*3.2. Spatial Distribution and Temporal Variation of MERRA-2 AOD*

There are 41 years from 1980 to 2020, totalling 492 months. With the monthly MERRA-2 AOD data, we obtained the mean value of MERRA-2 AOD during the period between 1980 and 2020. In Figure 2, the four-decade-long mean MERRA-2 AOD over the SCS was obviously high in the north and low in the south, and gradually decreased from the north to south. More obvious AOD variation could be seen in the north of the SCS. The highest AOD between 0.35 and 0.40 is over China's offshore region. This is due to the aerosol transport from land with higher AOD (over 0.40) to the ocean. The aerosol over the offshore region can be easily affected by the aerosol over land because of the short distance. The lowest AOD appeared in the southern part of the SCS where AOD was less than 0.15. In the middle of the SCS, AOD value was between 0.15 and 0.20. The mean MERRA-2 AOD value over Dongsha was between 0.275 and 0.30, while the AOD value over Taiping was between 0.15 and 0.175. AOD over Dongsha was higher than AOD over Taiping, which was consistent with the conclusion by Zhang et al. [43].

**Figure 2.** Spatial distribution of mean values of the 550 nm MERRA-2 AOD over the SCS in the period of 1980–2020.

As shown in Figure 3, the spatial distributions of mean MERRA-2 AOD over the SCS in eight 5-year periods showed similar distribution from 1980 to 2020. High AOD appeared in the north of the SCS, and low AOD appeared in the south of the SCS in the eight 5-year periods. In the period of 1980–1985, AOD in the north of the SCS was between 0.20 and 0.30, and AOD in the majority of the southern region of the SCS was between 0.15 and 0.20. In the period of 1986–1990, AOD in the south of the SCS was only between 0.10 and 0.15, which was obviously lower than that in 1980–1985. This could be due to the eruption of the El Chichon volcano in 1982 [48], which caused relatively higher AOD in 1980–1985. Because of the eruption of Pinatubo volcano in 1991 in the Philippines [49], AOD rose obviously in the period of 1991–1995, and was higher than that in 1986–1990 over the entire SCS. With the gradual deposition of volcanic pollutants, an obvious AOD decrease could be observed in 1996–2000. During 1996–2000, AOD was again below 0.15 in the south of the SCS. After 2000, rapid industrial development of China and the aerosol transport from the land to ocean contributed to the AOD increase over the SCS. AOD over the offshore region of China in the north of the SCS after 2000 was obviously higher than that before 2000. Affected by the high AOD over land, AOD over the offshore region of China could exceed 0.35 after 2000. The sea area to the west of Hainan Island, whose location could be seen in Figure 2, is about 0.40 in the period of 2001–2005. The AOD of this sea area exceeded 0.50 in 2006–2010 and 2011–2015, and then decreased to about 0.40 in 2016–2020.

**Figure 3.** Variation of MERRA-2 AOD in 5-year intervals in the period of 1980–2020.

Figure 4 illustrated the mean AOD in each month over the SCS between 1980 and 2020. In most months, similar spatial distribution could be observed. High AOD appeared in the north and low AOD occurred in the south. In addition, high AOD could be observed in spring and winter, while low AOD occurred in summer. In March and April, AOD over the north of the SCS exceeded 0.50, while in June and July AOD over the north of the SCS was only about 0.20. In September, higher AOD could be observed in the south than in the middle of the SCS. These could be explained by the following facts. With a monsoon climate, the SCS has a northeast monsoon in winter and spring, and a southwest monsoon in summer and autumn. In winter and spring, dust and pollution transported from mainland China and Japan in the northeast, while during summer and autumn, the SCS received biomass burning pollutants from Malaysia and Indonesia from the southwest [41].

**Figure 4.** Spatial distribution of mean MERRA-2 AOD in each month in the period of 1980–2020.
