*3.4. Spatial Distribution and Temporal Variation of MERRA-2 AOD of Different Aerosol Compositions*

Black carbon, organic carbon, dust, sulfate, and sea salt aerosol are the main components in the aerosol. MERRA-2 AOD is the sum of SO4AOD, BCAOD, OCAOD, DUAOD, and SSAOD. As shown in Figure 6, the mean BCAOD, OCAOD, DUAOD, SO4AOD, and SSAOD during 1980 and 2020 showed different characteristics over the SCS. SO4AOD and SSAOD had higher values than BCAOD, OCAOD, and DUAOD. BCAOD and DUAOD had the lowest values which were less than 0.02 over the entire SCS. OCAOD was also low, and was under 0.04 over most part of the SCS. Only in the north of the SCS, OCAOD exceeded 0.04. SO4AOD showed similar spatial distribution to the total AOD mentioned above, while the spatial distribution of BCAOD and DUAOD was completely different from that of total AOD. The spatial distribution of BCAOD over the SCS showed almost no difference, and so did DUAOD. The spatial distribution with most obvious difference over the SCS was SO4AOD. SO4AOD was high in the north and relatively low in the south, and gradually decreased from the north to south over the SCS. More obvious SO4AOD variation appeared in the north of the SCS. The highest SO4AOD (over 0.20) appeared in the north of the SCS, and the lowest SO4AOD (under 0.06) occurred in the south of the SCS. SO4AOD in the middle of the SCS was mainly between 0.06 and 0.08. The SSAOD was between 0.06 and 0.10 over most part of the SCS, and the highest SSAOD values were mainly between 0.08 and 0.10. Total AOD over SCS was mainly affected by SO4AOD and SSAOD.

The contribution of SO4AOD, BCAOD, OCAOD, DUAOD, and SSAOD to AOD was obtained through the equations

SO4AOD Percent = SO4AOD/AOD (1)

$$\text{BCAOD Percent} = \text{BCAOD} / \text{AOD} \tag{2}$$

$$\text{OCAOD Percent} = \text{OCAOD} / \text{AOD} \tag{3}$$

$$\text{DUAOD Percent} = \text{DUAOD} / \text{AOD} \tag{4}$$
