Spatiotemporal Variation Of Aerosol Optical-radiation Properties And Its Response To Human Activities In The Yangtze River Basin

Aerosols play an important role in the balance of energy budget,global climate change,regional atmospheric environment and human health.Due to the complex and diverse composition of aerosols,uneven spatial and temporal distribution and incomplete understanding of the interaction between aerosols and clouds,aerosols have become one of the most uncertain factors in current climate change research.Especially in recent decades,the rapid development of industrialization and urbanization in the Yangtze River Basin has made the region one of the main sources of anthropogenic aerosol emissions in the world.Therefore,it is of great scientific significance and application value to actively carry out research on the temporal and spatial characteristics of aerosol optical-radiative characteristics and its response mechanism to human activities in the Yangtze River Basin(YRB).In this study,based on multi-source spatial data,the temporal and spatial characteristics of aerosol optical properties and aerosol direct radiative effect in the YRB were firstly analyzed,and then from the perspectives of anthropogenic sources,socio-economic development,land use change,urban form and meteorological conditions,the mechanism of the influence of human activities on regional aerosol optical-radiative characteristics was further explored.The main research contents were as follows.Aqua-MODIS C6(DT3K,DT10K,DB10K and DTB10K),VIIRS?EDR and MERRA-2 aerosol optical depth(AOD)retrievals were compared with CE318 AOD in Taihu(TH),Hefei(HF),Wuhan(WH)and Kunming(KM)sites.The comparison results revealed that:(1)High-quality(QF=3)Aqua–MODIS C6 and VIIRS?EDR aerosol satellite remote sensing products were recommended in the Yangtze River Basin.(2)The high-quality DT3K and DT10K products were prone to overestimate AOD in spring and summer,with overestimation rates of 23%and 15%,respectively;on the contrary,high-quality DB10K products were prone to underestimate AOD in spring and summer,with an underestimation rate of about 12%.High-quality DTB10K products were best matched with ground observations,characterized by wider coverage and higher inversion accuracy(R=0.85,within EE%=55%and RMB=1.10);(3)Compared with Aqua-MODIS C6 products,high-quality VIIRS?EDR aerosol products had poor correlation with ground-based measurements(R=0.73),and were prone to underestimate(overestimate)AOD in summer(winter);(4)MERRA-2 products were prone to underestimate AOD in the whole year,with an underestimation rate of about13%.Using MERRA-2,DTB10K and DB10K products,the temporal and spatial variations of aerosol optical thickness(AOD),?ngstr?m Exponent(AE)and single scattering albedo(SSA)were investigated in the YRB from 1980 to 2017.The results show that the annual average AOD of the YRB was about 0.36.The high AOD values were concentrated over the Yangtze River Delta(YRD),Central China(CC)and Sichuan Basin(SB)in summer(0.425)and spring(0.393),while the low AOD values were observed over the Source of YRB(SYR)in winter(0.326)and autumn(0.291).The seasonal variations of AOD were closely related to the local meteorological conditions.There was a significant trend in AOD in the middle and lower reaches of the YRB(P<0.05):AOD increased slightly(0.0025 year^-1)during 1980-1999,increased significantly(0.0201 year^-1)during 2000-2008 and decreased significantly(0.0185 year^-1)during 2009-2016.MERRA-2,Terra and Aqua sensors observed that the rapid growth trend of AOD of the YRB was effectively curbed after 2008,which was closely related to a series of energy saving and emission reduction measures implemented by the Chinese government in recent years.Furthermore,AE values in the YRB were all greater than 0.7,and 53.31%of the AEs were greater than 1.3,indicating that the aerosols in the Yangtze River Basin are dominated by fine-mode particles.The AE showed a significant seasonal variation.In spring,the coarse particles(AE<1.3)were dominant;in summer,with the increase of AOD,the fine particles(AE>1.3)gradually increased;although the AOD in autumn decreased,the fine-mode aerosol particles(AE>1.3)still dominated;in winter with the gradual decrease in AOD,the coarse particles(AE<1.3)began to increase.Additionally,the spatial difference of SSA in the YRB was small,and the long-term trend of the SSA was not significant(P=0.38).Using CERES and MERRA-2 radiation products,the aerosol short-wave direct radiative effects on the top of atmosphere(TOA),surface(SFC)and atmosphere(ATM)under clear sky conditions were quantitatively estimated in the YRB during 1980-2016,and the uncertain sources of instantaneous ADRE were further explored.The results show that under the clear sky conditions,the aerosol exerted a cooling(warming)effect on the top of the atmosphere and the surface(in the atmosphere).Over the SYR,ADRE_TOA(>4 Wm^-2),ADRE_SFC(>-10 Wm^-2)and ADRE_ATM(<6 Wm^-2)were low,and their seasonal variations were not obvious.On the contrary,ADRE_TOA(<-10 Wm^-2)?ADRE_SFC(<-20 Wm^-2)and ADRE_ATM(>14 Wm^-2)were high over the YRD,CC and SB,and their seasonal variations were significant.The cooling(warming)effect of aerosol on the TOA and SFC(ATM)were significantly enhanced during 2000-2008,with an annual average growth rate of-0.3744 Wm^-2 year^-1(ADRE_TOA),-0.5652 Wm^-2year^-1(ADRE_SFC)and 0.2935Wm^-2 year^-1(ADRE_ATM);and significantly decreased during 2009-2016,with an annual average decline rates of 0.2731 Wm^-2 year^-1(ADRE_TOA),0.5868 Wm^-2 year^-1(ADRE_SFC)and-0.3145 Wm^-2 year^-1(ADRE_ATM).Additionally,there was a significant positive(negative)correlation between ADRE_ATM(ADRE_TOA and ADRE_SFC)and AOD(|R|>0.8,p<0.05),confirming that aerosol loading is the dominant factor in the long-term trend of ADRE.Furthermore,the total uncertainty of the instantaneous ADRE(TOA)at the top of atmosphere in the YRB was approximately±1.8 Wm^-2.They were primarily from five sources including uncertainties in calibrated CERES fluxes(±0.4 W m^-2),conversion between filtered to unfiltered TOA radiances(±0.4 W m^-2),radiance to flux conversion with aerosol effects(±0.4 W m^-2),cloud contaminations(±0.5 W m^-2),and uncertainties in clear-sky fluxes estimations(±1.6 W m^-2).Based on MLR(multiple linear regression model),GWR(geographically weighted regression model)and improved STIRPAT(Stochastic Impacts by Regression on Population,Affluence and Technology)model,the impact mechanism of human activities on AOD and ADRE in the YRB from 1980 to 2016 was explored from the perspectives of anthropogenic emissions,socio-economic development,land use change,urban form and meteorological conditions by using multi-source spatial data.The results showed that AOD,|ADRE_TOA|,|ADRE_SFC|and ADRE_ATM were all positively related to anthropogenic emissions over the YRD,CC and SB;but there were no significant correlations over the SYR.Additionally,AOD,|ADRE_TOA|,|ADRE_SFC|and ADRE_ATM were all closely correlated with the socioeconomic indicators.Notably,the correlation between natural aerosols(such as sand and sea salt)and socioeconomic indicators did not passed the 95%confidence test.Moreover,the socio-economic development indicators of mega-cities were not significantly correlated with AOD and ADRE,which may be due to the strong comprehensive development in the mega-cities,resulting in the complex and diverse sources of aerosols,and thus weakening the sensitivity of aerosol loadings to the socioeconomic indicators.Land types were closely related to AOD,|ADRE_TOA|,|ADRE_SFC|and ADRE_ATM.The AOD values in the construction land and cultivated land were the largest;in the forest land and water area were the second;and in the grassland and unused land were the smallest.Moreover,land use cover change(LUCC)had also a significant impact on the changes of AOD and ADRE.For example,during 1980-2010,the construction land in the YRD,CC and SB expanded dramatically,with an annual growth rate of 5.786%,which led to a significant growth of AOD and ADRE in these regions.Additionally,the compactness and fragmentation of urban form could be characterized by the patch area(CA),number of patches(NP),edge density of patches(ED)and largest patch index(LPI).AOD,|ADRE_TOA|,|ADRE_SFC|and ADRE_ATM were positively correlated with CA,NP and ED,but negatively correlated with LPI.The results show that a less fragmented and more compact urban form,helped alleviate local aerosol loadings by enhancing urban connectivity,reducing vehicle dependence and facilitating the use of bicycles and walking.Seasonal variations of AOD were closely related to local meteorological conditions.However,there were no significant trends in meteorological factors from1980 to 2016,indicating that these meteorological factors were unlikely to be the main reason for the significant increase of AOD.Notably,the temperature showed a significant upward trend(0.03 K year^-1),indicating that climate warming was one of the driving factors for the significant growth of AOD,but far less than the impact of anthropogenic emissions.