The past decade has seen a substantial amount of research on air-sea gas exchange and its environmental controls. These studies have significantly advanced the understanding of processes that control gas transfer, led to higher quality field measurements, and improved estimates of the flux of climate-relevant gases between the ocean and atmosphere. This review discusses the fundamental principles of air-sea gas transfer and recent developments in gas transfer theory, parameterizations, and measurement techniques in the context of the exchange of carbon dioxide. However, much of this discussion is applicable to any sparingly soluble, non-reactive gas. We show how the use of global variables of environmental forcing that have recently become available and gas exchange relationships that incorporate the main forcing factors will lead to improved estimates of global and regional air-sea gas fluxes based on better fundamental physical, chemical, and biological foundations.

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Advances in Quantifying Air-Sea Gas Exchange and Environmental Forcing*

When completely submerged, the leaves of some species retain a surface gas film. Leaf gas films on submerged plants have recently been termed 'plant plastrons', analogous with the plastrons of aquatic insects. In aquatic insects, surface gas layers (i.e. plastrons) enlarge the gas-water interface to promote O₂ uptake when under water; however, the function of leaf gas films has rarely been considered. The present study demonstrates that gas films on leaves of completely submerged rice facilitate entry of O₂ from floodwaters when in darkness and CO₂ entry when in light. O₂ microprofiles showed that the improved gas exchange was not caused by differences in diffusive boundary layers adjacent to submerged leaves with or without gas films; instead, reduced resistance to gas exchange was probably due to the enlarged water-gas interface (cf. aquatic insects). When gas films were removed artificially, underwater net photosynthesis declined to only 20% of the rate with gas films present, such that, after 7 days of complete submergence, tissue sugar levels declined, and both shoot and root growth were reduced. Internal aeration of roots in anoxic medium, when shoots were in aerobic floodwater in darkness or when in light, was improved considerably when leaf gas films were present. Thus, leaf gas films contribute to the submergence tolerance of rice, in addition to those traits already recognized, such as the shoot-elongation response, aerenchyma and metabolic adjustments to O₂ deficiency and oxidative stress.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2008.03769.x

Surviving floods: leaf gas films improve O2 and CO2 exchange, root aeration, and growth of completely submerged rice

[1] Air-sea gas exchange is an important part of the biogeochemical cycles of many climatically and biologically relevant gases including CO2 ,O 2, dimethyl sulfide and CH4. Here we use a three year observational time series of five noble gases (He, Ne, Ar, Kr, and Xe) at the Bermuda Atlantic Time series Study (BATS) site in tandem with a onedimensional upper ocean model to develop an improved parameterization for air-sea gas exchange that explicitly includes separate components for diffusive gas exchange and bubble processes. Based on seasonal timescale noble gas data, this parameterization, which has a 1s uncertainty of ±14% for diffusive gas exchange and ±29% for bubble fluxes, is more tightly constrained than previous parameterizations. Although the magnitude of diffusive gas exchange is within errors of that of Wanninkhof (1992), a commonly used parameterization, we find that bubble-mediated exchange, which is not explicitly included by Wanninkhof (1992) or many other formulations, is significant even for soluble gases. If one uses observed saturation anomalies of Ar (a gas with similar characteristics to O2) and a parameterization of gas exchange to calculate gas exchange fluxes, then the calculated fluxes differ by � 240% if the parameterization presented here is used compared to using the Wanninkhof (1992) parameterization. If instead one includes the gas exchange parameterization in a model, then the calculated fluxes differ by � 35% between using this parameterization and that of Wanninkhof (1992). These differences suggest that the bubble component should be explicitly included in a range of marine biogeochemical calculations that incorporate air-sea gas fluxes.

/pdf/noble-gas-constraints-on-air-sea-gas-exchange-and-bubble-xscpba8b7e.pdf

Noble gas constraints on air-sea gas exchange and bubble fluxes

Gas exchange across the air–water interface drives the flux of climate-relevant gases and is critical for biogeochemical processes in aquatic ecosystems. Despite the presence of mountain streams worldwide, we lack basic understanding of gas exchange through their turbulent surfaces, making global estimates of outgassing from streams and rivers difficult to constrain. Here we combine new estimates of gas transfer velocities from tracer gas additions in mountain streams with published data to cover streams differing in geomorphology and hydraulics. We find two different scaling relationships between the turbulence-induced energy dissipation rate and gas transfer velocity for low- and high-channel slope streams, indicating that gas exchange in streams exists in two states. We suggest that turbulent diffusion drives gas transfer velocity in low-energy streams; whereas turbulence entrains air bubbles in high-energy streams, and the resulting bubble-mediated gas exchange accelerates with energy dissipation rate. Gas transfer velocities in the high-energy streams are among the highest reported. Our findings offer a framework to include mountain streams in future estimates of gas fluxes from streams and rivers at the global scale. Bubble-mediated gas exchange in high-energy streams accelerates faster as energy dissipation intensifies than does turbulent-diffusion-driven gas exchange in low-energy streams, according to an analysis of new measurements and published data.

Distinct air-water gas exchange regimes in low- and high-energy streams

[1] Between Nov. 1 2007 and Jan. 31 2008, we calculated the air-sea flux of CO2, sensible heat, and water vapor in an Arctic polynya system (Amundsen Gulf, Canada) using eddy covariance equipment deployed on the research icebreaker CCGS Amundsen. During this time period, Amundsen Gulf was a dynamic sea ice environment composed primarily of first year ice with open water coverage varying between 1–14%. In all cases where measurements were influenced by open water we measured CO2 fluxes that were 1–2 orders of magnitude higher than those expected under similar conditions in the open ocean. Fluxes were typically directed toward the water surface with a mean flux of −4.88 μmol m−2 s−1 and a maximum of −27.95 μmol m−2 s−1. One case of rapid outgassing (mean value +2.10 μmol m−2 s−1) was also observed. The consistent patten of enhanced gas exchange over open water allows us to hypothesize that high water-side turbulence is the main cause of these events. Modification of the physical and chemical properties of the surface seawater by cooling and brine rejection may also play a role. A rough calculation using an estimate of open water coverage suggests that the contribution of these events to the annual regional air-sea CO2 exchange budget may make the winter months as important as the open water months. Although high, the uptake of CO2 fits within mixed layer dissolved inorganic carbon budgets derived for the region by other investigators.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JC006760

I.S. Leifer

Papers

Modelling of bubble-mediated gas transfer: Fundamental principles and a laboratory test

Breaking waves and air-sea gas transfer (LUMINY). Final report