Comparison of MISR-MODIS aerosol optical depth over the Indo-Gangetic basin during the winter and summer seasons (2000-2005)

Satellite aerosol data provide good temporal sampling and superior spatial coverage relative to ground-based stations, especially over the Indian subcontinent. We have used ground-based Kanpur Aerosol Robotic Network (AERONET) station aerosol optical depth (AOD), located in the central part of the Indo-Gangetic (IG) basin, to validate Multiangle Imaging SpectroRadiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) Terra level 3 AOD products for the 2001-2005 summer and winter seasons. MISR has been found to perform better than MODIS during both seasons, which we attribute to its unique design (viewing and spectral capability), despite its lower revisit frequency compared to MODIS. In both seasons, the MISR level 3 product distinctly shows a large gradient of AOD along north-south (N-S) and west-east (W-E) transects over the IG basin, compared to the southern part of the Indian subcontinent. An increase in satellite-derived aerosol loading over the 2000-2005 time period has been found over major cities located in the IG basin; this increase is also reflected in ground-based air quality data of the Central Pollution Control Board (CPCB). High MISR AOD during the winter season can be attributed primarily to anthropogenic activities, as major dust storms take place during summer (April-June). We have computed the spatial correlation between MISR and MODIS AOD during the 2000-2005 winter (November-January) seasons, when the effect of desert dust is smallest and the anthropogenic component over the IG basin is largest. The average pixel count with MISR-MODIS correlation coefficient > 0.6 during the winter season has been found to be higher over the IG basin and India (35.6% and 32.2% of pixels, respectively) compared to the world (20.4% of pixels). Since the MODIS AOD retrieval algorithm works best for dense and dark vegetative regions, whereas the MISR retrieval is generally less sensitive to surface type, we attribute this result to the relatively high vegetation cover (using MISR Normalized Difference Vegetation Index as a proxy) found in the IG basin and over India. (c) 2007 Elsevier Inc. All rights reserved.