Climatological variations in aerosol optical depth and aerosol type identification in Liaoning of Northeast China based on MODIS data from 2002 to 2019

The climatological variations of aerosol optical properties are significant in the study of aerosol effect on regional environment. The MODIS level 2 aerosol optical depth (AOD) products (MOD04/MYD04) at 10 km were used to identify the spatial distribution, interdecadal trends, and aerosol types from 2002 to 2019 in Liaoning province over Northeast China. The highest AOD was distributed in central Liaoning, with a peak value of approximately 0.6 and the fine mode fraction (FMF) was higher in eastern Liaoning about 0.8. The overall AOD for Liaoning in summer was significantly higher than that in other seasons, and the higher AOD values were observed in central Liaoning and the area around Bohai Rim (up to approximately 0.7). The frequency of 0.1 ≤ AOD < 0.2 and 0.2 ≤ AOD < 0.3 was highest in 14 cities of Liaoning. For FMF > 0.8, the cities were categorised into those with an increase frequency of 15% to 20%, and those with a decrease frequency of 5% to 10%. During 2014 to 2019, the interannual variation in AOD in Liaoning exhibited a significant downward trend, with a minimum AOD of approximately 0.25 in 2019. The FMF in Liaoning from 2002 to 2019 exhibited a fluctuating trend around 0.5. The AOD in Liaoning showed an increasing trend from 2002 to 2008 with a maximum of approximately 0.02/year and a clear decreasing trend from 2009 to 2019 of approximately −0.02/year. Maritime aerosol mostly occurred in cities in western Liaoning accounted for 20.58% to 36.52%. Dust aerosol had a considerable influence on central Liaoning accounting for 2.95% to 16.18%. In eastern Liaoning, continental background aerosol accounted for 36.69% to 43.30%. These results revealed the influence of AOD and fine particles on aerosol extinction in Northeast China and provided important information for the climate change in Northeast Asia.