Small amounts of ASO4I,J,K can be directly emitted from sources.
The majority of ASO4I,J,K is produced from gas-phase or aqueous oxidation of SO2. Gas-phase reaction resulting in sulfuric acid (which will condense to particles) is available in mech*.def:
SO2 + OH = SULF + HO2 + SULRXN # 4.50E-31^-3.9 & 1.30E-12^-0.7 &0.53 &1.1;
Aqueous reactions with H2O2 and O3 (and others) are handled in the cloud chemistry module. No aqueous-aerosol reactions currently produce sulfate. The aero module will indicate how condensed sulfuric acid is allocated among the sizes (I, J, etc). ISORROPIA does not produce ASO4* species, but does use them to predict other inorganic species (ammonium, nitrate, etc).