Optical Characteristics Of Aerosol In China Based On MERRA-2 Reanalysis And AERONET Ground-based Remote Sensing

Aerosols not only have important impacts on the environment,but the climate effects they bring are very complex and cannot be ignored.Aerosol optical characteristics reflect the ability of aerosol particles to absorb and scatter radiation.Therefore,understanding the aerosol optical characteristics is the basis for studying its climate effects.The new generation of reanalysis data MERRA-2 contains not only five types of aerosol optical depth(AOD)but also Angstrom exponent(?)products.Due to its high spatiotemporal resolution and abundant data products,it is helpful for the study of long-term aerosol optical properties.This article uses MERRA-2's hourly and monthly AOD at 550 nm,? between470-870 nm from 2000 to 2018,and five types of aerosol optical depth(black carbon,dust,organic carbon,sulfate,and sea salt)data,combined AERONET with the V3 version Level 2 data of AOD at 500 nm and ? between 440-870 nm of 19 stations in China from 2000 to 2018.After time-space matching,the applicability of MERRA-2's AOD and ? products in China is first verified,followed by the combination of long-term MERRA-2 reanalysis and AERONET ground-based observation data to discuss aerosol optical characteristics in China,and finally analyze the spatiotemporal variation of the optical depth of different types of aerosols.The main results are as follows:(1)MERRA-2's AOD data is applicable in China,but MERRA-2 underestimates AOD at most sites.Among all the stations,the best performing is Hong_Kong_Hok_Tsui station,the worst effect is the NAM_CO station.For each region,the best fitting effect is in the Pearl River Delta region,where the fitting coefficient is the largest,the scattered points are closest to the 1: 1 line,correlation coefficient(R)and standard deviation ratio(SDV)are 0.84 and 0.81,and mean absolute percentage error(MAPE)is 25.97%.(2)MERRA-2's ? products have applicability in some regions.The correlation coefficient of the two data is low.Overestimated ? at almost all sites.The most serious overestimation was NAM_CO station.The MAPE of NAM_CO station was as high as 1103.36%.For each region,MERRA-2 has the best effect in the Pearl River Delta,with root mean square error(RMSE)of 0.26,MAPE of 17.44%.MERRA-2 has the worst effect on the Qinghai-Tibet Plateau,obviously overestimating ?.(3)The analysis results of daily and annual changes in aerosol optical properties show that,MERRA-2 reproduces the daily variation of AOD better,while ? is relatively poor.The AOD in the Yangtze River Delta,the Pearl River Delta,the Loess Plateau,and the North China Plain showed an "inverted U-shape",while the daily variation of AOD in the Qinghai-Tibet Plateau area showed a "U-shape".The ? in the Qinghai-Tibet Plateau is significantly lower at noon,but the ? in the other regions does not change significantly.For annual variation,optical properties of the Yangtze River Delta and the North China Plain are similar,with the highest AOD in June and small ? in March and April.The AOD and ? in the Pearl River Delta are small in July.The Qinghai-Tibet Plateau has a low AOD year-round,with the highest AOD and the smallest ? in April.The AOD in the Loess Plateau is the largest in January,? has obvious seasonal changes.MERRA-2 can basically reflect the annual characteristics of aerosol optical characteristics,but deviations occur in some regions.(4)MERRA-2 shows that from 2000 to 2018,the highest AOD value is the Sichuan Basin,with an average value of about 0.8 or higher.AOD value is larger in spring and summer and smaller in autumn and winter.The Taklimakan Desert has the lowest AOD value in winter.The Sichuan Basin has high AOD in all four seasons,and relatively lower in summer.Beijing,Tianjin and Hebei can achieve 0.8 in summer.AOD in Southeast Tibet and the Taklimakan Desert increased significantly from 2000 to 2018,with growth rates of 0.01 and 0.006 per year,respectively.The growth rate of AOD in Guangxi and Guangdong in spring is about 0.013 per year,and the AOD in the Sichuan Basin and surrounding areas increases significantly in winter at a rate of 0.012 per year.(5)? gradually increases from west to east in space,but there are differences between seasons.In spring,? is the smallest.In the desert area,? is less than 0.3.Beijing,Tianjin and Hebei's ? is also less than 1,the predominant in spring are coarse particles.The coastal areas are dominated fine particle in four seasons,it is larger in autumn and winter,which is greater than 1.5.(6)For each type of aerosol optical depth and proportion,the sea salt AOD on the mainland is less than 0.01,accounting for about 5%,coastal area are higher,about 0.05,accounting for 10%-20% of the total AOD;Black carbon AOD is low in the west and high in the east.Black carbon AOD is about 0.01 in the west and accounts for a small proportion.The highest proportion is in Beijing,Tianjin,Hebei and parts of Henan,accounting for 10% of the total AOD;Sulfate AOD is as high as 0.5 or more in the Sichuan Basin,and its proportion gradually increases from west to east,Xinjiang accounts for about 20%;The highest dust AOD is in the Taklimakan Desert,about 0.33.Proportion is smaller in the eastern,Xinjiang accounts for about 75% of the total AOD;The organic carbon aerosol has the largest optical depth in Sichuan,Chongqing,Guangxi,and Hunan,about 0.1.In Southeast Tibet and Yunnan account for the largest proportion,about 33%.(7)The black carbon AOD is large in winter and the smallest in summer.The Sichuan Basin has high values of black carbon and sulfate AOD,which are above 0.07 and 0.6 in winter;The Taklimakan Desert has a high optical depth of dust aerosols in four seasons,the largest in spring and summer,about 0.45;The optical depth of organic carbon in spring was significantly larger than other seasons.From the perspective of the changing trend from 2000 to 2018,the optical depth of black carbon in northeast Sichuan showed a significant decrease trend,the rate of decrease was about 0.0005 per year;The increase rate of sulfate AOD is the largest in Chongqing,above 0.008;The optical depth of dust aerosols in the Taklimakan Desert has increased significantly at a rate of 0.004 per year;Organic carbon,black carbon,and sulfate AOD in southeastern Tibet all increased significantly,with sulfate AOD increasing the most at 0.005 per year.