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

Abstract

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, https://doi.org/10.1002gbc.20080) using surface‐ocean N2 gas measurements determined from observations of dissolved gas pressure and oxygen concentrations every 3 hr on a mooring in the northeast subarctic Pacific at Ocean Station Papa. The changes in N2 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 0.37 ± 0.14 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.

Annual Net Community Production of Particulate and Dissolved Organic Carbon From a Decade of Biogeochemical Profiling Float Observations in the Northeast Pacific

“…Although we could have used an oxygen budget to estimate NCP as well, nitrate, DIC, and TA budgets were chosen for this study due to our interest in euphotic zone NCP and the sensitivity of oxygen budgets on accurate gas exchange rates at the air-sea interface, which has been addressed in a previous manuscript using the same float data set (Plant et al, 2016). Furthermore, oxygen has been used as a tracer of NCP in the Northeast Pacific in many previous studies (Bushinsky & Emerson, 2015;Emerson & Stump, 2010;Emerson et al, 2008Emerson et al, , 2019Nicholson et al, 2008;Pelland et al, 2018;etc. ); thus, we believe our oxygen budget analysis provides no further insight to the contributions cited.…”

The Pressure of In Situ Gases Instrument (PIGI) for Autonomous Shipboard Measurement of Dissolved O2 and N2 in Surface Ocean Waters

“…We observed linear relationships between sensor and discrete O 2 , with offsets in the uncalibrated data typical of Aanderaa optodes whose accuracy decays over time (Bittig et al, 2018). The strong linearity between sensor and discrete data enables calibration with an average accuracy of 1%, which is required for successful field deployments (Emerson et al, 2019). Optode and GTD-derived N 2 also showed strong coherence with discrete Niskin bottle samples analyzed by mass spectrometry and could be validated to within ~1.2%.…”

“…Although the solubility properties of N 2 and O 2 somewhat differ, and N 2 is subject to minor biological influences (e.g., N 2 -fixation), ΔO 2 /N 2 approximates ΔO 2 /Ar under many conditions. Oxygen and N 2 measurements have been combined to infer gas dynamics (Zhou et al, 2014;Tortell et al, 2015) and NCP from refined O 2 budgets (Emerson et al, 2019), but NCP derivation from underway ΔO 2 /N 2 remains largely underexploited. The development of a robust system for continuous O 2 /N 2 measurement thus has the potential to significantly expand oceanic NCP estimates.…”

Seasonal and spatial variability in surface pCO2 and air–water CO2 flux in the Chesapeake Bay

“…High gas exchange rates and low primary production in winter sustained nearly saturated DO, even if pCO 2 % was oversaturated. Although bubble injection increasing DO% can be neglected when wind speed is lower than 10 m s −1 (Emerson et al 2019), sustained wind gusts higher than this value were observed intermittently, which could partially contribute to oversaturated DO% in surface waters. Besides, the supersaturated DO% in February was majorly controlled by high biological production.…”