New methods for the retrieval of chlorophyll red fluorescence from hyperspectral satellite instruments: simulations and application to GOME-2 and SCIAMACHY
Global satellite measurements of solar-induced fluorescence (SIF) from chlorophyll over land and ocean have proven useful for a number of different applications related to physiology, phenology, and productivity of plants and phytoplankton. Terrestrial chlorophyll fluorescence is emitted throughout...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-08-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/9/3939/2016/amt-9-3939-2016.pdf |
| Summary: | Global satellite measurements of solar-induced fluorescence (SIF) from
chlorophyll over land and ocean have proven useful for a number of different
applications related to physiology, phenology, and productivity of plants and
phytoplankton. Terrestrial chlorophyll fluorescence is emitted throughout the
red and far-red spectrum, producing two broad peaks near 683 and 736 nm.
From ocean surfaces, phytoplankton fluorescence emissions are entirely from
the red region (683 nm peak). Studies using satellite-derived SIF over land
have focused almost exclusively on measurements in the far red
(wavelengths > 712 nm), since those are the most easily obtained with
existing instrumentation. Here, we examine new ways to use existing
hyperspectral satellite data sets to retrieve red SIF
(wavelengths < 712 nm) over both land and ocean. Red SIF is thought to
provide complementary information to that from the far red for terrestrial
vegetation. The satellite instruments that we use were designed to make
atmospheric trace-gas measurements and are therefore not optimal for
observing SIF; they have coarse spatial resolution and only moderate spectral
resolution (0.5 nm). Nevertheless, these instruments, the Global Ozone
Monitoring Instrument 2 (GOME-2) and the SCanning Imaging Absorption
spectroMeter for Atmospheric CHartographY (SCIAMACHY), offer a unique
opportunity to compare red and far-red terrestrial SIF at regional spatial
scales. Terrestrial SIF has been estimated with ground-, aircraft-, or
satellite-based instruments by measuring the filling-in of atmospheric and/or
solar absorption spectral features by SIF. Our approach makes use of the
oxygen (O<sub>2</sub>) <i>γ</i> band that is not affected by SIF. The SIF-free
O<sub>2</sub> <i>γ</i> band helps to estimate absorption within the spectrally
variable O<sub>2</sub> B band, which is filled in by red SIF. SIF also fills in
the spectrally stable solar Fraunhofer lines (SFLs) at wavelengths both
inside and just outside the O<sub>2</sub> B band, which further helps to estimate red
SIF emission. Our approach is then an extension of previous approaches
applied to satellite data that utilized only the filling-in of SFLs by red
SIF. We conducted retrievals of red SIF using an extensive database of
simulated radiances covering a wide range of conditions. Our new algorithm
produces good agreement between the simulated truth and retrievals and shows
the potential of the O<sub>2</sub> bands for noise reduction in red SIF retrievals as
compared with approaches that rely solely on SFL filling. Biases seen with
existing satellite data, most likely due to instrumental artifacts that vary
in time, space, and with instrument, must be addressed in order to obtain
reasonable results. Our 8-year record of red SIF observations over land with
the GOME-2 allows for the first time reliable global mapping of monthly
anomalies. These anomalies are shown to have similar spatiotemporal structure
as those in the far red, particularly for drought-prone regions. There is a
somewhat larger percentage response in the red as compared with the far red
for these areas that are drought sensitive. We also demonstrate that
good-quality ocean fluorescence line height retrievals can be achieved with
GOME-2, SCIAMACHY, and similar instruments by utilizing the full complement
of radiance measurements that span the red SIF emission feature. |
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| ISSN: | 1867-1381 1867-8548 |