Abstract
Chlorophyll fluorescence is directly linked to the photosynthetic efficiency of plants. As satellite-based remote sensing has been shown to have the potential to derive global information about fluorescence it has become subject of various recently published studies stimulating an upsurge in this research field. This manuscript presents a simple and fast retrieval method for solar induced terrestrial plant fluorescence (SIF) which relies on only a few prerequisites. The spaced based remote sensing spectrometers used in this work typically exhibit an additive spectral feature, which is not fluorescence. This is often accompanying the actual SIF retrieval and can significantly deteriorate the results. To account for this effect a correction method has been developed and is combined with the retrieval. The method has been applied to ten years of SCIAMACHY data with promising results. The retrieved SIF values are lying between 0 and 4 mW [m-²,sr-¹,nm-¹]. However, most of the retrieved values are not exceeding 1.5 [m-²,sr-¹,nm-¹], agreeing with previous studies on the subject. Results have been retrieved for SCIAMACHY spatial resolution of 240 x 30 km² and gridded to 80 arc minutes. A clear seasonal variation could be shown utilizing 10 years of SCIAMACHY data (2002-2012). In absence of large area ground based validation data a final judgment of the results presented is not feasible. However, a direct comparison to data of others was showing similar results for most areas.
Highlights
After chlorophyll molecules in plants absorb light, most of it is used for photosynthesis to convert carbon dioxide to sugars and molecular oxygen, O2
We do not expect significant contribution of chlorophyll-a fluorescence within selected oceanic regions shown in Figure 3 because of the strong liquid water absorption in the Near InfraRed (NIR) spectral range and low concentration of Chl-a in such areas, we investigated the potential impact of chlorophylla fluorescence on the filling-in of Fraunhofer lines
In contrast to the retrieval of εa where the minimization problem given by Equation (15) can be solved using solely measured radiance and irradiance spectra, the retrieval of fluorescent emission strength (FES) requires the proper selection of a Fluorescence Reference (FR) spectrum to solve the minimization problem given by Equation (14), determination of a priori fluorescence emission spectrum Fb(λ) at the top of canopy layer, and auxiliary data to avoid cloud “contamination.”
Summary
After chlorophyll molecules in plants absorb light, most of it is used for photosynthesis to convert carbon dioxide to sugars and molecular oxygen, O2. Chlorophyll fluorescence spectra typically span the spectral range from the red to the near-infrared with two pronounced peaks at around 690 and 740 nm Within this wavelength range the additive signal emitted as SIF at canopy leaf or vegetation level alters the top of atmosphere (TOA) radiance. The result based on GOME-2 data (Joiner et al, 2013) were achieved with 25 spectral principal components from vegetation-free pixels of one day, which could compensate for the zero-offset effects It was shown, that other sources of in-filling such as rotational or vibrational Raman scattering do not play an important role in the Near InfraRed (NIR; Vasilkov et al, 2013) and maximum values will hardly exceed 0.1% in-filling of the solar and telluric absorption lines.
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