The Ground-based And Satellite-based Validation And Application Of AIRS-AMSU CH₄ Products

As the third largest greenhouse gas after carbon dioxide and water vapor in the atmosphere,the enhancement of methane(CH4)concentrations will accelerate the global warming.M-K mutation detection and other methods were used in this paper to analyze temporal,spatial variation and factors affecting CH4 concentrations in China's five major WMO/GAW background stations(Waliguan(WLG),Lin'an(LAN),Longfengshan(LFS),Shangdianzi(SDZ)and Shangri-La(XGLL)).The methane concentrations of WLG,LAN,LFS and SDZ sites are selected from January 2009 to December 2013,and the data of Shangri-La(XGLL)site is selected from July 2010 to December 2013.Near-surface CH4 retrieved by AIRS/AMSU(Hereinafter referred to as AIRS)is validated by ground-based CH4 at China's five major WMO/GAW atmospheric background stations firstly.Then AIRS/AMSU is compared with GOSAT.Finally,the temporal and spatial distribution of CH4 concentration in China was analyzed using AIRS Level 3 products.The results are as follows:(1)The analysis of the ground-based methane concentration shows that LAN station has the highest methane concentration,followed by LFS station,SDZ is in the middle,Shangri-La at least during the study period.The mean value of them is 1965 ppbv,1939 ppbv,1914 ppbv,1864 ppbv and 1861ppbv respectively.The average annual growth rate of methane concentration in WLG is 0.034%,the average annual growth rate in LAN,LFS,SDZ is 0.033%,0.025%,0.018%and 0.01%,respectively.LAN,LFS,SDZ have mutation point.The time of occurrence of the mutation point coincides with the time of the pollution process.The seasonal variations of each site are as follows:methane concentration at LAN background station reaches the valley in July;LFS presents "W" type;SDZ has the highest CH4 concentration in July and August,WLG is similar to SDZ;XGLL site is affected by monsoon in summer,showing a distinct single peak.LAN,LFS,SDZ,XGLL stations show the same diurnal trend:the lowest value occurs in the afternoon,the concentration is higher in the night and early morning;there is a peak at midday at WLG station,which is due to grazing in pastoral areas.(2)The comparison between AIRS retrieval CH4 and ground-based CH4 shows that average deviations between ground-based CH4 and AIRS near-surface CH4 at LAN,LFS,SDZ,WLG and XGLL stations are-93.12ppbv,-44.03ppbv,-30.75ppbv,-5.19ppbv and 9.65ppbv,respectively.Correlation coefficients between AIRS CH4 and ground-based CH4 are 0.5,0.53,0.21,0.49and 0.11,respectively.Standard deviations do not exceed 22ppbv.The values of AIRS CH4 are generally lower than those of ground-based,and the AIRS retrieval CH4 has the same monthly change with the ground-based CH4.The accuracy of AIRS retrieval CH4 is greatly affected by cloud.And it is affected by retrieval algorithms,seasonal changes and so on.(3)The results of intercomparision between AIRS and GOSAT show that AIRS and GOSAT retrieval profiles have the same trend.The GOSAT retrieval CH4 concentrations are slightly higher than the methane concentrations retrieved by AIRS near the ground.In 500—600hPa layer,they are in highest coincidence,the deviations are within 17.9ppbv.In 10—70hPa layer,the deviations are larger,and the AIRS retrieval CH4 is much higher than the GOSAT retrieval CH4,and the deviations can reach 254.5ppbv.(4)The distribution of CH4 in China is as follows:The near-surface CH4 concentration in China increases with increasing latitude.Higher methane concentrations in the eastern coast of China than in the west.Relatively high concentrations of methane in regions with petroliferous basins.The seasonal variation of the near-surface CH4 concentration in China is:the methane concentrations in autumn and winter are higher than those in spring and summer.The methane concentration in the Qinghai-Tibet Plateau reaches the maximum in the autumn,which is mainly due to the strong convergence and southwest monsoon.The seasonal change of methane concentration in southern China is not significant.The concentration of methane in the troposphere in China shows a significant increase from 2009 to 2013.In general,the vertical distribution of methane concentration in China is:as the height increases,it decreases.And as the height increases,the difference in methane concentration between regions also gradually decreases.