We use observations of the oxygen-17 excess of non-sea salt sulfate (Δ¹⁷O(nssSO₄^2-)) collected from two ship cruises in the subtropical northeastern Atlantic Ocean in August 2006 and Februrary 2007 to quantify sulfate formation pathways in the marine boundary layer (MBL).  The large observed Δ¹⁷O(nssSO₄^2-) values up to 8.0‰ suggest a large role for sulfate formation via S(IV) oxidation by O₃ in the MBL.  Model simulations with the GEOS-Chem global chemical transport model suggest that in-cloud oxidation of S(IV) by O₃ represents almost half (47% on average) of total in-cloud sulfate production on average, which can be accomplished with a cloud water pH of 5.4 - 5.5.  A model parameterization accounting for the impacts of sea salt aerosol on cloud droplet chemical heterogeneity and resulting impacts on in-cloud sulfate production rates improves the model's agreement with the Δ¹⁷O(nssSO₄^2-) observations in the MBL.  Including this parameterization in the model has little impact in the global sulfur budget due to the dominant role of continental anthropogenic emissions for global sulfur emissions.  The large observed Δ¹⁷O(nssSO₄^2-) argue against a significant role of halogens for sulfate formation in the MBL of the remote wintertime subtropical northeast Atlantic, but halogen oxidation on the order of 10 - 15% of total sulfate production is consistent with the summertime Δ¹⁷O(nssSO₄^2-) observations in the more polluted coastal region of the Iberian Peninsula.

nssSO₄^2- concentrations (μg m^-3) and Δ¹⁷O (‰) from aerosol samples collected in August 2006 (bordered circles) and February 2007 (circles.)  Sulfate concentrations are represented by the size of the circles, and the Δ¹⁷O values are represented by their color.

Helen Amos, Harvard University

Jordi Dachs, Institute for Chemical and Environmental Research, IIQAB-CSIC, Barcelona, Spain

Duncan Fairlie, NASA Langley

Fairlie, T.D., D.J. Jacob, J.E. Dibb, B. Alexander, M.A. Avery, A. van Donkelaar, and L. Zhang, Impact of mineral dust on nitrate, sulfate and ozone in transpacific Asian pollution plumes, Atmos. Chem. Phys. Discuss, 9, 24477-24510, 2009.

Allman, D.J., "Quantifying sulfate formation pathways downwind of the Sahara desert: Influence of dust aerosols on the marine boundary layer sulfur budget", M.S. Thesis, June 2009.