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

Abstract

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 μm < d < 1 μm) of NSS‐SO42− shows that (34±13)% of the total NSS‐SO42− exists in coarse mode particles with d>1 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.