Preliminary Study Of Satellite Remote Sensing Of Greenhouse Gases Methane Of Chinese Academy Of Meteorological Sciences

As one of the most important trace gases in the earth atmosphere, methane(CH4) plays an important role in the atmospheric chemistry, while it is also an im-portant greenhouse gas just next to carbon dioxide (CO2). IPCC4th assessmentreport shows: despite the about 200 times smaller atmospheric burden of methane(about 1.7ppmv) compared to CO2 this increase constitutes about 20%of the an-thropogenic climate forcing by greenhouse gases because on a per molecule basismethane is a much more e?ective greenhouse gas than CO2. CH4 is also one of themost important chemical activities of carbon-containing compounds. It can a?ectthe atmospheric ozone (O3) and HxOy chemical reaction because it's reaction withthe hydroxyl radical (OH) in the troposphere. Many ground-based stations observedresults shows: the atmospheric CH4 concentrations have more than doubled since thepre-industrial era; the growth rate of CH4 was slow down after 2000, but a signif-icant increase from 2006 to 2007. The growth of CH4 concentrations will produceimportant e?ects on the Earth's radiation balance and then give direct impact onclimate change. But the quantification of global methane emissions still has largeuncertain, as the CH4 emissions have a large spatial and temporal variation and ourground-based observations are limited. To better understand the emissions and sinksof methane, many scientists have carried out a large number of satellite remote sens-ing monitoring of the methane.Now it is just beginning in our country to make research on remote sensing ofthe atmospheric trace constituents by high-spectral satellite. In the next few years,we will pay close attention on the high-spectral satellite detector development, butalso urgently we need to make more research on a variety of trace atmospheric con-stituents (including the greenhouse gases methane) retrieval methods and ground-basevalidation work. In view of this, this paper makes an investigation on the UnitedStates, Canada and Europe's hyper-spectral remote sensing instruments, which areused to measure the atmospheric trace gases in orbit. Then give a introduction of theTES, ACE-FTS, MIPAS, IASI, MOPITT, SCIAMACHY and the ground-base FTIRinstruments, which can be used to detect methane in the atmosphere, though theinstruments parameters, indicators performance, data processing methods and theirCH4 product. Base on this, we did a summary of the current international retrievalmethods used for methane remote sensing focusing on analysis and comparison ofthe principal and characteristics of weighting-function DOAS (WFM-DOAS) method, optimal estimation method (OEM) and the artificial neural method (SA-NN). Con-sidering the direction of our satellite development, we make a research on the OEMretrieval method, which can be used to get methane profile information. After a studyon the CH4 absorption characteristics in the infrared and near-infrared band, we se-lect a appropriate spectral micro-window for algorithms research and developmentbased on the IASI infrared spectroscopy. To validation our methane retrieval results,we compared our CH4 total column results with the o?cial results, which get by theSA-NN method. Also, we compare our retrieval result with the ground-base FTIRmeasurements made by the National Satellite Meteorological Center of China Meteo-rological Administration. Finally, CH4 remote sensing products from foreign satelliteare used to drive the spatial and temporal distribution characteristics in China's re-gional.By the methane absorption characteristics analysis, we find there are three dis-tinct spectral windows (7.6um (1200～1400cm?1), 3.3um (2900～3150cm?1) and 2.3um(4100～4500cm?1)) can be used for satellite remote sensing. Also it always exist otherinterfering gases such as H2O, N2O, O3, CO in these CH4 absorption bands, whichwill largely a?ect the accuracy of the CH4 profile inversion. Satellite Nadir and Limbways remote sensing of high sensitivity of-band simulation results show that: 1) themethane infrared absorption band 7.6um is sensitivity mainly in 5 15km; the highsensitivity of 3.3um and 2.3 um bands is mainly in 0 5km. Taking into account thethree-band range of di?erent highly sensitive, in theory, the remote sensing method tocombine 4.7um and 3.3um or 4.7um and 2.3um bands can contain di?erent levels ofCH4 profile information, which is conducive to accurate inversion. 2) The Limb wayremote sensing is more sensitive to senior methane in the atmosphere. On one hand,usually the satellite limb observations is less a?ected by the lower atmospheric emis-sion spectra relative to the Nadir observations, which means the limb remote sensingis much more conducive to reducing the impact of interference gases; on the otherhand, Limb manner of remote sensing can use multiple scan angle (di?erent tangentheight) to do the observations, so the spectrum contains more information on thevertical distribution (higher degree of freedom) of our goal atmospheric constituents,which may drive a higher vertical resolution result. In theory, the way to combineboth the Nadir and Limb observation can do a better CH4 remote sensing. Spectralresolution is another important factor to a?ect the inversion accuracy. Compare andanalysis characterization and residue of di?erent resolution spectrum from model, alsoconsidering the theory CH4 absorption line broadening width by the atmosphere,thechoice of 0.0Xcm-1 order of magnitude of the spectral resolution remote is proposed to do the methane remote sensing inversion.In the methane profile retrieval algorithm research, considering the H2O, N2Ointerference in the CH4 7.6um absorption band covered by IASI infrared observationspectrum, we chose (1320～1360cm?1) as our retrieval micro-window and then developa program called SATFIT to do the methane profile and total column amount retrievalwork, which is based on the optimal estimation method. To validate the SATFIT re-trieval results, we compare SATFIT total amount of CH4 retrieval results on threedi?erent choose regions of the Northern Hemisphere high latitudes with o?cial SA-NN products, the results show good agreement in all these areas. Also comparingthe IASI retrieval result with NSMC FTIR initial observation, results shows the de-viation of CH4 total column amount is less than 20%. Based on the encounteredproblems (the inversion results of instability and inversion time-consuming) in theusing of SATFIT physical inversion, also taking into account the two main inversionmethods (statistical inversion and physical inversion) advantages and disadvantagesand the actual business needs to run, it is recommended to use statistical inversioncombined with the physical inversion approach to do the inversion data processing.Though the simulation analysis the di?erent height sensitivity of CH4 absorptionbands characteristics, we believe that the SCIAMACHY CH4 products used 2.3umband to do the retrieval re?ects lower CH4 concentration information in the atmo-sphere; the AIRS and IASI CH4 products used infrared 7.6um to do the retrievalre?ects the distribution characteristics of CH4 in the high-level; Theoretically, theTES which covers both the 3.3um and 7.6um CH4 absorption bands can give an ideallow-rise to high-level distribution characteristics of CH4 in the atmosphere. Based onthis, this paper gives the preliminary monitoring results of CH4 spatial and temporaldistribution in China's region by the SCIMACHY, AIRS, TES years of remote sensingproduct. The average mixing ratio of CH4 is higher in the southeast of China than inthe northwest. Qinghai-Tibet Plateau is the low CH4 concentration center. There issignificant season's variation for average mixing ratio of CH4 with a peak in summertime. But in the top of troposphere CH4 mixing ratio is higher in the northwestthan in the south and east of China. CH4 mixing ratio in the area of Qinghai-TibetPlateau shows a high value during summer monsoon. CH4 profiles from TES productshows a high value in the height of 180PHa. Trends analysis suggests that this highvalue increase during these years.It is di?culty to get accuracy atmospheric trace gases concentration from satelliteremote sensing: on one hand, the atmospheric trace elements in the satellite remotesensing spectral signal is very weak, so its detection requires a satellite sounder with both higher spectral resolution (0.0Xcm?1 order of magnitude) and higher spectralsignal to noise ratio; on the other hand, processing such a huge amount of data fromhyper-spectral satellite is more complicated and the atmospheric composition infor-mation retrieval technology is still not perfect. Our country's satellites still do nothave the ability to remote sensing CH4. The inversion study of atmospheric tracegases in this paper is only a beginning. With the developing of our satellites, we needto do more research on the atmospheric composition information retrieval technol-ogy. This paper's research could be a good base for future developing Chinese ownsatellite constitute instrument, expanding methane retrieval method and applicationof the production.