Isotopic constraints on the formation pathways of sulfate aerosol in the marine boundary layer of the subtropical northeast Atlantic Ocean

We use observations of the oxygen-17 excess of non-sea salt sulfate aerosol (Delta O-17(nssSO(4)(2-))) collected from two ship cruises in the subtropical northeast Atlantic Ocean in August 2006 and February 2007 to quantify the formation pathways of sulfate in the marine boundary layer (MBL). The large observed Delta O-17(nssSO(4)(2-)) values up to 7.3 parts per thousand suggest a large role for sulfate formation via S(IV) oxidation by O-3 in the MBL. Model simulations with the GEOS-Chem global chemical transport model suggest that in-cloud oxidation of S(IV) by O-3 represents over one-third (36-37%) of total in-cloud sulfate production on average. 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 Delta O-17(nssSO(4)(2-)) observations in the MBL. Including this parameterization in the model had little impact on the global sulfur budget due to the dominant role of continental anthropogenic emissions for global sulfur emissions in the present-day. The large observed Delta O-17(nssSO(4)(2-)) argue against a significant role of hypobromous (HOBr) or hypochlorous (HOCl) acid for sulfate formation in the remote MBL of the wintertime subtropical northeast Atlantic, but S(IV) oxidation by HOBr/HOCl on the order of 20% of total sulfate abundance is consistent with the summertime Delta O-17(nssSO(4)(2-)) observations in the more polluted coastal region of the Iberian Peninsula. Additional measurements of Delta O-17(nssSO(4)(2-)) are needed to quantify sulfate production mechanisms in the MBL over larger spatial and temporal scales.