Abstract
Moderate resolution imaging spectroradiometer (MODIS) aerosol optical depth (AOD) products have been widely used to characterize the temporal variations and spatial distributions of atmospheric aerosols. In the present study, we evaluate the performance of four Terra and Aqua MODIS Collection 6 (C6) quality assured AOD products in the Pearl River Delta (PRD) region, a humid subtropical region. The 10 km AOD products retrieved by the Dark Target (DT) and Deep Blue (DB) algorithms, the merged DT/DB (DTDB) 10 km product, and the DT 3 km AOD product were obtained for 2006–2015. These products were compared with Aerosol Robotic Network (AERONET) observations, and with each other. The Terra- and Aqua-derived AODs are quantitatively similar. However, there are significant differences among the four AOD products. The DT 10 km product correlates more closely with AERONET AOD observations than does the DB 10 km product. The latter tends to underestimate the AOD, whereas the former typically overestimates it for highly urbanized areas. The DTDB 10 km product is mainly derived from the DT 10 km product; it does not provide a gap-filled data set, because valid DB 10 km retrievals are not included in the merged product even when DT 10 km retrievals are unavailable. Therefore, the DT/DB merging protocol should be improved. The DT 3 km AOD product closely mimics the DT 10 km product; however, it contains fewer data than the DT 10 km product over water-contaminated areas. In addition, although the quality assured AOD products are recommended for use in quantitative applications by the MODIS aerosol science team, the sampling frequency of these products is generally lower than 25%. Thus, the sampling issues of these products should be considered in humid subtropical areas.
Highlights
Aerosols are tiny particles suspended in the atmosphere
The Dark Target (DT) 3 km product is highly correlated to Aerosol Robotic Network (AERONET) observations, the root mean square error (RMSE) of DT 3 km is slightly higher than that of DT 10 km, and the number of collocations for DT 3 km is typically fewer than that of DT 10 km; this is consistent with previously reported results [19,30]
The number of Moderate resolution imaging spectroradiometer (MODIS)/AERONET collocations at the “Zhongshan Univ” site is small, the results demonstrate the limitations of the DT algorithm over non-vegetated urban areas, which is consistent with previous reports [15,18]
Summary
Aerosols are tiny particles suspended in the atmosphere. Naturally and anthropogenically generated aerosols have gained increasing attention because of their influences on climate and human health [1,2]. A variety of sensors that are well-suited for aerosol studies have been launched, which enable aerosol properties to be characterized at local, regional, and global scales [3,4]. Moderate resolution imaging spectroradiometer (MODIS) instruments on board Terra and Aqua satellites provide daily global records of aerosol properties [5]. Because the signal received by the sensor is composed of contributions from both the atmosphere and the surface, several algorithms have been developed to retrieve aerosol properties and produce aerosol products from MODIS. The operational MODIS aerosol products are generated using the Dark Target (DT) and Deep Blue (DB) algorithms; both of these algorithms are continuously being updated [6,7]. Current operational MODIS aerosol products are grouped into Collection 6 (C6), with a standard resolution of 10 km.
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