| Popis: |
Precise measurements of CH 4 in a column of near surface air, and in partial columns above this, would be very valuable in identifying sources/sinks of atmospheric CH 4 , and its transport. For this purpose we have proposed a grating mapping spectrometer (GMS) for deployment as an Instrument of Opportunity (IOO) on the NPOESS that acquires data in the 2990 to 3050 cm -1 spectral region. It will provide measurements of CH 4 absorption of sunlight in the weaker CH 4 features in the region, and of thermal emission in the stronger CH 4 features in the region. It is the combination of the two that provides the vertical information. The IOO will acquire spectra on a crosstrack swath centered on nadir, and with 1/2 width of 55 degrees on each side of nadir (about 2800 km full width swath on the ground for a nominal 828 km satellite altitude). This with footprints that are about 3.1 km on a side at nadir. The small footprint facilitates cloud screening, and identification of CH 4 source hotspots. A capability to project the slit to nadir along the direction from satellite to sun will be utilized for over the ocean viewing in order to facilitate measurements in solar glitter. It will obtain spectra with resolution n -1 and sample spacing -1 . Based on the spectral characteristics and currently achievable very low-noise we do a linear error analysis (Rodgers, [1]) for the simultaneous retrieval of multi-column CH 4 , humidity, and surface parameters and 13 CH 4 total column. We show that useful multi-column CH 4 retrievals can be obtained, with good near surface sensitivity in sunlit conditions. We also show the 13 CH 4 column can be retrieved with precision better than 3%. Retrieval of 13 CH 4 column in the earth's atmosphere is analogous in difficulty to retrieval of the major CH 4 isotope column in the Martian atmosphere by a similar GMS deployed on a Mars orbiter. We show that H 2 O vertical information can be retrieved from these measurements and discuss the potential for ethane column retrieval. |
