Abstract
The aim of this study is to evaluate the accuracy of MODerate resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) products over heavy aerosol loading areas. For this analysis, the Terra-MODIS Collection 6.1 (C6.1) Dark Target (DT), Deep Blue (DB) and the combined DT/DB AOD products for the years 2000–2016 are used. These products are validated using AErosol RObotic NETwork (AERONET) data from twenty-three ground sites situated in high aerosol loading areas and with available measurements at least 500 days. The results show that the numbers of collections (N) of DB and DT/DB retrievals were much higher than that of DT, which was mainly caused by unavailable retrieval of DT in bright reflecting surface and heavy pollution conditions. The percentage falling within the expected error (PWE) of the DT retrievals (45.6%) is lower than that for the DB (53.4%) and DT/DB (53.1%) retrievals. The DB retrievals have 5.3% less average overestimation, and 25.7% higher match ratio than DT/DB retrievals. It is found that the current merged aerosol algorithm will miss some cases if it is determined only on the basis of normalized difference vegetation index. As the AOD increases, the value of PWE of the three products decreases significantly; the undervaluation is suppressed, and the overestimation is aggravated. The retrieval accuracy shows distinct seasonality: the PWE is largest in autumn or winter, and smallest in summer. The most severe overestimation and underestimation occurred in the summer. Moreover, the DT, DB and DT/DB products over different land cover types still exhibit obvious deviations. In urban areas, the PWE of DB product (52.6%) is higher than for the DT/DB (46.3%) and DT (25.2%) products. The DT retrievals perform poorly over the barren or sparsely vegetated area (N = 52). However, the performance of three products is similar over vegetated area. On the whole, the DB product performs better than the DT product over the heavy aerosol loading area.
Highlights
Atmospheric aerosols are small particles (0.001–100 μm) from both natural and human sources suspended in the atmosphere that can significantly influence the ecosystem [1], climate, and hydrological cycle [2] due to their effect on radiative forcing [3,4], precipitation and clouds [5]
The results showed the poorest performance for the Dark Target (DT) algorithm over the study area, with only 45.6% of the collections falling within the EE, and with larger RMSE and MAE errors than the Deep Blue (DB) and DT and DB merged AOD products (DT/DB) retrievals
The DB aerosol optical depth (AOD) retrievals were in good agreement with AErosol RObotic NETwork (AERONET)
Summary
Atmospheric aerosols are small particles (0.001–100 μm) from both natural and human sources suspended in the atmosphere that can significantly influence the ecosystem [1], climate, and hydrological cycle [2] due to their effect on radiative forcing [3,4], precipitation and clouds [5]. Satellite remote sensing is the most effective way of measuring characteristics of aerosol on the global scale. Remote sensing retrievals of aerosol properties generally achieve a low accuracy, Atmosphere 2019, 10, 548; doi:10.3390/atmos10090548 www.mdpi.com/journal/atmosphere. The mass of African dust transported in the atmosphere is large [16], and it has been suggested that the transported dusts have a substantial influence on the regional radiative budget [17]. China, India, and Pakistan’s mean aerosol optical depth measured at AErosol RObotic NETwork (AERONET) sites [18] exceeds global background levels by 4–5 times [19]
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