Prior knowledge-supported aerosol optical depth retrieval over land surfaces at 500 m spatial resolution with MODIS data

Abstract

Aerosol optical depth (AOD) values at a spatial resolution of 500 m were retrieved over terrain areas by applying a time series of Moderate resolution Imaging Spectroradiometer (MODIS) 500 m resolution data in the Heihe region (36–42° N, 97–104° E) of Gansu Province, China; in the Pearl River Delta (18–30° N, 108–122° E), China; and in Beijing (39–41° N, 115–118° E), China. A novel prior knowledge scheme was used in the algorithm that performs cloud screening, simultaneous AOD and surface reflectance retrieval from the MODIS 500 m Level 1B data. This prior knowledge scheme produced a new Ångström exponent α, utilizing a Terra pass time α and an Aqua pass time α to better satisfy the invariant α assumption. The retrieved AOD data were compared with AOD data observed with the ground-based, automatic Sun-tracking photometer CE318 at corresponding bands in the Heihe region and with Aerosol Robotic Network (AERONET) data in the Pearl River Delta and in Beijing. Validation experiments demonstrated the potential of applying the algorithm to MODIS 500 m AOD retrieval on land; validation showed the uncertainty of Δτ = ±0.1±0.2τ over various types of underlying land surface, including cities, where τ is the aerosol optical depth. The root mean square errors (RMSEs) were around 0.1 for inland regions and up to 0.24 for cities by the sea, such as Hong Kong and Zhongshan, China.

Acknowledgements

This work was supported in part by the Ministry of Science and Technology (MOST), China under Grant Nos 2008AA12Z109, 2010CB950802, 2010CB950803 and 2007CB714407 and by the State Key Laboratory under Grant No. O8Y01556KZ. The authors would like to thank Didier Tanre from the Université de Sciences et Techniques de Lille, France, for help and explanation of the general accuracy of AOD. MODIS data were available through the NASA MODIS LAADS Web. Many thanks to the PI investigators of the AERONET sites used in this article. This study was jointly supported by ‘WATER: Watershed Airborne Telemetry Experiment Research’ (Grant No. KZCX2-XB2-09) under the auspices of the CAS Action Plan for West Development Programme. Experimental data can be accessed via http://westdc.westgis.ac.cn/water and http://rsdc.bnu.edu.cn.