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Abstract
This study evaluates the new Aqua MODIS Dark Target (DT) Collection 6 (C6) Aerosol Optical Depth (AOD) (MYD04_3K) retrieval algorithm at 3 km resolution over Asian countries that have recently experienced severe and increasing air pollution. Retrievals showed generally low accuracy compared with the AErosol RObotic NETwork (AERONET), with only 55% of retrievals within the expected error (EE). The uncertainty appears mainly due to systematic overestimation at both low and high AOD levels. This is attributed to under-prediction of surface reflectance, similar to, but more severe than, the C6 DT product at 10-km resolution. This is because MYD04_3K observes more noise in the surface reflectance computations, due to retention of some bright pixels in the retrieval window which would be discarded at 10 km. Greatest uncertainty was observed at urban sites, especially those dominated by coarse aerosols. Results suggest that the DT at 3 km is less reliable than MODIS C6 AOD products at 10 km.
Highlights
Atmospheric aerosols are small particles from both natural and human sources suspended in the atmosphere
Asia is the largest source of aerosols and their precursor gases, with China, India, and Pakistan’s mean aerosol optical depth (AOD) measured at AErosol RObotic NETwork (AERONET) sites exceeding global background levels [4] by 4–5 times
To increase the number of statistical samples for validation, collocation was defined as the average of at least two AERONET AOD measurements between 12:00 and 14:00 local time and at least two pixels of MODIS AOD observations within a sampling window of 3 ˆ 3 pixels centered on the AERONET site, i.e., the average of a 9 km 9 km spatial region for the
Summary
Atmospheric aerosols are small particles from both natural and human sources suspended in the atmosphere. Since they have wide ranging impacts on climate forcing [1], visibility [2], and human health [3], their unprecedented increase over Asian countries in recent decades is a cause for concern. Satellite remote sensing has been used to quantify and monitor aerosol distributions over much larger areas than can be covered by ground stations, and their optical properties including aerosol optical depth (AOD) can be derived from image visible wavebands. The DT algorithm over land utilizes the generally bright appearance of aerosols over dark surfaces such as vegetation in the visible wavebands.
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