Whitecap bubbles, big and small, and their asynchronous contributions to sea salt aerosol production, and sea-air gas transfer

It has long been recognized that the bursting of the bubbles produced in a breaking wave is the primary source of the sea-salt aerosol found in the marine atmosphere (e.g., Blanchard, 1963). The rate at which these bubbles reach the sea surface and burst is directly related to the rate of oceanic whitecap production, and the determination of the near-cubic dependence of whitecapping on wind speed, along with a recognition of the other factors that influence whitecap production, has made it possible to parameterize this production (e.g., Monahan, Spiel, and Davidson, 1986) for climate modeling, etc. (IPCC, 2001). It has also been shown, as is to be expected, that the large bubbles, the major source of film droplets, surface and break quickly, while the somewhat smaller bubbles, the source of the jet droplets, take longer to rise to the sea surface and collapse (e.g., Woolf, Bowyer, and Monahan, 1987).

That the aggregation of bubbles in the bubble plumes beneath a whitecap also plays a major role in facilitating the air-sea transfer of gases such as CO2 , by forming a “low impedance vent” up to the immediate sea surface , was described some 25 years ago (Monahan and Spillane, 1984), and the near-cubic dependence of the gas transfer coefficient on wind speed for all but the calmest conditions, which follows from this physical model, has more recently gained widespread acceptance (e.g., Wanninkhof and McGillis, 1999). The role of the rapidly rising large bubbles, and the release of the buoyant potential energy associated with this action, in effectively stirring the immediate surface layer of the ocean right up through the “stagnant”, or laminar sub-layer, is easily seen to accommodate the sea-to-air, and air-to-sea, transfer of the wide range of gases found in our atmosphere and in solution in the sea, that are water-side controlled. It is also recognized that the 50 ìm-radius and larger bubbles, those that rise at meaningful velocities toward the sea surface and represent a disproportionate fraction of the aggregate bubble volume, can act as “gas elevators”, transporting gases that diffuse into the bubbles while submerged to the sea surface and hence to the atmosphere. Likewise, atmospheric gases trapped in bubbles when they form as a wave breaks, and are then transported to depth, are able to diffuse into the water column.

The bubble concentration in the plume beneath a whitecap decreases exponentially with depth, so the effect of the strong near-surface turbulence is to work against the vertical (upward) gradient in bubble concentration and this results in the net down-gradient (downward) transport of those much smaller bubbles whose buoyancy-associated vertical rise velocities are negligible. And it is these small bubbles, with their surface-tension-induced overpressure, and their relatively large area to volume ratio, that under most circumstances rapidly dissolve. The fact that many of these smaller bubbles (certainly those of less than 10 ìm–radius) entrained when a wave breaks never get back to the sea surface has implications for the effective transfer from water-to-air via the whitecap “vent” of certain gases of an amphiphilic nature. Any gas molecules that, even momentarily, adhere to the surface of these bubbles, and these are the bubbles with which most of the aggregate bubble surface area is associated, will contribute to a temporary reduction in the vertical gradient of gas concentration for a gas such as Dimethyl Sulfide, where the dissolved gas concentration typically exceeds the concentration in the overlying air. This will result in a net reduction in the effectiveness of the whitecap as a “low impedance vent”, and, as suggested by Vlahos and Monahan (2009), may explain the apparently anomalously low DMS transfer velocities, when compared with transfer velocities of other, non-amphiphilic gases, at the same wind speeds. It is to be noted that when these small bubbles dissolve, their surface load of DMS will be released back into the water column, and will help restore the prior gas concentration gradient, but by then the “low impedance vent” at the sea surface associated with the intensive large-bubble bursting event will long have disappeared, and the gas is again faced with the relatively ineffective mechanism of molecular diffusion to reach the immediate sea surface, the same situation it found itself in when there was no whitecap present.

Vlahos and Monahan (2009) considered the total partial pressure of an amphiphillic molecule in seawater containing a significant number of non-rising bubbles with a total surface area due to bubbles and ÖB. A molecule with an affinity for this water-bubble boundary layer may experience a solubility enhancement that may be estimated using it's octanol-water partition coefficient. The result is a change in the fugacity of the compound at that instant. For compounds with a Kow > 1 (such as DMS for which Kow = 6.3) the effective Henry's Law constant can be predicted using; Heff = H/ (1+ (Cmix/Cw) ÖB), where H is the dimensionless Henry's Law constant, Cmix/Cw is a dynamic solubility enhancement of the molecule due to bubbles and ÖB is the fraction of bubble surface area per m2 surface ocean. Heff may be substituted in gas transfer models to predict the air-sea gas flux over a range of wind speeds for comparisons with field data.

The shortcomings of such simple physical conceptions of the role of whitecap bubbles in aerosol production, and in the air-sea exchange of gases, are acknowledged, but they do have the virtue of making explicit the localized, transient, asynchronous, nature of the processes that, at all by the lowest wind speeds, control the air-sea flux of aerosols and gases.