Kinetics of Oxidation of Aqueous Sulfur(iv) by Nitrogen Dioxide

Although it is generally recognized that SOz and NOx (s NO + NOz) are the precursors of the strong acids (i.e., HzS04 and HN03) found in precipitation,! the detailed mechanisms for the oxidation of these compounds and their incorporation into atmospheric liquid water are not fully understood. In principle, the incorporation of acids into cloudor rainwater can take either or both of two pathways: (1) homogeneous gas-phase oxidation of SOz and NOx to produce gaseous HzS04 and HN03 (and possible subsequent formation of clear-air aerosol) followed by incorporation into liquid water, or (2) absorption of SOz and NOx, as well as Oz and possibly other oxidants (e.g., 03 and HzOz), into cloudor rainwater followed by chemical reactions in the liquid phase, either catalytic or noncatalytic. The former process has been the focus of much atmospheric research in the past decade, and a body of useful information has been attained, which appears adequately to describe and predict the chemistry and physics involved in clear air.Z-6 The latter pathway, on the other hand, is less adequately described. Nonetheless, some information obtained to date on the aqueous-phase oxidation of SOz7-13 and Noxl4,15 has clarified the role of aqueous-phase production of strong acids in the atmosphere. Oxidation of dissolved sulfur(IV) (: SOz(aq) + HS03+ so3=) has been shown to be effected by various oxidants such as Oz,7,8 03,9,10 HzOz,l0-12 and HNOz,l3 in some instances involving catalysts such as transition metal ions.l6-18. The potentially important oxidation of aqueous S(IV) by NOz, however, has not been studied in a rigorous fashion. Besides being a major atmospheric pollutant present in significant concentration, NOz is a relatively strong oxidant, comparable to Brz.l9a Historically one would also note that NOz (along with other nitrogen oxides and oxyacids) has been employed as a catalyst for the manufacture of HzS04 from SOz by the lead chamber process.l9b In the context of possible atmospheric significance, reaction of aqueous S(IV) and NOz has been studied by a number of investigators. Nash20,21 and Takeuchi et a1.22 studied the absorption of NOz by S(IV) solution; although fast reaction was indicated by strong absorption of NOz, reagent gas solubilities, mass transfer, stoichiometry, and reaction rate were not systematically investigated.