On the basis of theoretical considerations and experimentals in the temperature range 3000–3400 K evidence is presented supporting the formation of excited C 2 by absorption of solar photons. This phenomenon that we propose to name: solar-induced fluorescence (SIF) is described for the first time.

Chlorophyll natural fluorescence response to upwelling events in the Southern Ocean

Variability of solar‐induced (natural) fluorescence and chlorophyll were measured on scales of hours to weeks in the upper layer of a cyclonic eddy located south of the Antarctic Polar Front using a free‐floating drifter. The fluorescence signal was analyzed both in terms of chlorophyll concentration and as an indicator of energy distribution in the photosynthetic apparatus. Long‐term trends in fluorescence parallel changes in chlorophyll concentration. Considering a significant positive correlation between fluorescence and the relative depth of the eddy upper layer we hypothesize that the observed short‐term variations in natural fluorescence are a physiological response of phytoplankton to changes in the supply of limiting nutrients. This interpretation is consistent with the Southern Ocean iron limitation hypothesis.

Evidence for a simple relationship between natural fluorescence, photosynthesis and chlorophyll in the sea

To test the proposal that the natural or solar-induced fluorescence of chlorophyll a in the sea provides a purely optical measure of chlorophyll and the rate of photosynthesis in the sea, we studied the relationship between natural fluorescence and photosynthesis in several environments including the central South Pacific, the western Sargasso Sea, and two sheltered bays. The results of 76 such measurements between 2 and 150 m depth and covering a 1500-fold range in production indicate that photosynthesis is highly correlated ( r > 0.9) with natural fluorescence. Furthermore, a substantial portion of the remaining variability can be explained by an examination of the relationships of the quantum yields of photosynthesis and fluorescence as a function of light level. Specifically, the quantum yield of photosynthesis decreases more rapidly than the quantum yield of natural fluorescence with increasing irradiance. Predicted rates of primary production from measures of natural fluorescence and PAR fall within ±30% for half of the samples, and within ±75% for 90% of the cases (assuming that the carbon fixation measurements were without error). The prediction of chlorophyll from natural fluorescence and PAR exhibit similar correlations. This suggests that natural fluorescence measurements, either as a supplement to direct measurements or as independent optical measurements, provide a new and rapid means of estimating gross photosynthesis in the sea.

An in vivo fluorescence response in the Bay of Biscay in June

CONCLUSION AND SUMMARYSurface fluorescence measurements in June in the Bay of Biscay showed little correlation with chlorophyll a measurements due to the presence of a marked diurnal fluorescence rhythm although satisfactory correlations have been obtained at other times of year in the same region (see Fig. 10 of Pingree et al. 1982). By contrast, the beam attenuation coefficient values showed a good correlation with the chlorophyll a measurements and can be used to map the sea surface distribution of chlorophyll a when the diurnal rhythm is present, particularly when the phytoplankton is dominated by a single species. This conclusion may be of considerable practical significance when contemplating synoptic surveys in such situations. As Prezlin & Ley (1980) have observed there is growing awareness that fluorescence per unit chlorophyll a is not a constant that can be used reliably in mixed phytoplankton biomass estimates based on in vivo chlorophyll a fluorescence, and the problem is clearly compounded if some species exhibit a marked diurnal fluorescence rhythm. Furthermore, the present observations support the caveats of Falkowski & Kiefer (1985), that in situ moored fluorometers, the use of air-borne laser-induced fluorescence, or satellite-received passive, solar-induced fluorescence, only measure fluorescence, not chlorophyll per se.The fluorescence response near the shelf-break over a 24 h period following a drogued dahn buoy showed a variation of about a factor of 5 between the day and night fluorescence levels. Beam attenuation coefficient measurements and Coulter Counter analyses showed that these results could not be attributed to the migration of the phytoplankton.

Chlorophyll fluorescence effects in the red part of reflectance spectra of natural waters

Solar-induced fluorescence of algal pigments can be (remotely) measured in productive water masses. Detection of the peak related to this fluorescence in the red part of the reflectance spectra can then be used for the (remote) assessment of the biomass, particularly in the waters where other optical determination methods fail. To study the chlorophyll fluorescence effects in natural waters a two-flow radiative transfer model was developed and measurements were performed in turbid coastal waters. Several phenomena, including the influence of the suspended and dissolved materials on the height and the position (shift) of the peak, and the effects of the vertical stratification of the water column, are described. The observed shift of the reflectance maximum to the longer wavelength appears to be inherent to the presence of the particular material in the water. Reflectance values near the sea surface in the red part of the spectrum are only little influenced by the stratification.

Contamination of the reflectance of natural waters by solar-induced fluorescence of dissolved organic matter

Contamination of the reflectance of natural waters by solar-induced fluorescence of dissolved organic matter