Validation of Version 5.1 MODIS Aerosol Optical Depth (Deep Blue Algorithm and Dark Target Approach) over a Semi-Arid Location in Western India

We have examined the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 5.1 Level 2 Aerosol Optical Depth (AOD) from Deep Blue algorithm and Dark Target approach over Ahmedabad, India for 2002 to 2005. Deep Blue algorithm is observed to be able to retrieve AOD over the Rann of Kuchchh, a region of high surface reflectance which poses difficulty for the Dark Target approach. Microtops sunphotometer measured AOD was used to validate the MODIS AOD from the two algorithms. MODIS data from both Terra and Aqua platforms were used, and the comparison was done at 470, 550, and 660 nm wavelengths. The MODIS (Dark Target) Microtops correlation showed an improvement in all correlation parameters over that from MODIS Collection 5 at our study region. Considering the overall dataset, MODIS (Dark Target) Microtops showed a stronger correlation (R550 = 0.55 for Terra, R550 = 0.69 for Aqua) than MODIS (Deep Blue) Microtops (R550 = 0.43 for Terra, R550 = 0.50 for Aqua). A diurnal dependence of the error due to improper aerosol model assumption was noted with the slopes of MODIS-Microtops AOD correlation being lower for Terra than for Aqua. This feature was observed to affect the MODIS-Microtops AOD correlation for Dark Target approach more than the Deep Blue algorithm. Deep Blue derived AOD and Dark Target AOD from Aqua MODIS were found to be consistent, with the correlation between the AODs from the two algorithms being better for Aqua than Terra for all seasons, except monsoon. No dependence was observed of the correlation between MODIS-Microtops AOD on surface reflectance. Our recommendation is to use the Deep Blue AOD over the Rann of Kuchchh region and Dark Target AOD over other regions of Gujarat.