Validation And Spatiotemporal Analysis Of Satellite Aerosol Products Over Eastern China

Nowadays,the theory,algorithm and technology of satellite aerosol remote sensing are constantly improved,and the product versions are continuously updated.Terra MODIS,Aqua MODIS,and NPP VIIRS have all released the latest version of AOD(Aerosol Optical Depth)products in recent years.However,there are few validation studies focused on the new versions and the error sources of inversion algorithm,which is not conducive to the application and promotion of products.In this paper,the Eastern China is taken as the study domain,where the regional composite air pollution problems are prominent.Firstly,the AERONET(Aerosol Robotic Network)data was used to perform strict and sufficient validation,and explore the possible error sources;considering the validation results and time range of data available,Terra MODIS product was used to analyze the AOD characteristics from2000 to 2019.The main conclusions are as follows:(1)All satellite products show high precision,and the correlation coefficients R of all products are greater than 0.9;NPP shows the highest precision,with the highest correlation coefficient R and determination coefficient R~2,the lowest root mean square error RMSE and the mean bias,the slope of the linear fitting line is closest to 1,the highest proportion falls within the expected error EE;Terra and Aqua show similar performance;After quality assurance,the accuracies of all products have been greatly improved.Therefore,the products after quality assurance are recommended for aerosol research and application,especially for Terra and Aqua.(2)The error analysis of satellite AOD products shows that the error of all products increase with the increase of AOD,especially when AOD>1.5,the maximum bias exceeds±0.2;All satellite AOD products show poor precision under the situation of high AOD(AOD>0.8)and small aerosol particle sizes(1<(?)ngstr(?)m exponent<1.5).No obvious relationship between the error and(?)ngstr(?)m exponent when AOD is low(AOD<0.1).Therefore,more attention should be paid to the case of high AOD and fine particles;The accuracy is highest in urban areas(bright surface),while worst in water areas(dark surface),and there is a slight overestimation over cropland and forest.In the future,it should focus on improving the accuracy of products over relatively dark surfaces such as water bodies and forest,so as to better adapt to the needs.(3)According to the inter-annual,seasonal and monthly characteristics of AOD,obvious regional differences could be found.High-value areas are mainly concentrated in urban areas,industrial dense areas and eastern coastal areas;while low-value areas are mainly concentrated in areas with sparse population,high terrain,inconvenient transportation and high vegetation coverage.Statistical analysis of AOD in various provinces showed that the AOD of Shandong Province,Tianjin City,Jiangsu Province,Henan Province were higher(AOD>0.6),while Taiwan Province,Beijing City,Shanxi Province,Fujian Province were lower(AOD<0.4).(4)AOD temporal variations released that the AOD in Eastern China showed a trend of slowly increasing(+0.049/10a)and then rapidly decreasing(-0.205/10a)from2000 to 2019,with the year of 2011 as the turning point.Overally,the AOD decreased over the past 20 years(-0.042/10a),indicating that a series of air pollution control measures have achieved significant results in recent years.Spatially,the provinces with the most obvious downward trend are Hunan Province(-0.077/10a)and Hubei Province(-0.066/10a),while Shandong Province,Jiangsu Province,Hainan Province,Shanghai remained almost stable.Temporally,the decline rate was fastest in summer(-0.062/10a)and slowest in winter(-0.016/10a).AOD in Eastern China usually highest in summer,followed by spring and winter,and lowest in autumn,and the maximum and minimum values occur in June and November,respectively.(5)Different regions have their own unique AOD spatial distribution and seasonal variation characteristics.High AOD values are mostly concentrated in cities with developed economies,high levels of urbanization and industrialization;From the long time series,the annual average AOD(0.749)of the Huaihai Economic Region is much higher than other regions in all years,and the air pollution is the most severe,followed by the Yangtze River Delta(0.569),and the AOD values of the Pearl River Delta and Beijing-Tianjin-Hebei region are relatively close(0.493,0.489).In the past 20 years,AOD in all regions has shown a trend of increasing first and then decreasing,and as a whole,they showed a weak downward trend,especially in Pearl River Delta.(6)Atmospheric governance measures have achieved significant results by implementing the"Ten Atmospheres Measures".Compared with 2012,the area of AOD in Eastern China decreased by more than 10%accounted for 78.98%,and the area decreased by more than 30%accounted for 46.53%.Among them,Huaihai Economic Region is the fastest(-0.489/10a),followed by Pearl River Delta(-0.403/10a)and Beijing-Tianjin-Hebei(-0.288/10a),and Yangtze River Delta is the slowest(-0.261/10a).