Evaluation of the Aqua-MODIS C6 and C6.1 Aerosol Optical Depth Products in the Yellow River Basin, China

Abstract

In this study, Aqua-Moderate Resolution Imaging Spectroradiometer (MODIS) Collection (C6) and C6.1 Dark Target aerosol optical depth (AOD) retrievals at 3 km (DT3K) and 10 km (DT10K), Deep Blue AOD retrievals at 10 km (DB10K), and combined DT and DB (DTB) AOD retrievals at 10 km resolutions were validated from 2002 to 2014 against ground-based sunphotometer AOD measurements obtained from the Chinese aerosol remote sensing network (CARSNET). The CARSNET AOD data were obtained for sites at Mt. Waliguan (MW), Lanzhou (LZ), Ulate (UL), and Zhengzhou (ZZ) located in the Yellow River basin (YERB) region, China. Errors and agreement between satellite and ground data were reported using Pearson's correlation (R) and relative mean bias (RMB). Results showed that the DT3K C6.1 highest quality flag (QF = 3) AOD retrievals were well correlated with the sunphotometer AOD data, with an R of 0.82 and an RMB of 1.01. Overestimation and underestimation in DT AOD retrievals were observed for AOD > 1.1 and AOD < 1.1, respectively. A significant underestimation of 37% in DB10K AOD retrievals was observed across all the sites except ZZ, which was indicated by a low-value RMB (0.63). Spatial distribution maps showed high AOD values (>0.8) over the lower part of the YERB and low AOD values (<0.4) across the upstream part of the YERB. This might be due to a large number of aerosol emissions over the lower developed areas and a scarcity of aerosols over the upstream mountain areas. Overall, this study supports the use of DT10K C6.1 AOD retrievals over the western semi-arid and arid regions of the YERB and DTB10K AOD retrievals over the north-central water system and eastern plain regions of the YERB.Atmosphere 2019, 10, 426 2 of 18 climate-change assessment and air-quality monitoring, it is necessary and crucial to conduct long-term continuous aerosol observations at a regional scale [10][11][12][13].Scientists have made many efforts in establishing global and national aerosol observation networks for regular measurements of aerosol optical properties, such as the aerosol robotic network (AERONET) and the Chinese aerosol remote sensing network (CARSNET) [14]. These networks provide aerosol optical properties directly by using ground-based sunphotometers [15][16][17]. However, due to the high cost of construction and operation of ground-based sunphotometers, they are few in number at a global scale, especially in remote areas. Therefore, satellite remote sensing is expected to become an ideal technology for aerosol optical and radiation characteristic observations at regional and global levels. In recent decades, various satellite sensors including the Moderate Resolution Imaging Spectrometer (MODIS) have been used for regular aerosol monitoring [18][19][20].Recently, MODIS Collection (C6) aerosol products have been widely validated over land and ocean surfaces [21][22][23][24][25][26][27][28][29][30][31][32][33], including the Indian subcontinent [34][35][36], Southeast Asia [37], East Asia [38,39], Greece [40,41], and China [42][43][44]...

Comparison of MODIS- and CALIPSO-Derived Temporal Aerosol Optical Depth over Yellow River Basin (China) from 2007 to 2015

“…This phenomenon is caused mainly by the fact that the MYD04 C6.1 DT3K images had higher resolutions. On the other hand, the reason could be that C6.1 DT10K has few retrievals for the water system over the YERB, as mentioned by (Zhang et al 2019b). Figure 4 shows the validation of the MYD04 C6.1 DB10K AOD and CALIPSO AOD over the YERB area.…”

“…Since MODIS aerosol products are updated frequently, many scholars have carried out a large number of verification studies of new MODIS AOD products by comparing them with AERONET or CARSNET ground-based remote sensing data (Bilal et al 2018;Che et al 2015;Dubovik et al 2012;He et al 2017;Shi et al 2019;Wang et al 2019). Zhang et al (2019b) evaluated the performance of MODIS Collection C6.1 (C6.1) AOD products over the Yellow River basin (YERB) by comparing the AOD data from ground-based CARSNET site and obtained good results (Zhang et al 2019b). The study suggested that no single satellite AOD product performed satisfactorily over YERB.…”

“…The main reason might be that the location of the substellar point of CALIPSO is difficult to match precisely with the few ground-based sites, making it difficult to verify the CALIPSO aerosol products. Therefore, we decided to compare the MODIS and CALISPO products with each other to indirectly verify the performance of CALIPSO, based on the previous research results of MODIS-CARS-NET obtained from the studies of Zhang et al (2019a) and Zhang et al (2019b). On the other hand, the performance of CALIPSO was good in studying the properties of the lowest aerosol layers over YERB.…”

“…In recent years, as the environmental issues faced by China have grown to an unprecedented scale, researchers have placed much emphasis on studying the optical, physical, and distribution characteristics of aerosols in the Yangtze River delta, Beijing-Tianjin-Hebei, the Pearl River delta, and other important developed areas (Bilal et al 2019(Bilal et al , 2013(Bilal et al , 2014Deng et al 2008;Jie et al 2017;Liu et al 2008;Shen et al 2015;Xia et al 2016). Those authors obtained the long-term temporal variation and spatial distribution of the aerosols, but less research has been carried out with regard to the aerosol distribution characteristics over YERB (Zhang et al 2019b). The YERB is located in the north-central part of mainland China and lies in an east-west direction across the country.…”

“…The regional economies and industrial development of the YERB sub-regions have even more imbalance. Long-term accurate monitoring of aerosol optical physical characteristics and their spatiotemporal distributions over each sub-region of the YERB has important practical significance for the protection of the atmospheric environment over the YERB (Zhang et al 2019b). According to the characteristics of the YERB, this paper evaluates the performance of the CALIPSO AOD product over the YERB by comparing it with the Aqua MODIS C6.1 AOD products.…”