Kinetics of ammonia and ammonium ion inhibition of the atmospheric oxidation of aqueous sulfur dioxide by oxygen

This is the first study, which shows both NH3 and NH4+ to inhibit the autoxidation of aqueous SO2 in the pH range 7.0–8.5. The rate of the autoxidation, Raut, in both buffered and unbuffered media at a fixed pH is in conformity with the rate law: $$ R_{aut} ={{R_0 } \mathord{\left/ {\vphantom {{R_0 } {\left( {{\text{1 + }}B\left[ {\text{Inh}} \right]_{\text{T}} } \right)}}} \right. } {\left( {{\text{1 + }}B\left[ {\text{Inh}} \right]_{\text{T}} } \right)}} $$ where R0 is rate in the absence of the inhibitors, B is a pH dependent empirical constant and [Inh]T is the analytical concentration of NH3 or NH4+. Both ammonia and ammonium ions appear to inhibit the autoxidation either by scavenging SO4− radicals or by forming less-reactive /unreactive Co(II)-NH3 complexes or both. The atmospheric relevance of the inhibition by ammonia and ammonium ions is discussed.