Abstract
Due to the significant spatial variation of the performance of Moderate Resolution Imaging Spectrometer (MODIS) aerosol optical depth (AOD) retrievals, validation is very important for applications of MODIS AOD products at regional scales. This study presents a comparative analysis of Collection 6.1 MODIS AOD retrievals and ground measurements from five local sites and one Aerosol Robotic Network (AERONET) site in the Yangtze River Delta (YRD) region, which significantly complements the previous validation that utilized limited AERONET measurements. Generally, MODIS AOD retrievals showed a reasonable agreement with collocated ground measurements (R2 > 0.7), with 66% of Dark Target (DT) 10 km retrievals, 56% of Deep Blue (DB) 10 km retrievals, and 69% of DT 3 km retrievals falling within the expected error (EE = ±(0.05 + 0.2 × AOD)). Nevertheless, it was found that the DT AOD retrievals tended to be overestimated over urbanized and lakeside sites, while the DB AOD retrievals tended to be underestimated over all ground sites except for lakeside sites. Such patterns appeared to be linked with the systematic biases of the single-scattering albedo estimation in the AOD retrieval algorithms. Another significant finding of this study is that the uncertainties of the MODIS AOD retrievals were highly correlated with the land cover proportions of urbanized features and water (LCP_UW) in the surrounding region, especially for the DT products. An empirical correction method based on these correlations could substantially reduce the uncertainties of DT AOD products over high LCP_UW areas. The results not only highlight the significant impacts of both urban and water areas on the MODIS AOD retrieval algorithms but also create new possibilities to correct such impacts once the universal correlations between LCP_UW and the uncertainty measures are established.
Highlights
Our results showed that the slopes of the C6.1 Dark Target (DT) retrievals increased to ~0.9 for the 10 km product and ~1.0 for the 3 km aerosol optical depth (AOD) product, indicating that the systematic bias was effectively removed through the release of the new product
We provide a comprehensive comparison between the C6.1
We provide measurements a comprehensive between the C6.1
Summary
Licensee MDPI, Basel, Switzerland.Attribution (CC BY) license (https://creativecommons.org/licenses/by/ 4.0/).Atmospheric aerosols, which are defined as fine solid or liquid particles suspended in the atmosphere, play an important role in the Earth-atmosphere system through scattering and absorbing solar radiation (known as the aerosol direct effect) and modifying the microphysical properties and lifetimes of clouds (known as the aerosol indirect effect) [1]. Aerosol is also closely connected to many significant issues, including climate change, air quality, and public health [2,3,4]. However, owing to the wide spatial and temporal variability, complicated physical and chemical compositions, and short life cycle, atmospheric aerosol continues to be a large source of uncertainty in the climate system [5]. Although satellite
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