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Influence of sea-salt on aerosol radiative properties in the Southern Ocean marine boundary layer


There has been considerable debate about the relative importance of sea-salt and sulphate from non-sea-salt sources in determining aerosol radiative effects in the marine boundary layer. In the marine boundary layer, the most numerous aerosols are volatile sulphate particles smaller than about 0.08 µm (ref. 1) and most of the aerosol mass is in a few sea-salt particles larger than 1 µm. Yet intermediate-size aerosols between about 0.08 and 1 µm diameter are the most relevant to the radiative forcing of climate because they efficiently scatter solar radiation and also serve as cloud nuclei2. Indeed, Charlson et al.3 hypothesized that oceanic production of sulphate aerosols from the oxidation of dimethyl sulphide could be a powerful feedback in the climate system. It is generally assumed that marine aerosols smaller than about 1 µm are non-sea-salt sulphate, but a recent review cites indirect evidence that many aerosols in the sub-micrometre range contain at least some sea-salt4,5. Here we present direct observational evidence from a remote Southern Ocean region that almost all aerosols larger than 0.13 µm in the marine boundary layer contained sea-salt. These sea-salt aerosols had important radiative effects: they were responsible for the majority of aerosol-scattered light, and comprised a significant fraction of the inferred cloud nuclei.

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Figure 1: The chemical composition of particles at Cape Grim and Macquarie Island as a function of their diameter.
Figure 2: A typical negative ion mass spectrum of a single particle at Cape Grim.
Figure 3: Composition and backscattering of aerosols.
Figure 4: Size distribution and number of particles.


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This research is a contribution to the International Global Atmospheric Chemistry (IGAC) Core project of the International Geosphere-Biosphere Programme (IGBP) and is part of the IGAC Aerosol Characterization Experiments (ACE). The assistance of the Cape Grim Baseline Air Pollution Station staff and the Australian Bureau of Meteorology and the Commonwealth Scientific and Industrial Research Organization is acknowledged. Work at Macquarie Island was sponsored by the Office of Naval Research and electron microscope work at Cape Grim was supported by the NSF Atmospheric Chemistry Program. Work aboard the Discoverer was funded by the Aerosols Project of NOAA's Climate and Global Change Program. F.J.B. acknowledges an EPA fellowship and A.M.M. acknowledges a DOE fellowship.

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Murphy, D., Anderson, J., Quinn, P. et al. Influence of sea-salt on aerosol radiative properties in the Southern Ocean marine boundary layer. Nature 392, 62–65 (1998).

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