The reaction kinetics and thermodynamics of the reversible addition of S(IV) and several aldehydes were studied at low pH in order to determine which carbonyl-bisulfite adducts are potential S(IV) reservoirs in atmospheric water droplets. Benzaldehyde, glyoxal, glyoxylic acid, and hydroxyacetaldehyde were chosen as aldehyde substrates.
Spectrophotometric methods were employed to study the reaction kinetics. Between pH 1 - 3, the two rate-determining steps for adduct formation were the addition of HSO₃⁻ and SO₃²⁻ to the carbonyl carbon atom. The sulfite ion was a much more effective nucleophile than bisulfite; rate constants for sulfite addition are four to five orders of magnitude higher than for bisulfate. Below pH 1, some specific acid catalysis was also observed.
Adduct stability constants were determined by spectrophotometry and from microscopic reversibility relationships. Linear-free-energy relationships between carbonyl-bisulfate adduct stabilities and the Taft σ* parameter were found to hold for a limited set of aldehyde substrates. A relatively high correlation exists between bisulfate adduct stability constants and carbonyl hydration constants.
Criteria were formulated, which can be used to predict the potential effectiveness of a carbonyl to significantly stabilize S(IV) in droplets. Modeling calculations for an open atmosphere show that adduct formation rates are much slower than mass transfer and S(IV) oxidation rates under most fog- and cloud-water conditions. Formation rates of hydroxyacetaldehyde-, glyoxal-, and glyoxylic acid - bisulfate addition compounds are comparable to, or faster than, formation rates of hydroxymethanesulfonate, which has been identified in droplets. Equilibrium calculations suggest that these three addition compounds can also stabilize a significant excess of SO₂ in the liquid phase.
