Air-Sea Gas Transfer: Determining Bubble Fluxes With In Situ N2 Observations

Oxygen measurements by in situ sensors on remote platforms are used to determine net biological oxygen fluxes in the surface ocean. On an annual basis these fluxes are stoichiometrically related to the export of organic carbon from the upper ocean (the ocean's biological carbon pump). In situ measurements on remote platforms make it feasible to observe the annual biological oxygen flux globally, but the accuracy of these estimates during periods of high winds depends on model-determined fluxes by bubble processes created by breaking waves. We verify the importance of bubble processes in the gas exchange model of Liang et al. (2013, ) using surface-ocean N-2 gas measurements determined from observations of dissolved gas pressure and oxygen concentrations every 3hr on a mooring in the northeast subarctic Pacific at Ocean Station Papa. The changes in N-2 concentration during 10 separate monthlong periods in the winters between 2007 and 2016 indicate that bubble processes in the gas exchange model are over predicted by about a factor of 3 at this location. (The bubble mass transfer coefficients must be multiplied by 0.370.14 to match the observations.) These results can be used to adjust model-determined bubble fluxes to yield more accurate measurements of net biological oxygen production until the next generation gas-exchange models are developed. Plain Language Summary The flux of oxygen from the ocean to the atmosphere is related to the net biological flux of carbon to the ocean's interior. The biological carbon flux is the key process that determines the concentration of CO2 in the atmosphere and the oxygen concentration of deep ocean water. The net biological oxygen flux at the ocean-atmosphere interface can be determined by measuring the gradient of oxygen across the air-water interface and using this value in models of air-sea gas exchange rates. To determine the biological component of the oxygen flux, the models must be able to evaluate physical fluxes of gas transfer caused by bubbles that result from breaking waves at high wind speeds. Our paper uses measurements of nitrogen gas on a surface mooring in the northeast Pacific Ocean to evaluate the role of bubbles in air-sea gas exchange. With the information provided here oceanographers can use the verified gas exchange models to accurately determine biologically produced air-sea oxygen fluxes. This advance will make it possible to verify biological fluxes that are critical for determining the ocean's role in climate, predicted in global circulation models.