“…Kishcha et al, 2007;Mishchenko et al, 2007aMishchenko et al, , 2009Remer et al, 2008;Zhao et al, 2008;Li et al, 2009a;Herman et al, 2009;Li et al, 2009b;Streets et al, 2009;Yu et al, 2009;Cermak et al, 2010). Such studies are plagued by calibration issues (both within and between multiple sensors), signal to noise issues, and the aforementioned retrieval biases (e.g., Zhang and Reid, 2006;Li et al, 2009b). For example, using 23 years of Advanced Very High Resolution Radiometer (AVHRR) data, Mishchenko et al (2007b) studied the averaged aerosol optical depth (AOD) over global ocean and found a significant decreasing trend in AOD from 1991 to 2005 (GACP, Mishchenko et al, 2007b).…”