MOD04_L2 product files are stored in Hierarchical Data Format (HDF). HDF is a multi-object file format for sharing scientific data in multi-platform distributed environments. HDF files should only be accessed through HDF library subroutine and function calls, which can be downloaded from the HDF web site. Each of the 53 gridded parameters listed below is stored as a Scientific Data Set (SDS) within the HDF file.

Note: We are permitting small negative Aerosol Optical Depth values in order to avoid an arbitrary negative bias at the low AOD end in long term statistics. This is because MODIS does not have sensitivity over land to retrieve aerosol to better than ±0.05. This means in very clean conditions the algorithm cannot determine if the AOD = 0, 0.05 or -0.05. If we eliminate all the negative numbers and keep all the positive numbers, we introduce an artificial bias. Thus, we allow negative retrievals up to -0.05. To interpret these: If you are calculating long-term statistics, simply add the negtives into the mix and not worry about them. If you were looking at individual retrievals then count negative retrievals as 'very clean'. You could force them to be AOD = 0, for example. It really depends on the application. However, these small negative AOD values are good retrievals and do contain information.