The uptake of sulfur dioxide (SO2) on synthetic sea salt (SSS) and its components, NaCl and MgCl2⋅6H2O, was studied at 298 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. Significant uptake on dried salts was not observed, placing upper limits on the uptake coefficients, &ggr;, of <1¿10-4 for NaCl, <5¿10-4 for MgCl2⋅6H2O, and <8¿10-5 for SSS. However, SSS and MgCl2⋅6H2O that had not been dried before use showed significant uptake of SO2. The magnitude of the uptake depended strongly on the exposure time and the amount of water desorbing. Initially, the measured uptake coefficients for SO2 on SSS were as high as 0.09, but they rapidly decreased below 10-2 with a t-1/2 dependence as expected for approach to the equilibrium saturation concentration in an aqueous solution. The decreasing uptake coefficient slowly approaches zero over hours, consistent with reactions in a water layer with species such as CaCO3. The products of the reaction were shown by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) to include low solubility metal sulfites. These studies show that uptake of SO2 on sea salt particles, even below their deliquescence/effluorescence points, can be treated as if it is into an aqueous salt solution. ¿ 2000 American Geophysical Union