Retrieval of aerosol optical thickness at 500m resolution using MODIS images : a study in Hong Kong and the Pearl River Delta region

Abstract

Aerosol retrieval algorithms for the MODerate Resolution Imaging Spectroradiometer (MODIS) have been developed to estimate aerosol and microphysical properties of the atmosphere which help to address aerosol climatic issues at global scale. However, higher spatial resolution aerosol products for urban areas have not been well researched, mainly due to the difficulty of differentiating aerosols from bright surfaces in urban areas. Here, a new aerosol retrieval algorithm using the MODIS 500m resolution images is described, for the retrieval of aerosol properties over complex urbanized regions such as Hong Kong and the Pearl River Delta region. The rationale of our technique is to first estimate the aerosol reflectances by decomposing the Top Of Atmosphere reflectances from surface reflectance and Rayleigh path reflectances. For the determination of surface reflectances, a modified Minimum Reflectance Technique (MRT) is used, and MRT images are computed for different seasons. A strong correlation is shown between the surface reflectances of MRT images and MODIS land surface reflectance products (MOD09), with a value of ca. 0.87, and a difference of only 0.03 between MRT images and field measured ground reflectances. For conversion of aerosol reflectance to Aerosol Optical Thickness (AOT), comprehensive Look Up Tables (LUT) are constructed which consider aerosol properties and sun-viewing geometry in the radiative transfer calculations. Four aerosol types, namely mixed urban, polluted urban, dust, and heavy pollution, were derived using cluster analysis for three years of AERONET measurements in Hong Kong, and their aerosol properties were input to the LUT calculation. The resulting 500m AOT images were found to be highly correlated with ground measurements from the AERONET (r2 = 0.767) and MicrotopsII sunphotometers (r2 = 0.760) in Hong Kong. This study demonstrates the feasibility of aerosol retrieval at fine resolution over urban areas. Due to the high temporal resolution of MODIS, the 500m AOT images can be used to monitor cross-boundary aerosols and the development of pollutant sources in the Pearl River Delta region surrounding Hong Kong.