Accuracy of MODIS-derived AOD over an urban location in western India

Abstract

Correct assessment of aerosol properties is a pre-requisite for climate change study. On account of large heterogeneity in their properties both on spatial and temporal scales, satellite remote sensing is an ideal tool to study them. But the advantage offered by satellites is inhibited by contamination from surface reflectance and cloud interference. In the past, satellite remote sensing of aerosols was limited to over oceans, which offered a dark background. With the launch of MODIS instrument onboard Terra and Aqua satellites observations have been extended to over land. MODIS derives aerosol properties by making an assessment of surface reflectance at visible wavelengths based on mid-IR reflectance. This process has an empirical basis but the validity can only be verified by comparing the results against ground truth data. In this study MODIS derived AOD is validated against the ground based sunphotometer observations made at Ahmedabad (23.03° N, 72.53°E), an urban location in Western India, from 2002 to 2005. The local meteorology is summer from March till July, monsoon during July to September and winter from October to February. MODIS AOD data at 470 nm and 660 nm from both Terra and Aqua averaged over a 0.5×0.5 degree box centered at Ahmedabad are compared with the ground truth data. An overestimation up to 150% by MODIS during April- June and an underestimation up to 50% during October to March is found. An attempt to explain these differences in terms of seasonal variation in surface reflectivity and cloud contamination is presented and discussed.