Analysis Of Aerosol Optical Properties And Direct Radiative Forcing Characteristics Over China Based On Ground-based And Reanalysis Data

With ground-based and MODIS remote sensing data and MERRA-2 reanalysis product,this study carried out research about aerosol optical properties and direct radiative forcing over China.First,observation data from 2007 to 2015 of the sun photometer at northern suburb of Nanjing was used to analyze the aerosol optical property and direct radiative forcing in this area,then the accuracy of MERRA-2aerosol optical depth(AOD)over Yangtze River Delta was validated with sun photometer and sky radiometer,and then data from AERONET sites and MODIS was used to validate the accuracy of MERRA-2 AOD over the entire China.Based on the validation,aerosol optical properties and radiative forcing over China from 1980 to2017 were analyzed.The main conclusions were as follows:First,Northern suburb of Nanjing is dominated by fine mode aerosol.Single scattering albedo(SSA)of total and fine mode is found to decrease with the increase of wavelength,while SSA of coarse mode increases with the increase of wavelength.Aerosol direct radiative forcing offers cooling effect at the surface and top of atmosphere,and is high in spring and summer,low in autumn and winter.Second,good correlation could be found between MERRA-2 AOD and sky radiometer at Hefei and sun photometer at Nanjing in the Yangtze River Delta.AOD in the northern part of Yangtze River Delta was higher than that in the south.Highest AOD appeared in the northwestern part of Yangtze River Delta.AOD after 2000 was higher than that in the 1980 s and 1990 s.AOD in March,April and June was high,and was low in September,October,November and December.AOD comparison among large cities,medium-sized cities and remote areas showed the highest AOD appeared in large cities and the lowest was in remote areas.From 1980 to 1999,AOD in the three kinds of areas of Yangtze River Delta rose slowly.With the rapid development of economy,from 2000 to 2009,AOD in the three kinds of areas rose rapidly.Due to the environmental protection policy from the government,from 2010 to 2017,AOD in the three kinds of areas dropped gradually.From 1980 to 2009,AOD over the entire Yangtze River Delta showed an increase trend,while during the period between 2010 and 2017,AOD showed a decrease trend,with the highest decrease trend in the northwestern part of Yangtze River Delta surrounding Hefei.Third,there was good correlation between MERRA-2 AOD and AOD from AERONET sites and MODIS over China.High AOD appeared in the east,south and center of China,while low AOD was in the west and northeast of China.AOD after2000 was obviously higher than that in 1980 s and 1990 s.Slow AOD increase occurred in the 1980 s and 1990 s,and rapid increase appeared from 2001 to 2010,while gradual decrease could be observed between 2011 and 2017 over the entire China.Developed areas such as Yangtze River Delta,Jing-jin-ji Region and Pearl River Delta had the similar and more obvious AOD variation during the same period.From 1980 to 2009,AOD in most part of China showed an increase trend,while between 2010 and 2017 an AOD decrease occurred in almost the entire China.Through the analysis of spatial and temporal distribution of SO4 AOD,DUAOD,BCAOD,OCAOD and SSAOD over China from 1980 to 2017,we found that SO4 AOD and DUAOD accounted for major part of total AOD and that the variation of SO4 AOD was the driving force of the variation of total AOD.High DUAAOD occurred in the dust area in the northwest of China,while high BCAAOD mainly appeared in the east and center of China.DUAAOD and BCAAOD accounted for96.86% of total AAOD.From 1980 to 2017,the variation of total AAOD was mainly due to the variation of BCAAOD.Due to the increase of Black carbon emission,from1980 to 2007,BCAAOD showed an obvious increase trend over the entire China.From 2008 to 2017,a BCAAOD decrease trend could be observed in most part of China.Finally,aerosol direct radiative forcing showed a cooling effect at the top of atmosphere and surface over China,and a warming effect inside the atmosphere.The cooling or warming effect was more obvious in the east than the west of China.The correlation coefficient between AOD and aerosol direct radiative forcing was negative at the top of atmosphere and surface,and was positive inside the atmosphere.During the 5-year intervals from 1980 to 2017,the cooling effect of the aerosol direct radiative forcing at the top of atmosphere and surface was small in the 1980 s and1990s,and showed an increase after 2000,then reached the maximum during the period between 2006 and 2010,and gradually decreased from 2011 to 2017.From1980 to 2009,the cooling effect(warming effect)of aerosol direct radiative forcing showed an increase trend at the top of atmosphere and surface(inside the atmosphere)in the central and eastern China.From 2010 to 2017,the cooling effect(warming effect)of aerosol direct radiative forcing showed a decrease trend at the top of atmosphere and surface(inside the atmosphere)in the central and eastern China.