| As one of the main trace greenhouse gases,methane?CH4?has a heat absorption capacity of about 28 times that of carbon dioxide,which seriously affects the balance of radiation payments in the land gas system.Limited by the distribution and number of sites on measuring ground-level CH4 concentrations,there is a lack of large-scale and long-term ground-level scale observation.In recent years,satellite remote sensing has made important progress in the spatial distribution of atmospheric pollutants.In this study,a method for ground-level CH4 concentrations in China was constructed by using AIRS CH4 columns and an atmospheric chemical model MOZART-4.The spatial and temporal variations of ground-level CH4 concentrations in China from 2007 to 2019 were discussed.Based on the relationship between regional ground-level CH4 concentrations and emissions,the emdodied CH4 emissions in domestic and foreign trade are quantitatively analyzed by MRIO model.The main conclusions are as follows:Based on the output data of atmospheric chemical mode,the vertical distribution model of CH4 concentrations of multiple Gaussian functions is constructed,and the ground-level CH4 concentrations are estimated.The accuracy of model fitting results is high,and the proportion of R2 higher than 0.95 is over 90%,and the average RMSE is 2.92×1013 molec./cm3.In different seasons,the proportion of R2>0.95 reached more than 85%,and the simulation accuracy of spring and winter was higher than that of summer and autumn,because of the atmospheric conditions are relatively stable in winter and the atmospheric transport between the troposphere and the stratosphere is intense in summer due to the temperature rise.The estimated ground-level CH4 concentrations and ground-monitored data have a good consistency in the long time series,the fitting accuracy R2 is 0.680.87,there is no serious deviations in the spatial verification,the correlation coefficient p=0.80?p<0.01?,AE=60.69 ppb,and PAE = 3.18%.The spatial distribution characteristics of CH4 concentration and the distribution of source-sink are discussed by using the estimated ground-level CH4 concentrations.From 2007 to 2019,the averaged ground-level CH4 concentrations in China was 1827.7 ppb,showing a large spatial variation.On the whole,CH4 concentrations are higher in the north than south,and higher in the east than west.For different areas,due to regional sources and sinks,there are high values on the space of gathering circle,including the north China plain?Beijing,Tianjin and Hebei industrial circle area?,Shandong,Sichuan basin,Gansu and Xinjiang region and the Himalayan mountains in southern Tibet.The second highest values are located in the Yangtze river delta?Jiangsu-Shanghai,Anhui?,the Pearl river delta?Guangdong-southern Guangxi?,Henan,Hebei and other regions.The lowest values are distributed in the Qinghai-Tibet plateau,where the emissions from natural sources are the main ones,which are related to the distributions of natural wetlands and vegetations.The ground-level CH4 concentrations are greatly affected by human activities,including agricultural activities?rice field,straw burning?,energy exploitation and utilization,waste disposal,livestock breeding and rumination,etc.The satellite simulation of the ground-level CH4 concentrations in China shows obvious seasonal change.The concentration in winter and summer was 1842.63 ppb and 1828.43 ppb,respectively,with the lowest concentration in spring?1801.08 ppb?and the middle concentration in autumn?1813.75 ppb?,and the overall curve represents a "W" characteristic.Through the analysis of backward airflow trajectory in hot spots,low-altitude air transport not only increases the local CH4 concentrations?Xinjiang,Beijing-Tianjin-Hebei region,Sichuan?,but also strengthens the pollution situation in other areas?Yangtze river delta?through the long-distance transport.The results of the analysis of long time series show that the monthly CH4 growth rate is 0.44 ppb.month-1,and the seasonal increase is-16.48 ppb.month-1.The CH4 concentration in China showed a significant growth trend.According to the statistics of different regions,the CH4 growth rate in the eight regions of China in order is NW?0.61?> TP?0.59?> NC?0.57?> SW?0.52?> CC?0.51?= NE > YRD?0.47?> SC?0.44?.There is a strong spatial heterogeneity of methane emissions embodied in domestic and foreign trade in China.The correlation coefficient between ground-level CH4 concentrations and emissions in China was 0.81?p<0.01?.According to MRIO model,more than 16% of China's CH4 emissions were related to consumption through international trade activities.International exports from coastal provinces and economically developed regions contribute to higher CH4 ehmissions.In terms of interprovincial relations,Shanxi,Inner Mongolia,Henan and other provinces transferred from the province?producers?caused a large amount of CH4 emissions;the provinces of Jiangsu,Zhejiang and Guangdong have high CH4 emissions due to external transfers?consumers?.GDP was positively correlated with the negative net flow of CH4 emissions from provinces?R=0.57,p<0.01?.The areas with energy enrichment and heavy chemical industry concentration bear more emission risks for the economically developed areas and the backward areas with imperfect industrial structure.Through the analysis of hot spots such as the Yangtze river delta,Beijing-Tianjin,Guanghai and Chongqing,it is found that NC,NW and CC increase the CH4 emissions in local region through inter-provincial export trade,and the total amount is estimated to be 136.83×104 t,115.07×104 t and 95.17×104 t,while YRD and SC are imported from other provinces,which relatively improve the local air quality.Inter-provincial trade has improved air quality in the more developed coastal provinces but worsened it in the less developed inland provinces.All in all,the method that estimates the ground-level CH4 concentrations of the long time series by using satellite and model data is feasible.The ground-level CH4 concentrations have obvious seasonal and spatial variation characteristics,the variation trend of the long time series has distinct regional differences.Trade embodied CH4 emissions are affected by multiple factors such as regional economic development level,degree of industrialization and energy industry structure.The results describe the spatial distributions of greenhouse gases near the earth's surface in China and provide a scientific basis for relevant departments to formulate emission reduction policies. |
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