Retrieval of aerosol optical thickness using MODIS 500 × 500m2, a study in Hong Kong and Pearl River delta region

Aerosol detection and monitoring by satellite observations has been substantially developed over past decades. While several state-of-art aerosol retrieval techniques provide aerosol properties in global scale, the more detail characteristics remain unknown because most of the satellite sensors are limited to 1 km resolution observations. However, a new aerosol retrieval algorithm for the Moderate Resolution Imaging Spectroradiometer (MODIS) 500 m resolution data is developed to retrieve aerosol properties over land, which helps address the aerosol climatic issues on the local/urban scale. The rationale of our technique is to first estimate the aerosol reflectances by decomposing the top-of-atmosphere (TOA) reflectance from surface reflectance and Rayleigh path radiance. The modified Minimum Reflectance Technique (MRT) is adopted for the determination of the seasonal surface reflectances. A good agreement is revealed between the surface reflectances of MRT images and MODIS land surface reflectance products (MOD09), with a strong correlation of 0.9. Moreover, comprehensive look up tables (LUT) are constructed with the considerations of various aerosol optical properties and sun-viewing geometry in the radiative transfer calculations. The resulting 500 m aerosol optical thickness (AOT) data are highly correlated (r=0.94) with the AERONET sunphotometer observations in Hong Kong. This study demonstrates the applicability of aerosol retrieval at fine resolution in urban areas, which can assist the study of aerosol loading distribution and the impact of transient pollution on urban air quality. In addition, the MODIS 500 m AOT images can also be used to study the cross-boundary aerosols and feasible on locating the pollutant sources in the Pearl River Delta (PRD) region.