The subject of aerosol-cloud interactions has received prominent attention because of still higher uncertainty in estimating its climatic forcings and its possible climatic implications. Aerosol chemical and physical properties, cloud dynamic and turbulent characteristics, and the aerosol-cloud interactions should be considered together when evaluating the aerosol indirect effects, since the underlying mechanisms appear to be dependent upon each other, and accounting for them is impossible with the current understanding of aerosol indirect effect. Although the various studies on the aerosol indirect effect have been carried out all over the world, relatively few focuses have been put on the Northeast Asian region, which has suffered from a lot of anthropogenic air pollution and various kinds of aerosol compositions. Detailed and integrated careful observations are needed to understand the complex coupled mechanisms of aerosol-cloud interaction and its radiative forcing, but these strategies have not been applied to the region yet. The first step to aerosol indirect study in Northeast Asia is to have an overall understanding of the current state of aerosol and cloud optical properties derived from ground- and satellite-based remote sensings available since around early 2000, which eventually could provide the basic ground for both aerosol and cloud communities.

First of all, there seem to be no annual increasing/decreasing trends of monthly-average aerosol optical depth (AOD) from MODIS (MOD08) in the downstream region of China, which is also confirmed by the aerosol robotic network (AERONET). In general, AOD showed the strong horizontal gradient from China to Korea, with no relevant systematic association with the effective radius and optical depth of the liquid-phase cloud, which might be attributable to the masking synoptic meteorological variations and the coarse horizontal grid. Specifically comparisons of AOD and the effective radius demonstrated the significant negative correlation only in summer and over the Yellow Sea, indicating the seasonal and spatial sensitivity of aerosol-cloud interactions.