A systematic analysis of the salinity effect on air bubbles evolution

Abstract

The evolution of air bubbles after breaking waves plays an important role in gas and particle exchange between water bodies and the atmosphere. To improve our understanding of the impacts of salinity on this process, we systematically investigate the effect of salt concentrations ranging from 0 to 40 g/kg on the volume and size distributions of subsurface bubble plumes and surface foams in a laboratory breaking wave analog. Our experimental setup utilizes an intermittent plunging sheet to simulate breaking waves, while two synchronized digital cameras were used to monitor the temporal evolution of bubble plumes and surface foams. We first highlight the importance of plunging sheet intermittency on surface foam evolution. We then show that increasing salinity enhances the entrainment of submillimeter bubbles but has a less significant effect on larger supramillimeter bubbles. We observed that the foam area in saltier waters is consistently higher than that in freshwater throughout the foam decay phase. Furthermore, our investigation of surface bubble sizes shows that salinity has a more distinct effect on smaller (sub 2 mm) than on larger bubbles. This suggests that salinity may have a more pronounced impact on jet than on film drops ejection mechanisms. Finally, we conclude that the change in salinity within the typical oceanographic range is likely not a major factor for bubble-mediated interactions at the water surface during breaking waves. However, even low-salt concentrations greatly alter air entrainment characteristics in freshwater systems.Plain Language Summary When sea waves break, they entrain large volumes of air in the form of bubbles. These bubbles rise up to the surface and burst, ejecting gas and small droplets into the air. This ejection mechanism plays an important role in air quality and climate dynamics. The effect of water salinity on this process is still not well understood. We used an intermittent plunging waterfall inside a laboratory tank and digital cameras to monitor the evolution of bubbles within and at the surface of water. We found that the jet intermittency affects the foam evolution on the surface following the wave-breaking event. Increasing water salinity led to an increase in the entrained bubbles number, and potentially the air volume injected by the breaking waves. We also observed a larger and more persistent foam patch in more saline water than in freshwater. The size of the surface bubbles in the foam patch was also affected by the salinity indicating that the latter affects the bubble-bursting mechanism on the surface. We conclude that water salinity has a measurable effect on the air volume and bubble sizes injected by breaking waves and should be accounted for in future air-sea interactions studies.