Ammonium in the particle phase can be paired with various anions and thus in the form of ammonium nitrate, ammonium sulfate, ammonium bisulfate, or ammonium chloride. While the thermodynamic routines for the accumulation mode (ISORROPIA II) calculate each of these species, CMAQ only tracks total particulate ammonium, not the specific form. Thus, you can obtain ammonium and nitrate deposition. If you want to estimate ammonium nitrate, you could make assumptions/estimate it from stoichiometry. The accuracy of the approach will depend on the abundance of other aerosol species.