Abstract: Based on MODIS long term observations of aerosol daily product MOD04-L2, the temporal and spatial variations of aerosol optical properties and aerosol types in China from 2004 to 2018 are obtained using linear tendency estimation and AOD-AE aerosol classification methods. It is shown that during 2004-2018 : (1) high aerosol optical depth (AOD), values (550nm) are distributed in large urban agglomerations with low altitude, dense population and developed industries, while low AOD values are distributed in sparsely populated areas with high vegetation coverage mountains and grasslands. Areas such as Guizhou and the edge of Sichuan Basin are highlighted with high Ångstrom exponents (AE) and northwestern desert area with low values. (2) In 73 percent regions of ¨ China, AOD shows a decreasing trend during the observation period, and AE in the Eastern part of the “Hu Huanyong Line” also shows a decreasing trend. Especially, it is found that both AOD and AE decreased significantly during 2014-2018. (3) In terms of seasonal and monthly change trends, AE and clean continental aerosol are opposite to AOD, while urban industrial aerosol is the same as AOD. (4) The proportion of clean continental aerosols has increased year by year since 2014, indicating that air quality in China has gradually improved.

Key words: aerosol optical depth, ?ngstrom exponent, aerosol type, moderate-resolution imaging spectrora- ¨ diometer