Hydroxyalkylsulfonate formation: Its role as a S(IV) reservoir in atmospheric water droplets

Abstract

Kinetic and thermodynamic data obtained for the addition of S(IV) species with several aldehydes, including benzaldehyde, glyoxal, methylglyoxal, acetaldehyde, hydroxyacetaldehyde and glyoxylic acid, were used to predict their effectiveness as reservoirs for S(IV) in atmospheric water droplets. A linear free energy relationship between hydroxyalkylsulfonate stabilities and carbonyl hydration constants is presented which can be used to estimate the stability constants of other unknown carbonyl-bisulfite adducts. Formation rates of at least four of the adducts studied were greater than or comparable to the formation rate of α-hydroxymethanesulfonate (HMS). Under most fog- and cloudwater conditions, we have shown that these rates are slower than gas transfer processes. Equilibrium calculations in an open atmosphere indicate that hydroxyacetaldehyde, glyoxal, glyoxylic acid, and to a smaller extent, methylglyoxal, lead to potentially significant enrichment of S(IV) in the liquid phase, although HMS is an even better reservoir for S(IV). Scavenging of SO_2 from the gas-phase due to hydroxyalkylsulfonate formation becomes important at high liquid water contents, pH⩾5, and when an excess partial pressure of the aldehyde is present. The overall dissolution of RCHO and SO_2 into a droplet and the subsequent formation of the hydroxyalkylsulfonate also results in a net increase in acidity, which in a weakly buffered solution can be more than a unit drop in pH.