Sulfur dioxide uptake and oxidation in sea‐salt aerosol

Abstract

Measurements of SO2 and O3 uptake by sea‐salt and NaCl aerosol were made in a 600 m3 environmental chamber by measuring the rate of SO2 and O3 depletion during nebulization of seawater and NaCl solutions. The experiments were carried out with starting relative humidity between 80% and 92%, with SO2 concentrations between 35 and 60 ppb, and ozone concentrations between O and 110 ppb. For NaCl, no SO2 or O3 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 O3 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)‐O3 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 SO2 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.