FILLING-IN OF FAR-RED AND NEAR-INFRARED SOLAR LINES BY TERRESTRIAL AND ATMOSPHERIC EFFECTS : SIMULATIONS AND SPACE-BASED OBSERVATIONS FROM SCIAMACHY AND GOSAT

TERRESTRIAL AND ATMOSPHERIC EFFECTS: SIMULATIONS AND SPACE-BASED OBSERVATIONS FROM SCIAMACHY AND GOSAT J. Joiner1,∗, Y. Yoshida2, A. P. Vasilkov2, E. M. Middleton1, P. K. E. Campbell3, Y. Yoshida4, A. Kuze5, L. A. Corp6 NASA Goddard Space Flight Center, Greenbelt, MD, USA Science Systems and Applications, Inc., Lanham, MD, USA University of Maryland, Baltimore County, Joint Center for Environmental Technology (UMBC-JCET), Baltimore, MD, USA National Institute for Environmental Studies (NIES), Tsukuba-City, Ibaraki, Japan Japan Aerospace Exploration Agency (JAXA), Tsukuba-City, Ibaraki, Japan Sigma Space Corp., Lanham, MD USA ∗corresponding author, email: Joanna.Joiner@nasa.gov


Introduction
\Iapping of terrestrial vegetation fluorescence from space is of interest because it can potentially provide global information on the functional status of vegetation including light use efficiency and global primary productivity that can be used for global carbon cycle modeling. Space-based measurement of solar-induced chlorophyll fluorescence is challenging, because its signal is small as compared with the much larger reflectance signal. Ground-and aircraft-based approaches have made use of the dark and spectrally-wide 02-A (",,760 nm) and 02-B (",,690 nm) atmospheric features to detect the weak fluorescence signal [1]. lVlore recently, Joiner et a1. [2] and Frankenberg et a1. [3] focused on longer-wavelength solar Fraunhofer lines that can be observed with space-based instruments such as the currently operational GOSAT. They showed that fluorescence can be detected using Fraunhofer lines away from the far-red chlorophyll-a fluorescence peak even when the surface is relatively bright.
Here, we build on that work by developing methodology to correct for instrumental artifacts that produce false filling-in signals that can bias fluorescence retrievals. We also examine other potential sources of filling-in at far-red and NIR wavelengths. Another objective is to explore the possibility of making fluorescence measurements from space with lower spectral resolution instrumentation than the GOSAT interferometer. We computed the filling-in of the 866 nm Ca II line due to rotational-Raman scattering (RRS) using the LIDORT-RRS code of [5]. The filling-in owing to RRS is about a factor of 6 less than that due to an additive signal of 0.2mWm-2 sr-1 nm-1 . vVe also assessed the vibrational Raman scattering (VRS) contribution using the single scattering approximation and found that for typical values of surface albedo over land (>0.2), the filling-in is negligible.

Retrieval methodology
We use the same GOSAT fitting window as [2] (769.90-770.25 nm). We have a second fitting window between 758.45 and 758.85 nm, similar to that used by [3]. We scale the results from the 758 nm window by 0.696 and add them those from the 770 nm window to increase the signal-to-noise ratio (SNR) of the combined additive signal. We also derive an additive signal using the 866 nm Ca II solar line from SCIAMACHY with the spectral band 863.5-868.5 nm.
We use the following simplified model for the observed Earth spectral radiance l(),,) that assumes negligible atmospheric absorption and scattering: where is a reference spectrum ideally containing no filling-in from the source of interest (flu-. The main difference between our IJIJJLVQ.\oH and that of is that we use Qn<,,>~,e., radiance measurements made over the cloudy ocean as a reference rather than a measured or computed solar irradiance spectrum. There are several advantages of using cloudy Earth radiance spectra as a reference to derive a terrestrial additive signal as opposed to solar irradiance spectra (measured or computed) as detailed in 'Vhen comparing the derived additive signals with vegetation indices, we use a quantity called "scaled-F" defined as the retrieved divided by cos(SZA). This scaling roughly accounts for variations in due to the incoming (clear-sky) PAR. We note a significant filling-in (retrieved as F) at 866 nm over parts of the Sahara desert and the Saudi Arabian peninsula where vegetation is sparse. Filling-in over barren regions may be produced by luminescent minerals in soil and/or rock.

Conclusions
Our simulations indicate that terrestrial fluorescence filling-in of the 866nm Ca II line can be detected using hyperspectral instruments (spectral resolutions of the order of tenths of a nm) such as SCIAMACHY if the fluorescence at this wavelength is of the order of 0.1-0.2mWm-2 mn-1 sr-1 .
After corrections for instrumental artifacts, we retrieved an additive signal over land at 866 nm with SCIAMACHY. The magnitude of the derived additive signal at 866 nm is similar to that of our laboratory measurements. The spatial and temporal patterns of the detected additive signals at 866 nm are consistent with a vegetation source; they are similar to those of EVI and those derived from additive signals at 770 and 758 nm where fluorescence from chlorophyll-a in vegetation is stronger and expected to be the primary source of the signals.