Acidic tropospheric aerosols contain inorganic species such as sulfurous acid (H2SO3). As the main alkaline species, ammonia (NH3) plays an important role in the heterogeneous neutralization of these acidic aerosols. An aerosol flow-tube apparatus was used to obtain simultaneous optical and size distribution measurements using FTIR and SMPS measurements, respectively, as a function of relative humidity and aerosol chemical composition. A novel chemiluminescence apparatus was also used to measure ammonium ion concentration [NH4+]. The interactions between ammonia and hydrated sulfur dioxide (SO2 center dot H2O) were studied at different humidities and concentrations. SO2 center dot H2O is an important species as it represents the first intermediate in the overall atmospheric oxidation process of sulfur dioxide to sulfuric acid (H2SO4). This complex was produced within gaseous, aqueous, and aerosol SO2 systems. The addition of ammonia gave mainly hydrogen sulfite (SHO3-) tautomers and disulfite ions (S2O52-). These species were prevalent at high humidities enhancing the aqueous nature of sulfur(IV) species. Their weak acidity is evident due to the low [NH4+] produced. Size distributions obtained correlated well with the various stages of particulate compositional development.