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Keywords:

  • Cloud physics;
  • Condensation nuclei;
  • Nucleus production;
  • Sea-salt

Abstract

If recent measurements of the fluxes of sea-salt particles over the oceans as a function of wind speed are extrapolated to include the smallest particles detected over the oceans and in laboratory experiments on bursting bubbles, the flux F of particles having radii between r0 = 0.1 μm and r μm at 80% relative humidity may be represented by F = 2U3.5(e−0.2r0) – e−0.2r) m−2) s−1, where U is the near-surface wind speed in m s−1. The flux of particles of r ≤ 5 μm in winds of 16 m s−1 is 20 000 m−2s−1. The corresponding local rate of production by air bubbles bursting in the foam patches of breaking waves that occupy, on average, only 5% of the ocean surface at this wind speed is 40 cm−2s−1. The average concentration of particles over the sea area producing whitecaps is:

  • equation image

where vr is the deposition velocity for particles of radius r. In winds of 16 m s−1 the total concentration of sea-salt particles is calculated to be 40 cm−3. These, and the fluxes and concentrations calculated for other wind speeds, are consistent with measurements made at sea and in the laboratory by many scientists during the last 50 years.

Recently O'Dowd et al. have reported on sea-salt particles of dry radius as small as 0.03 μm, with radius 0.06 μm at 80% relative humidity and mass 5 × 10−16 g. If the above equation is extended to include such small particles, it predicts a total concentration of 60 cm−3 in winds of 17 m s−1, fairly close to the reported 70 cm−3.

In general these computed and observed concentrations of sea-salt particles are adequate to account for observed droplet concentrations in clouds over remote oceans uncontaminated by continental aerosols. However, on rather rare occasions droplet concentrations in maritime cumulus exceed 100 cm−3. Observations are cited to suggest that, in such cases, sea-salt nuclei may be augmented by biogenic particles originating at the sea surface and by sulphate/sulphuric acid nuclei produced by the absorption of derivatives of dimethyl sulphide in cloud droplets.

Locations such as Hawaii and Tasmania are occasionally contaminated by continental or local pollution which may account for the rare reports of droplet populations being as high as 400 cm−3.