Although sensor and algorithm design are identical between the two sensors, over time the sensors have decayed at different rates.  The rate of decay of Terra has been greater than Aqua. Consequently there is a small offset between Terra and Aqua AOD (.015 in Collection 5). Additionally, there is a decreasing trend of global land AOD in Terra Collection 5 (Levy et al., 2010), which is not observed in Aqua. This has been traced to a calibration issue (e.g. Wang et al., 2012), and is expected to be corrected in Collection 6. In addition, there is a calibration issue in Terra that causes striping over land in both Collection 5 and Collection 6.