A11F-01 INVITED
Overview of the Present stage of GOSAT Project
The Greenhouse gases Observing SATellite (GOSAT) project was started by Ministry of Environment, Japan Aerospace Exploration Agency and National Institute for Environmental Studies in 2004, and it will be launched in 2008. The critical design review is over and the proto-flight model is under integration. The main sensor is to observe the scattered sun light on Earth surface by an FTIR spectrometer at 1.6 mm and 2.0 mm with 0.2cm- 1 resolution. Additional oxygen band sensor at 760nm and thermal infrared covering 700-1800 cm-1 range are equipped on the same interferometer. The advantage of FTS observation is that it covers wide wavelength range including no absorption or water absorption saturation, as well as the wavelength of methane band at 1.7 mm. The signal without molecular absorption give us the information on the baseline power density, and the signal at strong H2O absorption range is a good index to evaluate a high altitude thin cloud; cirrus. The disadvantages are the viewing size is large, 10.8 km, and the cloud contamination change is large. The time required to obtain one spectrum is long, 4 second, in order to get a high quality spectrum better than 300 in SN ratio, which is a request of retrieval analysis. A pointing mechanism to fix the viewing point during the observation is required. This is used to switch between the nadir observation over land and the sun-glint observation over ocean without changing the satellite attitude. We use a Gauss-Newton method for retrieving column integrated CO2 and CH4 concentrations. The surface reflection spectrum is solved simultaneously assuming a relatively smooth spectrum with some parameters. The disturbances coming from path radiance caused by cirrus and aerosol are corrected. Some sensitivity analyses showed that assumptions of cirrus cloud-top height and cloud geometrical thickness had large influence on retrieved CO2 concentration, but we are challenging to solve this problem using the water saturated band and the oxygen band information. In order to examine the validity of the retrieval algorithm, we did two flight campaigns in Australia and Russia in 2007 using a similar spectrometer as the GOSAT flight model. Some results to evaluate the performance of GOSAT retrieval algorithm will be presented.
A11F-02 INVITED
The NASA Orbiting Carbon Observatory: Development Status
The Orbiting Carbon Observatory (OCO) will be launched into the Earth Observing System Afternoon Constellation (A-Train) in December 2008. This NASA Earth System Science Pathfinder (ESSP) mission will make spatially resolved measurements of the column-averaged CO2 dry air mole fraction, XCO2, over the sunlit hemisphere the Earth. These measurements will be analyzed with chemical tracer transport models to retrieve CO2 sources and sinks on regional scales and quantify their variability over the seasonal cycle. The observatory consists of a dedicated, 3-axis stabilized spacecraft bus that carries and points a single instrument. The instrument incorporates 3 bore sighted, high resolution grating spectrometers that will make coincident measurements of reflected sunlight in near-infrared CO2 and molecular oxygen (O2) bands. High spectral resolution (λ/Δλ>20,000) measurements within the CO2 absorption bands near 1.61 and 2.06 μm yield column abundance estimates that are most sensitive to the CO2 mixing ratios near the surface, where most sources and sinks are located. High resolution (λ/Δλ>17,000) measurements within the 0.765-μm O2 A-band spectra yield clear-sky surface pressure estimates with accuracies near 1 mbar over most of the sunlit hemisphere and constrain cloud and aerosol profiles to reduce uncertainties associated with multiple scattering. The instrument is currently being integrated and tested at the NASA Jet Propulsion Laboratory. Preliminary tests of the instrument focus indicate that all 3 spectrometers meet the stringent spectroscopic performance requirements needed to yield regional-scale XCO2 estimates with errors and systematic biases no larger than 0.3%. http://oco.jpl.nasa.gov
A11F-03
CO2 Retrievals Using Solar-Absorption Measurements by the OCO Spectrometer and an FTS
The Orbiting Carbon Observatory (OCO) mission (Crisp, D., et al., Adv. Space Res., 34, 700-709, 2004) will make the first global, space-based measurements of atmospheric CO2 with the precision and coverage needed to characterize CO2 sources and sinks on regional scales. During its 2-year mission, OCO will fly in a sun-synchronous orbit with a 16-day ground-track repeat time, just ahead of the EOS Aqua platform. OCO incorporates three bore-sighted high-resolution spectrometers (δν ≈ 0.3 cm-1) to measure reflected sunlight in the O2 A-band (0.76 μm) and two CO2 bands at 1.61 and 2.06 μm, respectively. Each sounding recorded in these three bands will be analyzed simultaneously to retrieve the column-averaged CO2 dry air mole fraction (XCO2) with a retrieval algorithm that incorporates an atmospheric radiative transfer model, an instrument simulator model, and an inverse method (Bösch, H., et al. J. Geophys. Res., 111, D23302, doi:10.1029/2006JD007080, 2006). In order to verify and improve the space-based CO2 measurements, the OCO project incorporates a comprehensive validation program based on ground-based Fourier Transform Spectrometers (FTS) measuring direct sunlight. These high-resolution measurements (δν = 0.011 cm-1; OPD = 45 cm) are ideally suited to OCO validation since their vertical sensitivities are very similar and the same O2 and CO2 absorption bands are used. As an important part of this strategy, solar-absorption spectra will be simultaneously acquired at JPL by an FTS and the OCO spectrometers during its first calibration tests planned in September, 2007. These measurements will be analyzed using the OCO retrieval algorithm as well as GFIT (an algorithm designed specifically for FTS analysis.) We will discuss the planned validation exercise (e.g., FTS vs OCO spectrometry, XCO2 inter-comparison), the solar-absorption measurement and present first results of the OCO instrument line shape (ILS) and the retrieved XCO2.
A11F-04
A Phillips-Tikhonov Based Carbon Dioxide Retrieval Algorithm: Technique and First Validation Results
Space borne remote sensing measurements of the atmospheric CO2 column have become feasible recently through the SCIAMACHY instrument aboard ESA's Envisat platform. Nadir observations of near-infrared solar radiation backscattered from the Earth's surface and atmosphere allow for CO2 column retrievals with sensitivity down to surface level. In the near future, dedicated space missions such as the OCO and GOSAT instruments aim at retrieving atmospheric CO2 columns with an accuracy that facilitates the determination of CO2 sources and sinks on regional scales. We present a retrieval algorithm based on a Phillips-Tikhonov regularization scheme that targets at the retrieval of CO2 column abundances from these different remote sensing platforms. The algorithm retrieves the vertical profile of CO2 by minimizing the least-squares difference between the observed spectrum and the forward model given the norm of the retrieved profile as side-constraint. In a first exercice, we retrieved CO2 abundances with 1 to 2 degrees of freedom from nadir measurements of SCIAMACHY in the near-infrared spectral range assuming a non-scattering atmosphere. These satellite retrievals are compared to coinciding direct sun observations over Park Falls, Wisconsin, USA, performed at very high spectral resolution by a ground-based FTS within the TCCON network. Applying our retrieval algorithm to the ground-based FTS spectra yields the vertical CO2 profile with 3 to 4 degrees of freedom. Our approach provides a full characterization of ground-based and satellite retrievals through the respective averaging kernel matrices making thorough validation studies possible. Preliminary results show promising agreement between the CO2 columns retrieved from both sensors. In the future, we plan to refine our algorithm by simultaneously retrieving CO2 abundances and aerosol properties in a scattering atmosphere. More extensive validation studies are necessary to improve the algorithm and to gain an estimate of the accuracy of the CO2 retrievals.
A11F-05 INVITED
The retrieval of the dry columns of greenhouse gases using SCIAMACHY
SCIAMACHY (Scanning Imaging Absorption spectrometer for Atmospheric CHartographY) was launched in 2002 aboard the ESA Envisat, which flies in a sun synchronous orbit in a descending node with an equator crossing time of 10.00 am. SCIAMACHY observes the back scattered, reflected and emitted radiation in 8 spectral channels from 214 nm to 2380 nm at spectral resolution form 0.2 nm to 1.4 nm. Carbon dioxide, CO2, and methane, CH4, are retrieved using their absorptions in the short eave infrared spectral region along with molecular oxygen, O2. The latest retrievals of the dry columns of CO2 and CH4 from SCIAMACHY and their seasonal behavior will be described and discussed. http://www.iup.uni-bremen.de
A11F-06
A Four-Year Global Climatology of Mid-Tropospheric Carbon Dioxide from AIRS
A 4-year zonally averaged mid-tropospheric climatology of carbon dioxide spanning -60 to +60 degrees, presently over ocean only, has been derived from the Atmospheric Infrared Sounder (AIRS) with estimated accuracies of about 0.5 ppm, based on in-situ aircraft measurements. This work uses AIRS channels with CO2 sensitivity functions that peak around 550 hPa, significantly lower than previous studies that use channels peaking in the 200-300 hPa region. Comparisons of this climatology with other data shows rectification of the boundary layer seasonal signal as a function of latitude, phase differences between boundary layer and mid-tropospheric seasonal cycles, and possible variations of 4-year growth rates with latitude. Comparisons to the NOAA Marine Boundary Layer smoothed data highlight transport processes for carbon dioxide into the mid-troposphere.
A11F-07
Laser Sounder for Measuring Atmospheric CO2 Concentrations: Demonstrations and Analysis
Accurate measurements of tropospheric CO2 abundances with global-coverage and monthly temporal resolution are needed to quantify processes that regulate CO2 exchange with the land and oceans. The NASA Orbiting Carbon Observatory (OCO) is a space mission focused on atmospheric CO2 for measuring total column CO2 and O2 by measuring the spectral absorption in reflected sunlight. The recent Decadal Survey for Earth Science by the US National Research Council has recommended a subsequent laser-based CO2 mission called ASCENDS. We have been developing a laser technique for the remote measurement of tropospheric CO2 concentrations from space. Our immediate goal is to develop and demonstrate the lidar technique and instrument technology that will permit measurements of the CO2 column abundance over horizontal paths and from aircraft at the few- ppmv level. Our longer-term goal is to demonstrate the needed capabilities of the technique, develop an approach and instrument design for ASCENDS. Our approach is to use the 1570-nm band and a two channel laser absorption spectrometer (ie DIAL used in altimeter mode), which continuously measures at nadir from a near-polar circular orbit. It uses several tunable fiber laser transmitters allowing simultaneous measurement of the absorption from a CO2 absorption line in the 1570 nm band, O2 extinction in the oxygen A-band, and aerosol backscatter in the same measurement path. It directs the narrow co-aligned laser beams toward nadir, and measures the energy of the laser echoes reflected from land and water surfaces. During the measurement, the lasers are tuned on and off a selected CO2 line and an O2 line (near 765 nm) at kHz rates. The lasers have spectral widths much narrower than the gas absorption lines. The receiver uses a telescope and photon counting detectors, and measures the background light and energies of the laser echoes from the surface along with scattering from any clouds and aerosols in the path. The gas extinction and column densities for the CO2 and O2 gases are estimated from the ratio of the on and off line signals via the DIAL technique. We use pulsed laser signals and time gating to isolate the laser echo signals from the surface, and to reject photons scattered from thin clouds and aerosols. Generally for laser measurements high SNRs are required, the CO2 estimates can be sensitive to small drifts or other errors in the instrument, and the absorption estimates need to be quite stable for hours. We have constructed a breadboard version of the CO2 sensor, which uses a low-power fiber laser and a 20 cm diameter telescope. We have used it to make measurements of CO2 absorption in the laboratory and over 206 and 400-m long open horizontal paths. These have been in several sessions extending over multiple days, and have allowed us to assess the measurement stability and to compare its estimates to readings from an external CO2 sensor. We have also calculated several characteristics of the technique for space including its expected measurement performance for different modulation types, and have performed an initial space mission accommodation study. We will describe these results in the presentation.
A11F-08
Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS)
We have developed and is in the process of validating the in-flight performance of an airborne prototype of the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) payload. The Team is partnership of ITT Space Systems Division, the NASA Langley Research Center, the University of New Hampshire Institute for the Study of Earth, Oceans and Space, and Atmospheric and Environmental Research Inc, The team has conducted several flights, the results of which have been used to fine tune the configuration and operation of the instrument, the parameters for the forward modeling studies and the retrieval algorithms. The airborne instrument suite measures; a) the number density of CO2 in the column of air beneath the aircraft, b) length of the column using a laser altimeter, and c) ambient air pressure and temperature. Within the next few months we will have installed a 3rd lidar tuned to O2 for making a direct measurement of number density of O2. The retrieval algorithm converts these measured parameters into column CO2 mixing ratio. The precision of the lidar measurements are a function of the product of power-aperture-integration time. As installed today, the lidar are configured to operate at the same received power level as baselined for an affordable LIDAR placed in low earth orbit. ITT and NASA have run several mission formulation and trade studies which confirm the baseline payload for ASCENDS meets the science requirement of 0.5% precision measurement of CO2 mixing ratio along a 100km horizontal segment and fits the mass-power-volume constraints (with margin) for a Taurus class payload. The established Technology Readiness Level 6 of the nearly commercial-off-the-shelf fiber transmitter system ensures that the cost and schedule constraints can also be met with margin. We will present the principals of operation of ITT's proprietary and patented lidar system for ASCENDS. The modeled performance and the measured performance will also be discussed, along with an assessment of the current technology readiness and planned activities for 2008.