SSAOD Percent = SSAOD/AOD (5)

**Figure 6.** Spatial distribution of mean MERRA-2 values of (**a**) SO4AOD, (**b**) BCAOD, (**c**) OCAOD, (**d**) DUAOD, and (**e**) SSAOD over the SCS during the period of 1980–2020.

Figure 7 showed the contributions of SO4AOD, BCAOD, OCAOD, DUAOD, and SSAOD to the total AOD over the SCS, respectively. SO4AOD Percent and SSAOD Percent were obviously higher than BCAOD Percent, OCAOD Percent, and DUAOD Percent. BCAOD Percent and DUAOD Percent had the lowest values which were less than 10% over the entire SCS. OCAOD Percent was mainly between 10% and 20%, and only a small part in the middle of the SCS was under 10%. SO4AOD Percent decreased from north to the south over the SCS. The highest values of SO4AOD Percent appeared in the north of the SCS, which were between 50% and 60%. Relatively lower values of SO4AOD Percent, between 30% and 40%, occurred in the middle and south of the SCS. Different from SO4AOD Percent, SSAOD Percent was low in the north, and higher in the middle and south of the SCS. In the north of the SCS, SSAOD Percent was between 10% and 20%, which was the lowest over the SCS. Most SSAOD Percent values over the SCS were between 30% and 50%. The highest SSAOD Percent values (between 50% and 60%) appeared in a small part in the middle of the SCS.

As shown in Figure 8a, an obvious annual mean TAOD increase followed by a decrease could be observed over NSCS during the period of 1980–2020, and the same was for SO4AOD which was higher than BCAOD, OCAOD, DUAOD, and SSAOD in this region. The TAOD maxima at the beginning of the 1980s and 1990s was due to the El Chichon eruption in Mexico in 1982 [48] and the Pinatubo eruption in the Philippines in 1991 [49]. With the deposition of the pollutants, TAOD gradually decreased after 1982 and 1991, respectively. He et al. [51] studied the MODIS AOD over the entire area of mainland China between 2002 and 2015, and found an upward tendency pre-2008 due to booming economy and a downward tendency post-2008 as a result of the Chinese government's environmental protection policy which lead to emission reduction. In this study, the increase of TAOD (SO4AOD) during the period of 1980–2007 was 0.0029 (0.0013) per year, while the decrease of TAOD (SO4AOD) from 2008 to 2020 was −0.0033 (−0.0029) per year over NSCS. The mean value of SO4AOD during the period of 1980–2020 accounted for 47.9% of TAOD over NSCS. The R between TAOD and SO4AOD from 1980 to 2020 was about 0.93, indicating that the variation of TAOD was mainly due to the variation of SO4AOD in this area. In Figure 8b,c, no significant variation trend of TAOD and SO4AOD could be observed from 1980 to 2020. This could be due to the far distance of MSCS and SSCS from mainland China. SSAOD was higher than SO4AOD in MSCS and SSCS. SSAOD accounted for 46.5% and 41.7%, while SO4AOD accounted for 37.9% and 35.6% in MSCS and SSCS, respectively.

**Figure 8.** Variation of annual mean values of TAOD, BCAOD, OCAOD, DUAOD, SO4AOD, and SSAOD over (**a**) NSCS, (**b**) MSCS, and (**c**) SSCS during the period of 1980–2020.
