Global Greenhouse Gas Observation by Satellite - Planet Action

More documents

Recommendations

Info

GOSAT Sensors and Observation Methods 2 GOSAT Sensors and Observation Methods GOSAT will observe infrared light reaching its sensors from the earth’s surface and the atmosphere and give spectra which can be used to calculate the column abundances of CO 2 and CH 4 . The column abundances are expressed as the total number of molecules of target gases over the unit surface area or as the ratio of target gas molecules to the total number of molecules in dry air per unit surface area. GOSAT will orbit the earth in roughly 100 minutes at an altitude of approximately 666 km and return to the same orbit in three days (Fig. 6). The observation instrument onboard the satellite is called the Thermal And Near-infrared Sensor for carbon Observation (TANSO). TANSO is composed of two sensors: a Fourier Transform Spectrometer (FTS) and a Cloud Aerosol Imager (CAI). Tables 1 and 2 summarize the target species, bands, and other specifications of the two sensors. TANSO-FTS utilizes optical interference, which is induced by splitting the incoming light into two optical paths to create an optical path difference between the two, and then recombining them. A light intensity distribution as a function of wavelength (spectrum) can be obtained by conducting a mathematical conversion called the Fourier transform on the signals observed while changing the optical path difference little by little. Bands 1 to 3 of FTS will provide the spectra of sunlight reflected from the earth’s surface in the daytime and band 4 will observe light emitted from the atmosphere and the earth’s surface throughout the day and night. The characteristics of sunlight reflection differ greatly between land and water surfaces. Seawater and freshwater absorb light which makes detection of the reflection difficult. However, in certain directions, sunlight is reflected as specular reflection and glitters brightly, so the sensor will target such points for observation over large water surfaces. GOSAT TANSO-CAI will observe the state of the atmosphere and the ground surface during the daytime in image form. The imagery data from TANSO-CAI will be used to determine the existence of clouds over a wide area including the field of view of FTS. When aerosols or clouds are detected, the characteristics of the clouds and the aerosol amounts are identified. This information is used to correct for the effects of clouds and aerosols on the spectra obtained by FTS. Over a three-day period, TANSO-FTS will take spectra from several tens of thousands points distributed uniformly over the surface of the earth. Since analysis can only be done on cloud-free areas, only approximately 10 percent of the total number of observation points can be used for calculating the column abundances of CO 2 and CH 4 . Even so, the number of data points significantly surpasses the current number of ground measuring points (currently under 200), and will serve to fill in areas where measurement has not been conducted to date. Table 1. Specifications of the Fourier Transform Spectrometer (FTS) sensor Band 1 Band 2 Band 3 Band 4 Spectral coverage [μm] 0.758~0.775 1.56~1.72 1.92~2.08 5.56~14.3 Spectral resolution [cm -1 ] 0.6 0.27 0.27 0.27 Target species O 2 CO 2 · CH 4 CO 2 · H 2 O CO 2 · CH 4 Instantaneous field of view/ Field of observation view at nadir Single-scan data acquisition time * 1 μm = 1/1000 mm Instantaneous fi eld of view: 15.8 mrad Field of view for observation (footprint): diameter of app. 10.5 km 1.1, 2.0, 4.0 seconds Table 2. Specifications of the Cloud and Aerosol Imager (CAI) sensor Band 1 Band 2 Band 3 Band 4 Spectral coverage [μm] 0.370~0.390 (0.380) 0.668~0.688 (0.678) 0.860~0.880 (0.870) Target substances Cloud, Aerosol Swath [km] 1000 1000 1000 750 Spatial resolution 0.5 0.5 0.5 1.5 at nadir [km] 1.56~1.68 (1.62) Sunlight Descending 666 km 10km diameter Ascending Figure 6. Conceptual diagram of GOSAT observation and the satellite orbits (three days, 44 orbits) 5000km(on the Equator) 3 Greenhouse gases Observing SATellite Project
90 45 0 -45 -90 0 30 60 90 120 150 180 210 240 270 300 330 360 Longitude(deg.E) Copyright (c) 2007 NIES Analysis Methods of GOSAT data 3 Analysis Methods of GOSAT data The data taken by the FTS and CAI sensors will be processed as shown in Figure 7. FTS-observed values provide spectra while CAI-based data will be used to generate cloud and aerosol information. These data will be combined together to calculate the CO 2 and CH 4 column abundances at observation points with no or only thin clouds and aerosol layer present. Furthermore, an atmospheric transport model will be used with the obtained distribution of column abundance of CO 2 to estimate the global distribution of CO 2 fl ux as well as the three-dimensional distribution of CO 2 concentrations. CO 2 and CH 4 absorb light with particular wavelengths. Therefore, by measuring how much light was absorbed by these molecules, the amounts of CO 2 and CH 4 existing through the optical paths can be calculated . Fig. 8 provides an example of spectra that are expected to be obtained from observation by FTS. The spectra structures like teeth of a comb indicate the absorption by gases, such as CO 2 and CH 4 , and the depths correlate with their column abundances. Radiance (W/m 2 /micron/str) Radiance(W/m 2 /micron/str) Radiance (W/m 2 /micron/str) Sensors TANSO-FTS sensor (provided by JAXA) TANSO-CAI sensor (provided by JAXA) April terly basis, and mapped out globally. The next step in data processing is the estimation of the flux of CO 2 using the acquired column abundances of CO 2 . We effectively “reverse” the atmospheric transport model to trace the origins of the CO 2 detected by GOSAT, and estimate the flux of CO 2 on a sub-continental scale (Fig. 5). The current method of estimating flux of CO 2 depends solely on ground-based observation data, which results in significant errors in estimations for regions such as Africa and South America, where observation points are scarce. GOSAT is August Observation data Interferogram Cloud/aerosol distribution Processed products 1.6 1.7 Wavelength(μm) Spectrum 3D distribution of CO 2 Column abundances of CO 2 and CH 4 Radiance(W/m 2 /micron/str) Latitude(deg.N) 3 2 1 0 FTS SWIR Pseudo Data Sun Zenith Angle = 80 deg. Count = 26346 ? Low density High density ? 355 357 359 361 363 365 367 369 371 373 (ppm) Among spectra obtained with FTS, only those spectra with no clouds in the field of view of the FTS will be identified with the use of images CO 2 flux Figure 7. Outline of GOSAT data processing from CAI. This is possible because the spatial resolution of CAI is high enough to detect cloud contamination in the field of view of FTS. The spectra with no clouds will then be analyzed using a numeric calculation capable of acquiring observation data almost uniformly around the globe and hence is expected to reduce errors in estimated CO 2 flux. Furthermore, using the distribution of method called the retrieval method based on Wavenumber (cm -1 ) 20000 15000 12000 10000 8000 7000 6000 5000 4000 the characteristics of absorption by gas, and 20 CO 2 flux obtained in this manner and Absorption band of water vapor(H2O) 15 Absorption band of carbon dioxide(CO2) the column abundances of CO 2 and CH 4 will be the atmospheric Absorption band of methane(CH4) 10 Absorption band of oxygen(O2) derived. The CO 2 absorption bands near 1.6μm transport model, it Absorption band of ozone(O3) 5 and 2.0 μm are quite important because they will be possible to 0 0.5 1.0 1.5 provide us with a large amount of information Wavelength (μm) 2.0 2.5 simulate the global 13200cm near the earth’s surface where the changes in 12900cm -1 6400cm-1 5800cm -1 5200cm 3 4800cm -1 O2 H2O CO2 CO2 CO2 distribution of CO 2 10 2 1 H2O CO 2 concentrations are most apparent. The absorption band near 14 μm is used for obtainsions. 1 in three dimen- CO2 CH4 0 0 0 0.76 0.77 1.6 1.7 1.95 2.00 2.05 Wavelength (μm) Wavelength (μm) Wavelength (μm) Band 1 Band 2 Band 3 ing information mainly at altitudes of 2 km and Wavenumber (cm -1 ) 2000 1500 1200 1100 1000 900 800 700 10 above. H2O 8 CH4 O3 CO2 CO2 Figure 8. Examples of 6 spectra obtainable through 4 Once enough points of data are accumulated, GOSAT observation and 2 the acquired column abundances of CO 2 and 0 5 6 7 8 9 10 11 12 13 14 15 the absorption Wavelength (μm) CH 4 can be averaged on a monthly and quar- bands of CO Band 4 2 and CH 4 4
Page 1 and 2: Global Greenhouse Gas Observation b
Page 3: Goals of the GOSAT Project 1 Goals
Page 7 and 8: Data Distribution 5 Data Distributi

Global Greenhouse Gas Observation by Satellite - Planet Action

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?