Sulfur dioxide uptake and oxidation in sea-salt aerosol

Measurements Of SO2 and O-3 uptake by sea-salt and NaCl aerosol were made i n a 600 m(3) environmental chamber by measuring the rate Of SO2 and O-3 dep letion 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 0 and 110 ppb. For NaCl, no SO2 or O-3 uptake was observed. For sea-salt ae rosol, uptake in the range of 0.21 and 1.2 millimoles of S per liter of (ne bulized) seawater was observed. Surprisingly, no O-3 uptake was observed ev en though the residence time of the aerosol in the chamber was long compare d to the time required for the predicted S(IV)-O-3 reaction to occur. Sever al S(IV) oxidation schemes are considered to explain these observations. Th e 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 > similar to5.5. This rapid decrease with pH explains why SO2 uptake wa s not observed in the NaCl aerosol and observed at a level approaching the seasalt alkalinity in the case of sea-salt aerosol.