Measurements of SO2 and NSS-SO42- were made over the Atlantic Ocean on board the RV Polarstern from October 9 to November 2, 1996, as part of the ALBATROSS campaign. The measurements were performed between 66.7 ¿N and 37.8 ¿S with a mean longitude of approximately 30 ¿W. The most frequent background values for SO2 were found to be 13 parts per trillion by volume (pptv) (0.54 nmol m-3 at standard ambient temperature and pressure (SATP)) in the Southern Hemisphere, and 15 pptv (0.62 nmol m-3 SATP) in the Northern Hemisphere. The mean values for total NSS-SO42- in particles with a d>0.2 μm were (5.99¿2.93) nmol m-3 (SATP) in the Southern Hemisphere, and (8.93¿5.29) nmol m-3(SATP) in the Northern Hemisphere. An analysis of the size-fractionated aerosol samples (d>1 μm and 0.2 μm1 μm. The main fraction of this NSS-SO42- is most likely produced by the oxidation of dissolved SO2 via heterogeneous reactions occurring in the aqueous phase of coarse mode marine aerosols. A case study on the kinetics of this oxidation pathway was conducted during ALBATROSS. October 12, 1996, the ship sailed in the plume of a volcano on Iceland during its eruption from September 30 to October 13, 1996, as indicated by trajectory analysis and by the measurements of NSS-SO42-, SO2, CO, and Hg. An empirical physicochemical approach considering the atmosphere as a natural flow reactor is used for the presented case study. The determined pseudo-first-order reaction rate constant for the oxidation of SO2 on marine aerosols is 3.31¿10-4 s-1 at 25 ¿C. Assuming that the occurrence of coarse mode marine aerosols is the rate-limiting variable of the reaction, the second-order reaction rate constant is found to be 1.32¿10-6 cm3 s-1 particle-1 at 25 ¿C. These values are in good agreement with results of previous field experiments as well as with the results of model studies. ¿ 2000 American Geophysical Union