The Growing Awareness of the Various Roles Bubbles Play in Air-Sea Exchange

The role of bursting bubbles in the injection into the atmosphere of jet- and film-droplets has been recognized for over half a century. The fact that the bubbles in the plume beneath a whitecap greatly increase the local effective air-sea interfacial area has long been known, as has been the bias towards gas invasion resulting from the hydrostatic pressure such submerged bubbles are subject to, and for the smaller bubbles, from the additional pressure resulting from surface tension combined with their small radii of curvature. The role of the larger, rising, bubbles in effectively destroying the stagnant layer at the surface of a whitecap, and thus creating a “low impedance vent” that vastly increases the air-sea gas exchange through such a surface, was postulated more than a third of a century ago. From a consideration that bubble buoyancy increases with bubble volume, i.e. as R³, while the drag on a bubble goes up with the bubble area, i.e. as R², it has long been recognized that larger bubbles have significant rise velocity within the bubble plume, while the smallest bubbles act as passive tracers in the same environment. It has recently been pointed out that within the turbulent bubble plume beneath an active whitecap, while the larger bubbles are rising to effectively create for a brief period of time the “low impedance vent”, the net motion of the smallest bubbles in this turbulent environment is down the bubble concentration gradient, i.e. downward in the water column. For gases such as DMS, whose molecules have an affinity for air-sea interfaces, it has been suggested that many such quasi-dissolved molecules in such an environment, during the time that the whitecap is acting as a low impedance vent, are sequestered at the surface of the myriad smaller, net-downward-moving, bubbles, and thus the air-sea exchange of such gases is impeded.