Coastal zones are inhabited by three-quarters of the global population and contribute to more than half of the global gross domestic product (GDP) despite accounting for only 5% of the Earth's land mass. Within coastal zones, river deltas are alluvial plains formed by the deposition of sediment and often occur where inland water flow leaves river estuaries and meets salty, stagnant ocean water. River deltas are home to more than half-a-billion people and are a crux of human civilization. The morphology of river deltas highly depends on the oceanographic forces of tides and waves and is also regularly reshaped by deltaic sediment cycling, which can be influenced by anthropogenic activities, such as damming and deforestation, resulting in the gain or loss of land area (Nienhuis et al., 2020;Syvitski et al., 2009). Quantifying the individual importance of tides, waves, and fluvial sediment supply in river delta formation is critical for policymakers to understand potential threats to deltas and develop adaptation strategies.As one of the largest deltas in Asia, the Pearl River Delta (see Figure 1a) is known for its high population density and economic level. The GDP of the Pearl River Delta reached 1,084.5 billion USD by 2017, accounting for nearly 9.1% of the GDP of mainland China (Statistics Bureau of Guangdong Province, 2019).

Assessments of long-term changes of air quality and global radiative forcing at a large scale heavily rely on satellite aerosol optical depth (AOD) datasets, particularly their temporal binning products. Although some attempts focusing on the validation of long-term satellite AOD have been conducted, there is still a lack of comprehensive quantification and understanding of the representativeness of satellite AOD at different temporal binning scales. Here, we evaluated the performances of the Moderate Resolution Imaging Spectroradiometer (MODIS) AOD products at various temporal scales by comparing the MODIS AOD datasets from both the Terra and Aqua satellites with the entire global AErosol RObotic NETwork (AERONET) observation archive between 2000 and 2017. The uncertainty levels of the MODIS hourly and daily AOD products were similarly high, indicating that MODIS AOD retrievals could be used to represent daily aerosol conditions. The MODIS data showed the reduced quality when integrated from the daily to monthly scale, where the relative mean bias (RMB) changed from 1.09 to 1.21 for MODIS Terra and from 1.04 to 1.17 for MODIS Aqua, respectively. The limitation of valid data availability within a month appeared to be the primary reason for the increased uncertainties in the monthly binning products, and the monthly data associated uncertainties could be reduced when the number of valid AOD retrievals reached 15 times in one month. At all three temporal scales, the uncertainty levels of satellite AOD products decreased with increasing AOD values. The results of this study could provide crucial information for satellite AOD users to better understand the reliability of different temporal AOD binning products and associated uncertainties in their derived long-term trends.

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