**1. Introduction**

Aerosol is an important constituent of the atmosphere, and plays a key role in the radiation balance, regional and global climate change, and human health [1–11]. Aerosol is released to the atmosphere through both natural and anthropogenic processes. To have a better understanding of aerosol, a large number of studies about aerosol have been conducted by researchers around the world [12–25]. However, at present, aerosol optical property is still one of the largest sources of uncertainty in estimating climate change. AOD represents extinction of light over the vertical column of atmosphere. As an important measure of aerosol loading, AOD is a crucial metric in assessment of global climate change and the impact of aerosol on precipitation, radiation, and clouds [26]. Ground-based

**Citation:** Sun, E.; Fu, C.; Yu, W.; Xie, Y.; Lu, Y.; Lu, C. Variation and Driving Factor of Aerosol Optical Depth over the South China Sea from 1980 to 2020. *Atmosphere* **2022**, *13*, 372. https://doi.org/10.3390/ atmos13030372

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 20 January 2022 Accepted: 21 February 2022 Published: 23 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

measurement with high accuracy is an effective method to obtain AOD, and is always used to validate satellite and reanalysis data.

Ground-based observation networks mainly include AERONET, China Aerosol Remote Sensing Network (CARSNET), and Sky Radiometer Network (SKYNET). AERONET could provide consistent measurements of aerosol optical properties, and was known as a global sun-photometer network [27–31]. Established by the China Meteorological Administration (CMA) in 2002, CARSNET is an independent aerosol monitoring network with the same type of sun-photometer as AERONET [32–35]. SKYNET equipped with pyranometer and sun/sky photometer is a radiation network [36]. Ground-based observation has high accuracy, but it offers limited temporal and spatial coverage. Compared with sparse ground-based observation, satellite remote sensing can provide measurements covering large areas. For example, the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products are usually used in aerosol analysis. Although MODIS data can cover a large area, the study period is only about 20 years. In addition, there are limitations under cloudy conditions for satellite data. Reanalysis product plays an important role in the areas which lack measurements or only have short-term observations. MERRA-2 aerosol products not only can cover large areas, but also provide long-term aerosol data from 1980 to present. Moreover, MERRA-2 also can offer aerosol data under cloudy conditions.

Due to rapid industrial development, air pollution has become a serious problem in the regions where people live. A lot of researchers have carried out studies about aerosol characteristics over land. Yu et al. [37] did research about aerosol optical properties in Beijing with AERONET measurements between 2002 and 2007, finding an obvious seasonal variation with higher AOD values in spring and summer. Kang et al. [38] conducted research about aerosol optical property in Nanjing through ground-based measurements from September 2007 to August 2008, and arrived at the conclusion that aerosol over Nanjing mainly contained the mixed type, urban-industrial type, and biomass burning type. Che et al. [39] studied aerosol pollution in eastern China with ground-based measurements between 2011 and 2015 from seven observation sites, pointing out that—different from northern China—AOD in the Yangtze River Delta during July and August is lower than in January and February because of particle dispersion. Due to air mass transport, natural and anthropogenic aerosol can spread through the atmosphere over a large spatial scale. The transport of atmospheric aerosol from continental areas to oceans leads to the spatiotemporal heterogeneity of the atmospheric aerosol characteristics over oceans, which can influence the regional climate [40]. Therefore, aerosol study over the oceans is also important.

As a low-latitude sea, the South China Sea (SCS) is located in the south of China and is adjacent to several Asian countries. The SCS is a major sea route which connects the Pacific Ocean and the Indian Ocean. With abundant marine resources, the SCS is the largest marginal sea of China. Large amounts of atmospheric aerosols from these surrounding regions transport to the SCS [41]. The deposition of aerosol over sea has an important impact on the seawater. In order to comprehensively understand the significant role of aerosol in climate change and the biogeochemical cycle, variation of aerosol properties over sea needs to be studied. At present, much attention has been given to the SCS. Sun et al. [42] conducted research about variability of AOD in the SCS and concluded that AOD had an obvious dependence on the wind speed. Zhang et al. [43] investigated the aerosol properties over the SCS with ground-based observations in two sites, including Taiping and Dongsha. The study found that AOD in Taiping is less than 0.2 and that AOD in Dongsha was between 0 and 0.6. Li et al. [44] studied the aerosol properties over the northern area of the SCS between 9 August and 7 September 2016 with a shipborne micro-pulse lidar, finding that AOD was lower over the southwest side than the northeast side of the cruise region.

Although investigations about aerosol properties over the SCS have been conducted, the study period of most studies is not long enough. It is essential to conduct investigation on the long-term variation of aerosol loadings over the SCS. In this study, variations of

MERRA-2 AOD, BCAOD, OCAOD, SO4AOD, DUAOD, and SSAOD from 1980 to 2020 were analyzed to offer a better understanding of aerosol property over the SCS.
