Sea-salt and dust aerosols impact atmospheric chemistry by scattering and absorbing UV radiation and by serving as sites for sites for reactive uptake of oxidants [Martin et al., 2003].  In addition, their alkalinity can affect chemical processing for pH dependent reaction rates [Chamedies and Stelson, 1992].  However, there has been disagreement in the literature on the importance of sea-salt aerosol alkalinity for the sulfur budget in the MBL.  The cartoon to the right describes the sulfur chemistry in the marine boundary layer (MBL) in the GEOS-CHEM model, including uptake and oxidation of SO₂ in sea-salt aerosols. 

Sulfate aerosols were collected and measured on board two cruises in the Indian Ocean as part of the INDOEX campaign.  Cruise tracks are shown in the figure to the left.  Oxygen isotope measurements can distinguish between sulfate formed in alkaline sea-salt aerosols versus other processes, and allow for the quantification of the impact of sea-salt aerosol alkalinity on the sulfur budget in the MBL and the resulting climate implications.

Using the INDOEX measurements as a constraint, we quantified the impact of sea-salt aerosol alkalinity on the sulfur budget in the MBL.  The figure to the right shows the yearly mean decrease (increase) in SO₂ (sulfate) concentrations in the MBL with the addition of sea-salt aerosols as a site for oxidation of SO₂.

The rate of sulfate formation via gas-phase oxidation of SO₂ also shows significant decreases, > 20% over much of the world's oceans.  Since gas-phase oxidation has the largest climate effect (relative to aqueous-phase oxidation), this warrants inclusion of sea-salt chemistry in global models estimating the impact of sulfate on climate in the MBL, and the degree to which marine phytoplankton regulate climate over the oceans through emission of dimethyl sulfide (DMS) [Charleson et al., 1987].

Joël Savarino, Laboratoire de Glaciologie et Geophysique de l'Environment, Grenoble, FRANCE

Mark H. Thiemens, University of California, San Diego

Charles C.W. Lee, URS Corp., L.A., CA

Martin, R.V., D.J. Jacob, and R.M. Yantosca, Global and regional decreases in tropospheric oxidants from photochemical effects of aerosols, J. Geophys. Res., 108(D3), 4097 (2003).

Chamedies, W.L., and A.W. Stelson, Aqueous-phase chemical processes in deliquescent sea-salt aerosols: A mechanism that couples the atmospheric cycles of S and sea salt, J. Geophys. Res., 97(D18), 20565 (1992).

Charleson, R.J., J.E. Lovelock, M.O. Andreae, S.G. Warren, Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate, Nature, 326, 655 (1987).