Filling-in of near-infrared solar lines by terrestrial fluorescence and other geophysical effects: simulations and space-based observations from SCIAMACHY and GOSAT

J. Joiner,L. A. Corp,A. Kuze,Y. Yoshida,E. M. Middleton,A. P. Vasilkov,P. K. E. Campbell

doi:10.5194/amt-5-809-2012

Abstract

Abstract. Global mapping of terrestrial vegetation fluorescence from space has recently been accomplished with high spectral resolution (ν/Δν > 35 000) measurements from the Japanese Greenhouse gases Observing SATellite (GOSAT). These data are of interest because they 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. Quantifying the impact of fluorescence on the O2-A band is important as this band is used for photon pathlength characterization in cloud- and aerosol-contaminated pixels for trace-gas retrievals including CO2. Here, we examine whether fluorescence information can be derived from space using potentially lower-cost hyperspectral instrumentation, i.e., more than an order of magnitude less spectral resolution (ν/Δν ~ 1600) than GOSAT, with a relatively simple algorithm. We discuss laboratory measurements of fluorescence near one of the few wide and deep solar Fraunhofer lines in the long-wave tail of the fluorescence emission region, the calcium (Ca) II line at 866 nm that is observable with a spectral resolution of ~0.5 nm. The filling-in of the Ca II line due to additive signals from various atmospheric and terrestrial effects, including fluorescence, is simulated. We then examine filling-in of this line using the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) satellite instrument. In order to interpret the satellite measurements, we developed a general approach to correct for various instrumental artifacts that produce false filling-in of solar lines in satellite measurements. The approach is applied to SCIAMACHY at the 866 nm Ca II line and to GOSAT at 758 and 770 nm on the shoulders of the O2-A feature where there are several strong solar Fraunhofer lines that are filled in primarily by vegetation fluorescence. Finally, we compare temporal and spatial variations of SCIAMACHY additive signals with those of GOSAT and the Enhanced Vegetation Index (EVI) from the MODerate-resolution Imaging Spectroradiometer (MODIS). Although the derived additive signals from SCIAMACHY are extremely weak at 866 nm, their spatial and temporal variations are consistent with chlorophyll a fluorescence or another vegetation-related source. We also show that filling-in occurs at 866 nm over some barren areas, possibly originating from luminescent minerals in rock and soil.

Highlights

Fluorescence from terrestrial vegetation has been measured in the laboratory and with ground- and aircraft-based instruments for several decades
Many studies have shown that in high light conditions and when plants are under stress, fluorescence is correlated with photosynthesis and light use efficiency (LUE) (e.g., Flexas et al, 2002; Louis et al, 2005; Meroni et al, 2008; van der Tol et al, 2009; Zarco-Tejada et al, 2009; Daumard et al, 2010; Amoros–Lopez et al, 2008)
Other research has indicated that chlorophyll fluorescence supplies information content about photosynthetic function that is complementary to reflectance-based spectral vegetation indices including the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI), linked to chlorophyll content, and the Photochemical Reflectance Index (PRI), related to changes in xanthophyll cycle pigments (Meroni and Colombo, 2006; Middleton et al, 2008, 2009; Rascher et al, 2009; Meroni et al, 2008; Daumard et al, 2010; Guanter et al, 2007; Zarco-Tejada et al, 2009; Joiner et al, 2011; Frankenberg et al, 2011b; Guanter et al, 2012)

Summary

Introduction

Fluorescence from terrestrial vegetation has been measured in the laboratory and with ground- and aircraft-based instruments for several decades (see e.g., the review of Meroni et al, 2009, and the many references therein). It is important to quantify the impact of fluorescence on atmospheric absorption bands, such as the O2-A band, that are used for cloud- and aerosol-characterization for other trace-gas retrievals, including CO2 (e.g., Frankenberg et al, 2011a). These fluorescence measurements, with global coverage from satellites, are of interest in their own right because the signal originates from the core complexes of the photosynthetic machinery where energy conversion of absorbed photosynthetically active radiation (PAR) occurs. Fluorescence in vegetation occurs because a portion of the sunlight absorbed by chlorophyll cannot be used for carbon fixation. Other research has indicated that chlorophyll fluorescence supplies information content about photosynthetic function that is complementary to reflectance-based spectral vegetation indices including the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI), linked to chlorophyll content, and the Photochemical Reflectance Index (PRI), related to changes in xanthophyll cycle pigments (Meroni and Colombo, 2006; Middleton et al, 2008, 2009; Rascher et al, 2009; Meroni et al, 2008; Daumard et al, 2010; Guanter et al, 2007; Zarco-Tejada et al, 2009; Joiner et al, 2011; Frankenberg et al, 2011b; Guanter et al, 2012)

Methods

Results

Conclusion