**4. Conclusions**

We analyzed spatial and temporal variation of AOD and optical depth of different aerosol types during the period of 1980–2020 with MERRA-2 dataset, and the difference of spatial and temporal distribution of AOD was obtained. SO4AOD and SSAOD dominated the variation of AOD over the SCS. An AOD upward trend followed by a downward trend occurred in the north of the SCS in this period. Mean MERRA-2 AOD from 1980 to 2020 was high in the north and low in the south over the SCS, and gradually decreased from the north to the south in this region. Due to rapid economic development, AOD after 2000 reached a higher value than that in 1980s and 1990s. Mean AOD in each month from 1980 to 2020 was analyzed. Higher AOD was in spring and winter, and low AOD appeared in summer. Similar spatial distribution could be observed between SAOD and AOD over the SCS, and AAOD was much lower than SAOD in this region. SO4AOD and SSAOD had higher values than BCAOD, OCAOD, and DUAOD over the SCS. Over the north of the SCS, SO4AOD accounted for more than 50% of TAOD, while BCAOD and DUAOD accounted for only less than 10% of TAOD all over the SCS. TAOD over NSCS showed an upward trend from 1980 to 2007 and a downward trend from 2008 to 2020. Similar annual variation between TAOD and SO4AOD over NSCS from 1980 to 2020 could be observed, and the R between them reached 0.93. This indicated that variation of the SO4AOD was the driving force of the variation of TAOD over NSCS from 1980 to 2020. The spatial distribution of AOD variation trend over the SCS in two periods was also analyzed. Different AOD variation trends over the different areas of the SCS appeared during the two periods including 1980–2007 and 2008–2020. AOD increase could be observed over most of the SCS between 1980 and 2007, while AOD decrease was observed over most of the SCS from 2008 to 2020.

**Author Contributions:** Conceptualization, E.S.; Software, E.S., W.Y. and C.F.; Methodology, C.F. and C.L.; Writing—original draft, E.S.; Investigation, W.Y. and Y.X.; Supervision, Y.L.; Writing—reviewing and editing, C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Key Laboratory of South China Sea Meteorological Disaster Prevention and Mitigation of Hainan Province (SCSF202010).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** MERRA-2 aerosol optical property data used in this study is from https: //goldsmr4.gesdisc.eosdis.nasa.gov/data/MERRA2\_MONTHLY/M2TMNXAER.5.12.4/2020/, accessed on 19 January 2022.

**Acknowledgments:** We thank NASA for making the MERRA-2 aerosol reanalysis publicly accessible. Thanks are also extended to the anonymous reviewers for their useful comments and suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

