Quantitative mapping of air pollution Sifakis and Paronis
Homogeneous simply means “the same”. In the paper you have read about AOT (Aerosol Optical Thickness, I'm assuming?), the software used to process the data requires the user to select a grid cell size – at a sufficiently small size, the properties of the atmosphere can be assumed to be the same everywhere inside the cell, i.e. homogeneous, to allow the software to perform its calculations. There's little point in selecting a grid cell size smaller than the distance scales that are interesting and/or meaningful, or smaller than the scale on which anyone could/would measure the properties of the atmosphere you are interested in.  As an extreme example, there would be little point in finding/calculating the Aerosol Optical Thickness for every square centimetre across a geographical region!