Discriminating between the film drops and jet drops produced by a simulated whitecap

Abstract

Film drops and jet drops, generated when whitecap bubbles burst, have long been regarded as the origin of most of the seasalt aerosol. Nevertheless, the contribution of each drop type, throughout the aerosol size spectrum, has remained in doubt. Four properties that distinguish film and jet drops are (1) elapsed time from wave breaking to drop production; (2) site of production of the drops relative to the active whitecap plume; (3) electrostatic charge per drop, and (4) dependence on seawater temperature of the number of drops produced. Interpreting the aerosol measurements made in the whitecap simulation tank at University College, Galway, in light of properties 1, 3, and 4, leads to the conclusion that the bulk of the aerosol produced, during the decay of a whitecap throughout the radius (at 80% RH) range 0.25–2 μm is film drops, while the majority of the drops produced with radii greater than 2 μm is jet drops. The crossover from a predominantly film‐drop aerosol to a mainly jet‐drop aerosol is a gradual one, with both mechanisms contributing equally to the 2‐μm‐radius (4‐μm‐radius as a seawater drop) droplet population.