Using eddy covariance to estimate air–sea gas transfer velocity for oxygen
Graphical abstract
Introduction
Air–sea exchange of oxygen is a research area of increasing scientific interest. Oxygen in the ocean is of crucial importance, as it is strongly linked to both the carbon and nitrogen cycles (Arrigo, 2005, Keeling et al., 2010) and is a key component in eutrophication (Karlsson et al., 2002). Gas transfer velocity describes the efficiency of the gas transfer process across the air–sea interface. Over the years numerous laboratory and field studies have been made to study gas transfer velocity. Several expressions have been proposed, relating the gas transfer velocity to different physical variables such as sea spray and bubbles (Monahan and Spillane, 1984, Wallace and Wirick, 1992, Woolf, 1997, Asher and Wanninkhof, 1998, McNeil and d’Asaro, 2007), surface films (Broecker et al., 1978, Frew, 1997, Salter et al., 2011), water-side convection (MacIntyre et al., 2002, Rutgersson and Smedman, 2010, Rutgersson et al., 2011) and rain (Ho et al., 1997, Ho et al., 2004, Zappa et al., 2009, Takagaki and Komori, 2007); still, wind speed holds as the most robust single parameter to describe the transfer velocity (Liss and Merlivat, 1986). The relative importance of these different processes for air–sea gas exchange is likely to vary for gases of different solubility and is still not fully understood (Garbe et al., 2014). It is well known that transfer velocities for water vapor and heat (corresponding to very high solubility) have a weaker wind speed dependence than CO2 (e.g. Rutgersson et al., 2001). Studies on the transfer velocities for acetone (Yang et al., 2014) and DMS (Huebert et al., 2010, Marandino et al., 2007) gases with relatively high solubility, has shown less wind speed dependence than the transfer velocity for CO2. By introducing eddy covariance measurements of oxygen (having a solubility 60 times lower than CO2), knowledge about the effect of a variety of processes on the transfer velocity could fundamentally increase.
The onset of breaking waves is expected to enhance the water-side turbulence (Kitaigorodskii, 1984, Woolf, 2005); in addition, it has been suggested that bubbles injection occurring in the near surface waters enhance the transfer process (Keeling, 1993, Woolf, 1993). Monahan and Spillane (1984) suggested a relation whereby the whitecap coverage scales to the cube of the wind speed. A cubic wind speed dependence for the transfer velocity, however, has only been verified for a few field studies (e.g. Wanninkhof and McGillis, 1999, Kihm and Körtzinger, 2010). Woolf (1997) proposed a relation of the transfer velocity to wind speed, using a theoretical model including injection of air bubbles in the bulk of the water. The importance of bubbles injection from wave breaking to the gas transfer velocity has been suggested to be inversely proportional to the gas solubility (Keeling, 1993, Asher et al., 1996, Woolf, 1997, Asher and Wanninkhof, 1998). However, the transfer velocities for different gases have also shown to be highly sensitive to void fraction and bubble distribution (Woolf, 1993, Woolf et al., 2007) within the water column.
Here we use the eddy covariance (EC) method to directly measure the air–sea gas flux of O2 and estimate a gas transfer velocity for oxygen. Only few previous studies exists using atmospheric EC data for oxygen (Andersson et al., 2014). Previous estimates of the gas transfer velocities of oxygen have instead been made using a budget method (Kihm and Körtzinger, 2010) or eddy covariance fluxes in water (McNiel and d’Asaro, 2007). Kihm and Körtzinger (2010) found a stronger wind speed dependence for the transfer velocity of O2 compared to prominent parametrizations for CO2. However, for most cases the gas transfer velocity of O2 is assumed to follow the same wind speed dependence as other nonreactive gases with low solubility. We will here use three different types of data to estimate the transfer velocity for oxygen. In all data sets the transfer velocities are determined from the measured EC O2 flux. For data set 1 (cf. 4.1) and 2 (cf. 4.2) measured air–sea gradient of oxygen are used in the computation of the transfer velocities, while for period 3 the concept of PQ (Photosynthetic Quotient cf. 3.3) was applied to determine the oxygen concentration in water within the flux footprint.
Section snippets
Theory
The air–sea flux, F, of a non-reactive gas such as O2 can be described bywhere k is the transfer velocity, Cw denotes the gas concentration in the bulk water, α the Ostwald solubility constant, and Ca the gas concentration in the air near the water surface. This transfer efficiency is also commonly described in terms of the resistance R, which is inversely proportional to k. The two-layer film model (Liss and Slater, 1974) describes the total resistance as the sum of the air resistance
Östergarnsholm
The measurements used in this study are taken at the Östergarnsholm site in the Baltic Sea. This station is located at 57°27′N, 18°59′E (Fig. 1). The site has been running semi-continuously since 1995. The site includes a land-based 30-m tower with the base 1 ± 0.5 m above mean sea level, situated on the southern tip of a flat island in the Baltic Sea and mooring instruments mounted on a buoy 1 km SE of the tower.
The tower is equipped with both slow response instrument for profiles and high
Case2013
Data was collected from June 19 to June 23, 2013. During the field campaign winds between 3 and 12 m s− 1 were measured, with two periods when conditions were stationary and with wind from the marine undisturbed sector, 80°–160° (Fig. 2a). The first period was found at the start of the measurements and covers 12 h of data showing mostly positive (upward directed) oxygen fluxes in the range 0.6–6.4 μmol m− 2 s− 1 (Fig. 2b), in agreement with the oxygen saturation in water varying between 105 % and 108 % (
Conclusions
Based on results from eddy covariance measurements in the Baltic Sea (Östergarnsholm), the first atmospheric air–sea fluxes and gas transfer velocities for O2 based on EC measurements are presented. These data are subject for corrections, where the density correction is the major one, introducing a mean uncertainty of 23% to the O2 flux. Within the periods that fulfil selection criteria's regarding wind direction and stationarity however, the O2 fluxes display an anti-correlation with the
Acknowledgments
We would like to give a special thank to Heidi Pettersson with staff at the Finnish Meteorological Institute for their help with deployment of the mooring instrument.
References (54)
- et al.
Using a high-frequency fluorescent oxygen probe in atmospheric eddy covariance applications
J. Atmos. Ocean. Technol.
(2014) Marine microorganisms and global nutrient cycles
Nature
(2005)- et al.
The effect of bubble-mediated gas transfer on purposeful dual-gaseous tracer experiments
J. Geophys. Res.
(1998) - et al.
The influence of bubble plumes on air–water gas transfer velocities
J. Geophys. Res.
(1996) - et al.
The influence of wind on CO2 exchange in a wind wave tunnel, including the effects of monolayers
J. Mar. Res.
(1978) - et al.
Forward-in-time and backward-in-time dispersion in the convective boundary layer: the concentration footprint
Bound.-Layer Meteorol.
(2007) - et al.
Hypoxia-related processes in the Baltic Sea
Environ. Sci. Technol.
(2009) - et al.
Hypoxia is increasing in the coastal zone of the Baltic Sea
Environ. Sci. Technol.
(2011) The role of organic films in air–sea gas exchange
- Garbe, C. S., A. Rutgersson, J. Boutin, B. Delille, C. W. Fairall, N. Gruber, J. Hare, D. Ho, M. Johnson, G. de Leeuw,...
The effect of rain on air–water gas exchange
Tellus
Influence of rain on air–sea gas exchange: lessons from a model ocean
J. Geophys. Res.
To what extent can we believe measurements on a land-based tower to represent upwind open sea conditions
Boreal Environ. Res.
Footprint estimation for scalar flux measurements in the atmospheric surface layer
Bound.-Layer Meteorol.
Linearity of DMS transfer coefficient with both friction velocity and wind speed in the moderate wind speed range
Geophys. Res. Lett.
Temporal and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian and Baltic waters: a review
Oceanogr. Mar. Biol.
On the role of large bubbles in air–sea gas exchange and supersaturation in the ocean
J. Mar. Res.
Ocean deoxygenation in a warming world
Mar. Sci.
Air–sea gas transfer velocity for oxygen derived from float data
J. Geophys. Res.
On the fluid dynamical theory of turbulent gas transfer across an air–sea interface in the presence of breaking wind waves
J. Phys. Oceanogr.
A three-dimensional backward Lagrangian footprint model for a wide range of boundary-layer stratifications
Bound.-Layer Meteorol.
Photosynthetic quotients, new production and net community production in the open ocean
Deep Sea Res. Part A
Air–sea gas exchange rates: introduction and synthesis
Fluxes of gases across the air–sea interface
Nature
The critical importance of buoyancy flux for gas flux across the air–water interface
Eddy correlation measurements of the air/sea flux of dimethylsulfide over the North Pacific Ocean
J. Geophys. Res. Atmos.
Parameterization of air–sea gas fluxes at extreme wind speeds
J. Mar. Syst.
Cited by (5)
On physical mechanisms enhancing air-sea CO2 exchange
2022, BiogeosciencesDirect Measurement of CO<inf>2</inf> Air-Sea Exchange Over a Desert Fringing Coral Reef, Gulf of Eilat (Aqaba), Israel
2022, Journal of Geophysical Research: OceansCritical Observations of Gaseous Elemental Mercury Air-Sea Exchange
2021, Global Biogeochemical CyclesAir-sea gas transfer in high Arctic fjords
2017, Geophysical Research Letters