Sulfur dioxide uptake and oxidation in sea‐salt aerosol

Measurements of SO 2 and O 3 uptake by sea‐salt and NaCl aerosol were made in a 600 m 3 environmental chamber by measuring the rate of SO 2 and O 3 depletion during nebulization of seawater and NaCl solutions. The experiments were carried out with starting relative humidity between 80% and 92%, with SO 2 concentrations between 35 and 60 ppb, and ozone concentrations between O and 110 ppb. For NaCl, no SO 2 or O 3 uptake was observed. For sea‐salt aerosol, uptake in the range of 0.21 and 1.2 millimoles of S per liter of (nebulized) seawater was observed. Surprisingly, no O 3 uptake was observed even though the residence time of the aerosol in the chamber was long compared to the time required for the predicted S(IV)‐O 3 reaction to occur. Several S(IV) oxidation schemes are considered to explain these observations. The Cl‐catalyzed aerobic mechanism as formulated by Zhang and Millero [1991] from empirical data best explains our observations. The Cl‐catalyzed S(IV) reaction decreases rapidly with decreasing pH, making it important only at pH>∼5.5. This rapid decrease with pH explains why SO 2 uptake was not observed in the NaCl aerosol and observed at a level approaching the sea‐salt alkalinity in the case of sea‐salt aerosol.