Alfred H. Woodcock

THE PRODUCTION, CONCENTRATION, AND VERTICAL DISTRIBUTION OF THE SEA‐SALT AEROSOL

Early interest in the sea-salt aerosol centered on its role in the formation of but in recent years, interest has spread to the role of sea salt in the evaporation of water from the surface of the seal6 and in the enrichment of heavy r n e t a l ~ , ’ ~ viruses,2 bacteria,’ and inert organic material,6 and to the harmful effects of sea salt on ships’ turbines.49 This paper is both a brief review of what is known about atmospheric sea salt, its origin and concentration, and a medium to present new data on its vertical distribution. No attempt will be made to discuss the influence of the continents on the salt aerosol. What follows assumes a quasi steady state between the Ocean and the atmosphere, with a long air trajectory over warmer water. Though space limitations prevent us from even mentioning many fine studies of the salt aerosol, we would be remiss if we did not refer readers to the pioneering work of Eriksson,2‘*22 J ~ n g e , ’ ~ and T~ba.~’-’‘ The salt aerosol is thought to originate primarily from the bursting of bubbles, and so we begin with bubble size distributions in the upper few meters of the sea.

Salt Particles and Raindrops in Hawaii

Abstract Particles of sea salt of different size in the marine air of Hawaii exhibit different ratios of iodine to chlorine, whereas raindrops of various sizes in Hawaii warm orographic showers contain iodine and chlorine in a relatively constant ratio. The mean value of the I/Cl ratio for raindrops (∼2.0 × 10−3) corresponds to the mean found among the smaller of the salt particles of weight range ∼10−12–10−14 gm. On the other hand, the mean I/Cl value among the giant salt particles of weight range of ∼10−8–10−12 gm is shown to be ∼3 × 10−4. It appears that raindrop growth occurs through coalescence among cloud droplets formed on the 10−12–10−14 gm salt particles. This result is interpreted to mean that in warm oceanic tradewind clouds giant salt nuclei may not, after all, be essential to the formation of raindrops.

Thermals over the sea and gull flight behavior

The flight performance of Herring Gulls relative to specific atmosphere and ocean conditions over the western North Atlantic indicates that large groups of gulls are able, through cooperative flight maneuvers, to induce ascending convective flow (thermals) in which they make extended soaring flights. These group flights in gull-induced thermals are limited to winds of 0 to ~ 1 m s−1 and to sea-minus-air temperature differences (δT) of ~3 to 6‡C.As wind speed increases from ~ 2 to 5 m s−1, thermals are naturally induced, and the minimum δT required for soaring is inversely related to wind speed. At higher winds (~5 to 13 ms−1), the minimum positive δT and minimum wind speed required for thermal soaring are directly related, thus indicating an apparent maximum efficiency for the natural production of thermals at wind speeds of about 5 m s−1 and δT of 1 to 2 ‡C.

Winds subsurface pelagic Sargassum and Langmuir circulations

Abstract The long survival of the free-floating plant Sargassum in the wind-mixed surface waters of the oceans is poorly understood. Evidence is presented here that the amounts of Sargassum collected by net tows at the sea surface decrease with increased wind speed. From this new finding and additional evidence in the marine literature, it is hypothesized that the plant is adapted to a cyclic submergence in the wind-induced vortical currents of these waters, returning to the surface only when the currents are less than the plants rise rate. Such adaptation, readily testable, indicates the plants capacity to maintain its density and rise rate independent of depth, thus adding to our understanding of its survival at sea.

Salt-Induced Convection and Clouds

Abstract The amounts of condensation on airborne sea-salt particles are estimated for different humidity and rate-of-rise conditions; the latent heat thereby released may be an important factor in the transport of moist air parcels from neat the sea surface to cloud-base altitudes. These estimates are used to compute a series of tentative temperature lapse-rates which may be found within certain ascending moist air parcels during wind storms and other conditions in marine air. Finally it is demonstrated that the addition of 20 to 40 mg of finely divided sea-salt particles per kg of air to the lower atmosphere in oceanic areas should result in sufficient heating to cause ascending motions and perhaps to cause cloud formation.