Observations of Rising Methane Bubbles in Trondheimsfjord and Its Implications to Gas Dissolution

Gas dissolution reduces the release of methane to the atmosphere from subsea sources. Being able to predict and assess the methane flux to the atmosphere requires knowledge on gas dissolution and mass transfer. This can be obtained by studying the size evolution of bubbles rising in water. New data of bubble size evolution have been obtained by releasing, tracking, and filming methane bubbles with an ROV in the Trondheimsfjord from depths varying between 100 and 300m. Released bubbles had an initial diameter between 5 and 7mm and were tracked until they reached a diameter of roughly 2mm. The new data were compared against theory, applying established correlations for the mass transfer coefficient. There was an inconsistency between experiment and theory. Thus, new correlations for the mass transfer are proposed. The new correlations are consistent with both the new experiments and previously published experiments. They indicate that the conditions in the ocean can be labeled as partly contaminated with respect to mass transfer. Plain Language Summary Methane bubbles released from the ocean can reach the atmosphere and affect the methane concentration in the atmosphere. Methane is a potent greenhouse gas and highly combustible. How much gas enters the atmosphere depends on how much gas is dissolved in the ocean. The shrinking of bubbles and the development of bubble size are signatures of the gas dissolution. There is a lack of knowledge on methane dissolution in seawater. An experiment outside Trondheim, Norway, has been conducted where an ROV released methane bubbles and recorded images of the bubbles as they rose upward. These images have been analyzed to shed light on gas dissolution.