*3.5. Variation Trend of MERRA-2 AOD*

In Figure 9a, an AOD increase could be observed over most part of the SCS from 1980 to 2007. In this period, highest AOD increase (between 0.005 and 0.01 per year) could be seen in the north of the SCS, and the AOD increase (between 0 and 0.005 per year) occurred in the north and middle of the SCS. *p*-values of less than 0.05 mainly appeared in the north of the SCS. Hse et al. [52] studied the global trends of AOD over ocean with SeaWiFS data, and also found annual AOD increase trend over ocean from 1998 to 2010. In Figure 9b, AOD decreased over most of the SCS. AOD decrease (between −0.01 and −0.005 per year) appeared over the north of the SCS, and AOD decrease (between −0.005 and 0 per year) was over the north and middle of the SCS. *p*-values of less than 0.05 mainly occurred in the north of the SCS and a small part in the middle of the SCS. The AOD increase or decrease was more significant over the coastal region of mainland China due to the aerosol transport from land to ocean.

**Figure 9.** Variation trend of MERRA−2 AOD over the SCS during the period (**a**) 1980−2007 and (**b**) 2008−2020 (the small black dot represents *p* < 0.05).
