The following parameters are given, with a being a subset of aa. However, all the non-overlapping elements of aa with a are already kicked out of emis2. Hence, the remaining aa and a are basically the same.

emis1(r,a,t)
emis2(r,aa,t)

Now I want to define the dimensions of emis2 dependent from a, not aa - so that emis2(r,a,t).

Do you have any idea how I can do that? Maybe by mapping the remaining elements from aa to a?

Since a is a subset of aa, you can index paa by a in the assignment above. The result is independent of paa(‘blahblah’), i.e. of the contents of paa for elements in aa - a.