Quantifying air-sea gas exchange using noble gases in a coastal upwelling zone

C C Manning,D P Nicholson,M E Squibb,R H R Stanley

doi:10.1088/1755-1315/35/1/012017

C C Manning, D P Nicholson + Show 2 more

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https://doi.org/10.1088/1755-1315/35/1/012017

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Abstract

The diffusive and bubble-mediated components of air-sea gas exchange can be quantified separately using time-series measurements of a suite of dissolved inert gases. We have evaluated the performance of four published air-sea gas exchange parameterizations using a five-day time-series of dissolved He, Ne, Ar, Kr, and Xe concentration in Monterey Bay, CA. We constructed a vertical model including surface air-sea gas exchange and vertical diffusion. Diffusivity was measured throughout the cruise from profiles of turbulent microstructure. We corrected the mixed layer gas concentrations for an upwelling event that occurred partway through the cruise. All tested parameterizations gave similar results for Ar, Kr, and Xe; their air-sea fluxes were dominated by diffusive gas exchange during our study. For He and Ne, which are less soluble, and therefore more sensitive to differences in the treatment of bubble-mediated exchange, the parameterizations gave widely different results with respect to the net gas exchange flux and the bubble flux. This study demonstrates the value of using a suite of inert gases, especially the lower solubility ones, to parameterize air-sea gas exchange.

Highlights

Noble gases dissolved in seawater are biologically and chemically inert, making them excellent tracers of numerous physical processes that control gas saturation states in the ocean [1,2,3]
Conclusions and future work This study is complementary to others that have demonstrated the value of using oceanic inert gas measurements in tandem with models to quantify air-sea gas exchange fluxes [2, 3, 6]
We demonstrated that short-term, high-frequency measurements of inert gases and diapycnal diffusivity can be used to quantify air-sea gas exchange in coastal regions

Summary

Introduction

Noble gases dissolved in seawater are biologically and chemically inert, making them excellent tracers of numerous physical processes that control gas saturation states in the ocean (e.g., bubble-mediated and diffusive gas exchange, temperature change, atmospheric pressure change, ice melting, diapycnal mixing, deepwater formation and ventilation) [1,2,3]. The dependence of bubble flux on solubility can be explained as follows: for lower solubility gases the atmospheric concentration is high relative to the water concentration, and when air bubbles dissolve in the water, the bubbles will generate a larger percent increase in the gas concentration, compared to a higher solubility gas. The saturation state of the higher solubility (heavier) noble gases changes more dramatically in response to surface heating/cooling, and mixing of water masses with different temperatures, due to the stronger dependence of the solubility on temperature compared to the lower solubility (lighter) noble gases. Parameterizations of physical processes from inert gases can be applied to bioactive gases, to obtain more accurate estimates of processes including biological productivity [3, 6] and denitrification [7, 8]

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Conclusion