Climatology and trends of aerosol optical depth with different particle size and shape in northeast China from 2001 to 2018.

Aerosol generated from the economic development and extensive urbanization in northeast China (NEC) could influence aerosol optical properties and affect the regional air quality. The level 3 aerosol optical depth (AOD) of different particle size and shape (spherical or nonspherical) obtained by Multiangle Imaging Spectroradiometer (MISR) version 23 were used to estimate their seasonal, annual, and decadal distribution and contribution in NEC from 2001 to 2018. The highest AOD of approximately 0.3 was found in the central Liaoning urban agglomeration, and the lowest AOD occurred in the mountainous area of NEC; the proportion of spherical AOD in NEC region was more than 90%. The contribution of large AOD was higher in spring, ranging from 28.8% to 29.8%. In spring and summer, small and medium AODs were concentrated in central Liaoning (approximately 0.2-0.3 and 0.06-0.08, respectively). The annual variation in the AOD of different particle size was significantly higher in Liaoning than in Jilin and Heilongjiang. The annual proportions of small and spherical AODs were approximately 60% and 90%, respectively. The annual occurrence of clean conditions with AOD < 0.05 was most common in northern Heilongjiang (approximately 20%). In NEC, the annual occurrence frequencies of 0.05 < AOD < 0.15 and AOD > 0.6 were the highest (approximately 50%) and the lowest (less than 1%), respectively. Interdecadal AOD revealed a positive trend from 2001 to 2008 and a negative trend from 2009 to 2018. The frequency of occurrence trend at different AOD levels also changed from positive to negative between these two periods. The findings in this study are based on the first aerosol retrieval of the newly released MISR in NEC. The results provide a comprehensive understanding of the regional and climatological aerosol extinction with different AOD of size and shape as well as various level bins in NEC.