There are three Aquarius L2 attributes/approaches that can be used to differentiate ascending from descending data in the orbital files:
1) ZANG: A straightforward way to separate ascending ascending from descending data in the L2 file is the parameter zang in the navigation block.
zang (8 byte float, array size Number of Blocks): long_name = "Intra-Orbit Angle"; valid_range = (0., 360.); units = “degrees"; angle within orbit from South pole passage at mid-block times.
zang < 180 => Ascending
zang >180 => Descending
2) BLOCK number: You could also use the file attribute Number of Blocks and use that to select just the first half of the blocks (ascending) or the second half (descending).
3) LATITUDE value: an ascending-descending transition would be where you go from the increasing latitudes to decreasing latitudes.
Probably use of #1. ZANG is the simplest method.