Removal of SO2 from the marine boundary layer over the Atlantic Ocean: A case study on the kinetics of the heterogeneous S(IV) oxidation on marine aerosols

Measurements of SO2 and NSS-SO42- were made over the Atlantic Ocean on boar d the RV Polarstern from October 9 to November 2, 1996, as part of the ALBA TROSS campaign. The measurements were performed between 66.7 degrees N and 37.8 degrees S with a mean longitude of approximately 30 degrees W. The mos t frequent background values for SO, were found to be 13 parts per trillion by volume (pptv) (0.54 mnol m(-3) at standard ambient temperature and pres sure (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 particl es with a d > 0.2 mu m were (5.99 +/- 2.93) nmol m(-3) (SATP) in the Southe rn Hemisphere, and (8.93 +/- 5.29) nmol m(-3) (SATP) in the Northern Hemisp here. An analysis of the size-fractionated aerosol samples (d > 1 mu m and 0.2 mu m < d < 1 mu m) of NSS-SO42-. shows that (34 +/- 13)% of the total N SS-SO42- exists in coarse mode particles with d > 1 mu m. The main fraction of this NSS-SO42-. is most likely produced by the oxidation of dissolved S O, via heterogeneous reactions occurring in the aqueous phase of coarse mod e marine aerosols. A case study on the kinetics of this oxidation pathway w as conducted during ALBATROSS. October 12, 1996, the ship sailed in the plu me 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 N SS-SO42- SO2, CO, and Hg, An empirical physicochemical approach considering the atmosphere as a natural flow reactor is used for the presented case st udy. The determined pseudo-first-order reaction rate constant for;the oxida tion of SO, on marine aerosols is 3.31 x 10(-4) s(-1) at 25 degrees C. Assu ming that the occurrence of coarse mode marine aerosols is the rate-limitin g variable of the reaction, the second-order reaction rate constant is foun d to be 1.32x10(-6) cm(3) s(-1) particle(-1) at 25 degrees C. These values are in good agreement with results of previous field experiments as well as with the results of model studies.