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Titel |
A Lidar Approach to Measure Atmospheric CO2 Concentrations from Space for the NASA ASCENDS Mission |
VerfasserIn |
Jianping Mao, S. Randolph Kawa, James B. Abshire, Haris Riris |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250044536
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Zusammenfassung |
The National Research Council of the U.S. National Academies has recommended a future
space mission called Active Sensing of CO2 Emissions over Nights, Days, and Seasons
(ASCENDS) in its Decadal Survey. This mission aims “to produce global atmospheric
column CO2 measurements without seasonal, latitudinal, or diurnal bias using simultaneous
laser remote sensing of CO2 and O2.” NASA Goddard Space Flight Center is developing a
lidar approach as a candidate toward this mission.
Our lidar measurement approach uses the 1570 nm CO2 absorption band and
samples one strong absorption line in this band, chosen for minimum temperature
sensitivity, at typically 8 wavelengths. The lasers measure pointed at nadir to better
see through clouds and higher signal-to-noise ratios, meanwhile allowing glint
measurements over the water and enhanced signals over land surfaces and vegetation with
"opposition effect". The pulsed laser wavelengths are extremely narrow and can be
locked within a MHz for high-fidelity measurements. The lasers are pulsed and the
surface echo signal can be well separated from the atmosphere backscatter by using
time gating in the receiver. Thus, this pulse approach can dramatically reduce the
influence of atmospheric scattering, which otherwise is a major error source in CO2
retrievals.
The technique uses all wavelengths to model and correct for (flatten) any residual
wavelength variability in the lidar’s measurement response. It then uses the ratio of the
on-line to off-line signal to retrieve CO2 column density. Measurements at different on-line
wavelengths have vertical weighting functions peaking at different altitudes of interest for
atmospheric CO2. The primary wavelengths near the line midpoints have vertical weighting
functions which peak nearly uniformly in the planetary boundary layer, which responds
nearly uniformly (regardless of boundary layer mixing) to CO2 sources and sinks at
the surface. The wavelength samples nearer the line peak have weighting function
peaking in the mid-troposphere, yielding CO2 transport information in the free
atmosphere.
In order to compute the column mixing ratio, a simultaneous O2 column density
measurement is needed. Our approach uses a line region in O2 A-band near 760 nm for this
with a column differential absorption method. It measures column absorption and line shape
at several on-line and off-line wavelengths near line pair centered at 764.7 nm. This
pair was also selected for minimum sensitivity to atmospheric temperature. The
on-line wavelength is selected in the valley between the O2 doublet lines for the
least temperature sensitivity, vertical weighting function peaking at the surface.
The offline regions are selected on the wings of the line pair. Measurements and
analysis have shown the online absorption is almost a linear function of surface
pressure.
In addition to atmospheric temperature profiles, atmospheric water vapor profiles
are also needed to compute the dry air column. Water vapor data are especially
important in the tropics and summer seasons, and should been known better than
10%.
For a given sensor design the surface returned pulse strength and hence signal-to-noise
ratio (SNR) depends strongly on laser peak power, orbit altitude, the atmospheric
transmission and surface reflectivity. The relative error of retrievals inferred from on-line to
off-line ratio decreases to less than 0.003 or 0.3% (measurement precision target) when laser
peak power increases to 3000 watts or greater. Meanwhile, lower orbit altitude, higher surface
reflectivity and atmospheric transmission will enhance SNR and reduce retrieval errors.
Modeling results using MODIS BRDF-adjusted surface reflectance and CALIPSO aerosols
and clouds optical depth data will be presented and discussed in the presentation. |
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