JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C00F04, doi:10.1029/2010JC006854, 2011
Toward a universal relationship between wind speed and gas
exchange: Gas transfer velocities measured with 3He/SF6
during the Southern Ocean Gas Exchange Experiment
David T. Ho,1 Rik Wanninkhof,2 Peter Schlosser,3,4,5 David S. Ullman,6 David Hebert,6,7
and Kevin F. Sullivan2,8
Received 8 December 2010; revised 1 April 2011; accepted 15 April 2011; published 28 July 2011.
[1] Two 3He/SF6 dual‐gas tracer injections were conducted during the Southern Ocean
Gas Exchange Experiment (SO GasEx) to determine gas transfer velocities. During the
experiment, wind speeds of up to 16.4 m s−1 were encountered. A total of 360 3He
and 598 SF6 samples were collected at 40 conductivity‐temperature‐depth (CTD) rosette
casts and two pumped stations. The gas transfer velocity k was calculated from the decrease
in the observed 3He/SF6 ratio using three different approaches. Discrete points of wind
speed and corresponding k were obtained from the change in 3He/SF6 ratio over three time
intervals. The results were also evaluated using an analytical model and a 1‐D numerical
model. The results from the three approaches agreed within the error of the estimates
of about ±13%–15% for Patch 1 and ±4% for Patch 2. Moreover, 3He/SF6 dual‐tracer
results from SO GasEx are similar to those from other areas in both the coastal and open
ocean and are in agreement with existing parameterizations between wind speed and
gas exchange. This suggests that wind forcing is the major driver of gas exchange for
slightly soluble gases in the ocean and that other known impacts are either intrinsically
related to wind or have a small effect (<20% on average) on time scales of the order of
days to weeks. The functionality of the wind speed dependence (quadratic or cubic) cannot
be unequivocally determined from SO GasEx results.
Citation: Ho, D. T., R. Wanninkhof, P. Schlosser, D. S. Ullman, D. Hebert, and K. F. Sullivan (2011), Toward a universal
relationship between wind speed and gas exchange: Gas transfer velocities measured with 3He/SF6 during the Southern Ocean
Gas Exchange Experiment, J. Geophys. Res., 116, C00F04, doi:10.1029/2010JC006854.
1. Introduction
[2] In order to constrain the magnitude and variability of
natural and anthropogenic CO2 uptake by the ocean, and
to determine the air‐sea fluxes of other climate relevant
gases such as DMS, it is essential to quantify gas transfer
velocities (k) and relate them to environmental forcing.
Considerable effort has been spent on determining empirical relationships between k and wind speed (u10) [Liss
1
Department of Oceanography, University of Hawai‘i at Mānoa,
Honolulu, Hawaii, USA.
2
OCD, AOML, NOAA, Miami, Florida, USA.
3
Lamont‐Doherty Earth Observatory, Earth Institute at Columbia
University, Palisades, New York, USA.
4
Department of Earth and Environmental Sciences, Columbia
University, New York, New York, USA.
5
Department of Earth and Environmental Engineering, Columbia
University, New York, New York, USA.
6
Graduate School of Oceanography, University of Rhode Island,
Narragansett, Rhode Island, USA.
7
Now at Fisheries and Ocean Canada, Bedford Institute of
Oceanography, Dartmouth, Nova Scotia, Canada.
8
CIMAS, University of Miami, Miami, Florida, USA.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JC006854
and Merlivat, 1986; Wanninkhof, 1992; Wanninkhof and
McGillis, 1999; Nightingale et al., 2000b; Ho et al., 2006]
since wind plays a central role in the generation of turbulence through the transfer of momentum to waves and currents at the water surface, and wind speed is widely measured.
Relationships between k and u10 are used extensively in
numerical models aimed at improving our understanding of
the carbon cycle, and they are also used in combination with
climatologies of surface ocean pCO2, wind speed, and sea
surface temperature to determine CO2 uptake by the ocean
[e.g., Takahashi et al., 2009].
[3] Over the years, different methods have been employed
to determine gas transfer velocities in the ocean, including
the use of opportunistic tracers such as the 222Rn deficit
method [e.g., Peng et al., 1979; Roether and Kromer, 1984;
Bender et al., 2011], natural and bomb 14C [e.g., Wanninkhof,
1992; Naegler et al., 2006; Sweeney et al., 2007], deliberate
tracers such as 3He/SF6 [e.g., Watson et al., 1991], and
micrometeorological techniques like eddy covariance, eddy
accumulation, and gradient flux techniques [e.g., McGillis
et al., 2001b; Huebert et al., 2004]. Each of the methods
has characteristic time and space scales associated with it,
and has unique attributes and shortcomings [Wanninkhof
et al., 2009]. For experiments on time scales of hours to
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days, 3He/SF6 and eddy covariance have emerged as prevalent methods for measuring gas exchange over the ocean.
[4] During the Southern Ocean Gas Exchange Experiment
(SO GasEx) [Ho et al., 2011], gas transfer velocities were
measured with 3He/SF6, and also determined from eddy
covariance of CO2 (J. B. Edson et al., Eddy‐covariance measurement of CO2 gas transfer velocity during the 2008 Southern
Ocean Gas Exchange Experiment (SO GasEx): Wind speed
dependency, submitted to Journal of Geophysical Research,
2011) and DMS [Yang et al., 2011]. In this contribution, we
focus on the 3He/SF6 component, as it provides integrated
measurements of gas exchange over a period of a day or
more, and spatial scales of a few tens of kilometers. In
contrast, eddy covariance provides measurements on time
scales of 20–30 min and spatial scales of a few kilometers.
However, since the signal to noise of individual measurements is low, averaging techniques (such a bin averaging
over discrete wind speed intervals) are frequently employed.
[5] One observation of note from previous 3He/SF6
experiments is that the two most complete data sets are from
two different environments, that is, a composite data set
from the coastal ocean (North Sea) [Nightingale et al.,
2000b] and a single study over the open ocean (Southern
Ocean) [Ho et al., 2006]. Yet, the relationships between
wind speed and gas exchange from those two areas are
similar. Furthermore, two Southern Ocean 3He/SF6 experiments, SAGE [Ho et al., 2006] and SOFeX [Wanninkhof
et al., 2004] show some differences. Because the large
experimental uncertainties in the SOFeX data preclude
development of a robust relationship with wind, data from
SO GasEx are critical to explore if the SAGE results taken
near New Zealand are representative for different sectors of
the Southern Ocean, or if the appreciable differences
between SOFeX and SAGE are caused by effects that are
not directly quantifiable by relating gas transfer velocity to
wind speed (e.g., fetch).
2. Methods
2.1. Study Site
[6] The 3He/SF6 dual‐tracer experiment was conducted as
part of SO GasEx, which took place in the southwest
Atlantic sector of the Southern Ocean (nominally at 50°S,
40°W), near South Georgia Island from 29 February to
12 April 2008. For more information about the study site,
see the work of Ho et al. [2011].
2.2. Wind Speeds
[7] Two sets of wind speed measurements are compared:
one from a satellite, QuikSCAT, and the other from the ship.
The ship‐based wind speeds and directions were constructed
from three sonic anemometers installed on the foremast of
the ship. The relative wind direction was used to select the
sonic anemometer(s) expected to be least affected by flow
distortion, and the measurements were corrected for both the
motion of the ship [Edson et al., 1998], and for flow distortion using an empirical correction. For more details on the
wind speed measurements made from the ship during SO
GasEx, see the work of Ho et al. [2011]. The colocated
satellite winds were selected using QuikSCAT Level 2B
12.5 km data within ±0.5 degrees and 1 h of the ship
location for each overpass (12.5 km swath). Rain flagged
data were omitted from the analysis. Details on the retrieval
of the vector winds at 10 m reference height u10 can be
found in the QuikSCAT Science Data Product User Manual
(ftp://podaac.jpl.nasa.gov/ocean_wind/quikscat/L2B/doc/
QSUG_v3.pdf). The comparison between ship and QuikSCAT wind speeds was performed to confirm that there are
no significant biases between the wind speed products, so
that the gas exchange/wind speed relationships determined
in this study can be applied to other regions using the
(global) QuikSCAT product.
2.3. The 3He/SF6 Technique
[8] The 3He/SF6 dual‐tracer technique has been employed
in coastal and open ocean experiments during the past two
decades to obtain integrated measurements of gas transfer
velocities [e.g., Watson et al., 1991; Wanninkhof et al., 1993,
1997, 2004; Nightingale et al., 2000a, 2000b; Ho et al.,
2006]. 3He and SF6, two inert gases with different Schmidt
numbers (Sc = kinematic viscosity of water divided by
diffusion coefficient of the gas in water), are injected together
into the mixed layer at a constant ratio, and the 3He/SF6 ratio
is monitored over time. The assumption is made that patch
dilution (e.g., horizontal mixing) is the dominant process
that affects the individual tracer concentrations but this
process does not alter the 3He/SF6 ratio. The ratio of k for
3
He and SF6 can be expressed as
k3He
¼
kSF6
Sc3He
ScSF6
1=2
;
ð1Þ
where kSF6 and k3He are the gas transfer velocities and ScSF6
and Sc3He are the Schmidt numbers for SF6 and 3He,
respectively. ScSF6 and Sc3He were determined from the
measurements of King and Saltzman [1995] for SF6, and
Jähne et al. [1987] for 4He, which can be used to calculate
Sc3He if the fractionation between 3He and 4He is known.
Jähne et al. [1987] determined the fractionation to be 15 ±
3%, which agrees with the ratio of the reduced masses
(12.8%). A more recent result based on molecular dynamics
simulations suggests the number to be 4.96 ± 0.83% [Bourg
and Sposito, 2008]. It is possible that because the molecular
dynamics simulations neglect quantum isotope effects, the
fractionation could be higher than that based on these
simulations. Further laboratory measurements or molecular
dynamics simulations including quantum isotope effects
will be necessary to determine the exact fractionation. Here,
we use a fractionation of 12.8% (based on the ratio of the
reduced masses) to be consistent with Wanninkhof et al.
[1993, 1997, 2004] and Nightingale et al. [2000b]. k(600),
the gas transfer velocity at a Sc number of 600, would be
about 11% higher if no correction for the difference in
diffusivity between 3He and 4He is applied [e.g., Ho et al.,
2006] (see equation (2) below). Likewise, if ScSF6 from
Wanninkhof [1992] were used instead of values from King
and Saltzman [1995], k(600) would be about 3% lower at
the sea surface temperatures of ca. 5–6°C for SO GasEx.
This analysis illustrates the level of uncertainty in the gas
transfer velocity due to lack of accurate basic physical data
for 3He and SF6.
[9] The gas transfer velocity for 3He k 3He can be determined by combining the advection‐diffusion equations for
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3
He and SF6, vertically averaged over the mixed layer, and
equation (1) [Wanninkhof et al., 1993]:
k3He
!
ln 3 Heexc SF6
d
;
¼ h
dt 1 ScSF Sc3 1=2
6
He
ð2Þ
where h is the characteristic gas exchange depth, which is
the depth where the 3Heexc/SF6 ratio changes appreciably
from the constant values above. This depth corresponds
closely to the depth where SF6 reaches 50% of its averaged
concentration in the top 20 m. SF6 and 3Heexc are the SF6
and excess 3He concentrations (i.e., 3He in excess of solubility equilibrium) in the mixed layer, respectively. In this
contribution, the terms 3Heexc and 3He will be used interchangeably. The characteristic gas exchange depth during
SO GasEx, referred to as mixed‐layer depth below, is
generally somewhat deeper than the mixed‐layer depth
defined by density, particularly during periods of rapid
shoaling of the density‐defined mixed layer as will be seen
to occur during the periods of both tracer patches. The gas
transfer velocities measured during SO GasEx are normalized to k(600), where 600 corresponds to Sc of CO2 in
freshwater at 20°C:
k ð600Þ ¼ k3He
600
Sc3He
1=2
:
ð3Þ
2.4. Tracer Injection
[10] The 3He and SF6 injection was achieved in a similar
way to previous tracer release experiments [Upstill‐Goddard
et al., 1991]. On the aft deck of the ship, a large 4800 L steel
tank was filled with seawater and infused with SF6 and 3He
prior to injection into the surface mixed layer. After the tank
was filled with seawater, a 1 L headspace was created and
flushed continuously with SF6. The SF6 was circulated
through the water via a length of diffuser tubing using a
pump until SF6 in the water reached the desired concentration. A few hours before injection, a 1 L 3He headspace was
created and circulated with a pump through the water until
the gas dissolved. This was repeated until the desired amount
of 3He was infused into the tank (ca. 10 L at STP for each
patch). During tracer injection, the top of the tank was fitted
with a weather balloon. As water was pumped out of the tank
with a peristaltic pump, the weather balloon was gravity fed
with seawater from a header tank to ensure that no headspace
developed, thus preventing 3He and SF6 from exchanging
with the headspace and altering the 3He/SF6 ratio.
[11] The tracer release was completed in a hexagonal
pattern within a Lagrangian framework, achieved by following a central GPS‐enabled drifter with a line‐of‐sight
VHF transmitter to adjust the ships track for surface water
advection during the release. The drifter marked the center
of the water parcel and the ship followed a track marked by
waypoints at the vertices of expanding hexagons, each
centered on an up‐to‐date position of the GPS drifter.
[12] The ship speed during the injections was ca. 7.5 km h−1.
The first injection took place on 8 March 2008 (yearday 68),
and lasted 13.5 h at a flow rate of 6 L min−1, creating a patch
with an area of ca. 50 km2. The second injection, in a different water parcel, was on 21 March 2008 (yearday 81),
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and lasted 6.5 h at a flow rate of 10.2 L min−1, creating a
patch with an area of ca. 12.5 km2. Nominal injection depth
for both patches was about 5–6 m.
[13] Because the volume of SF6 and 3He infused water
injected was the same for both patches and Patch 2 was
smaller, it had higher SF6 and 3He concentrations. Thus, it
was easier to survey and could be followed for a longer
period.
2.5. Underway SF6 Measurements
[14] An underway SF6 system, described in detail by Ho
et al. [2002] was used to monitor the advection and dispersion of the tracer patch. In addition to determining the
boundaries of the water parcel for the Lagrangian experiment, the system was used to determine the center of the
patch (i.e., the area of maximum tracer concentration)
for the twice‐daily conductivity‐temperature‐depth (CTD)
casts. For more information about underway SF6 measurements made during SO GasEx, see the work of Ho et al.
[2011].
2.6. The 3He Samples
[15] 360 3He samples (ca. 40 mL each) were drawn from
40 CTD casts and 2 submersible pump stations to be used in
conjunction with SF6 to calculate the gas transfer velocity
and to determine the extent of horizontal and vertical mixing. Typically, 8 to 10 depths per station were sampled,
covering the mixed layer, thermocline, and deeper water
below. The samples were stored in copper tubes closed at
both ends by means of stainless steel pinch‐off clamps. The
3
He measurements were performed in the Lamont‐Doherty
Earth Observatory noble gas laboratory. 3He and other gases
were extracted from the copper tubes and transferred to
flame‐sealed glass ampoules with low helium permeability
containing activated charcoal using a vacuum extraction
system [Ludin et al., 1998]. The 4He concentration and the
3
He/4He ratio were measured on a dedicated VG‐5400 He
isotope mass spectrometer. Prior to introduction into the
mass spectrometer, He was separated from all other gases by
a series of cold traps. 4He was measured using a Faraday
Cup and 3He was measured using a channel electron multiplier (Channeltron). Neon was measured in parallel on a
quadrupole mass spectrometer [Ludin et al., 1998]. Precision was about 0.5% in 3He for samples with very high 3He
excesses (100% < 3He < 1000%), and 0.2 to 0.5% for
samples with lower 3He excesses (−1.7% < 3He < 100%).
Precision of the 4He and Ne measurements was about 0.2
to 0.5, and 0.5 to 1%, respectively. 35 samples were lost
during extraction or measurement, and 111 samples had
anomalous 4He and Ne concentrations suggesting excess air
in the samples or fractionation during measurement. These
samples were not used in the interpretation.
2.7. Discrete SF6 Samples
[16] Discrete SF6 samples were collected in 550 mL
borosilicate bottles with ground glass stoppers for shipboard
analysis. 598 samples were collected during 40 CTD casts,
56 samples were collected from the underway seawater
line at 19 locations, and 11 samples were collected at the
2 submersible pump stations. Over 40 pairs of duplicate
samples were drawn at various times from the same Niskin,
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HO ET AL.: UNIVERSAL GAS EXCHANGE RELATIONSHIP
and the agreement between these duplicates was typically
better than 2%. The SF6 (and 3He) samples from the two
pumped stations were contaminated with ambient air and are
not used in the analysis here.
[ 17 ] An analytical system patterned after Law et al.
[1998], consisting of a purge‐and‐trap inlet coupled to a
GC‐ECD, was used to quantify the vertical SF6 distribution.
The dissolved gases in 269 mL of sample water were purged
with UHP N2 and then trapped on a Carboxen 1000 trap
held at −68°C. After several minutes of purging, the trap
was isolated and heated to 150°C. The purged gases were
swept onto a molecular sieve 5A column where the SF6
was separated from other gases. SF6 was measured with a
GC‐ECD. The detector was calibrated using six standards
with concentrations of 5.7, 55.1, 112, 167, 345, and 1109 ppt.
Custom software (LabVIEW) was used for instrument
control, acquisition of the ECD output, and reintegration of
the chromatographic peaks. 56 discrete samples were taken
from the uncontaminated seawater line and used to adjust
the final calibration of the underway SF6 system.
2.8. Models of 3He/SF6
[18] The temporal change in SF6 and 3He in response to
vertical mixing and air‐sea gas exchange during SO GasEx
can be examined by (1) looking at the differences in mixed
layer averaged 3He/SF6 profiles from each CTD station or
(2) considering all the 3He/SF6 pairs measured together. We
do both here by using an analytical model to examine the
former, and a 1‐D numerical model to examine the latter.
The objective of both is to compare the model‐predicted
tracer ratio (3He/SF6) to the observations in order to determine the optimal relationship between k(600) and 10 m
(neutral stability) winds (u10N).
2.9. Analytical Model
[19] Using the assumption that air‐sea gas exchange is the
only process that alters the 3He/SF6 ratio in the mixed layer,
the change in 3He/SF6 ratio during SO GasEx can be
modeled by an analytical solution to equation (2):
3
Heexc =SF6
t
¼
3
Heexc SF6 t1
1=2 k3 Dt 1 ScSF6 Sc3He
;
exp He
h
ð4Þ
where (3Heexc/SF6)t is the 3He to SF6 ratio at time t and
(3Heexc/SF6)t−1 is the tracer ratio at the previous time step.
k3He is prescribed from wind speeds measured during SO
GasEx and existing parameterizations between u10N and
k(600). The 10 min averaged wind speeds from the ship are
used, so that each time step is 10 min. By using the average
of all 3He/SF6 points in the mixed layer at each time step
and using equation (4), we are assuming that the average
ratios represent the mixed‐layer profiles. While inspection
of the profiles suggests that this is a reasonable assumption,
there are periods where the 3He/SF6 ratio at the surface is
lower and this approach will lead to an anomalously low k.
[20] The ability of commonly used parameterizations,
including the piecewise linear parameterizations of Liss and
Merlivat [1986], quadratic relationships of Wanninkhof
[1992], Nightingale et al. [2000b], and Ho et al. [2006],
cubic relationship of Wanninkhof and McGillis [1999], and
the hybrid parameterization of Wanninkhof et al. [2009], to
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predict the 3He/SF6 measured during SO GasEx could be
evaluated by examining the relative root mean square error
(rRMSE) between the observed and modeled 3He/SF6 ratios:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
uN n
u P Rmod Rnobs 2
u
t
Rnobs
rRMSE ¼ n¼1
;
N
ð5Þ
where Rnobs and Rnmod are the observed and modeled 3He/SF6
tracer ratios, respectively, N is the number of stations
sampled, and the summation is over the averaged 3He/SF6
ratios in the mixed layer for each station.
[21] The data are also used to determine the optimal
coefficients (a2 and a3) for a quadratic (k = a2u210N) and
cubic (k = a3u310N) parameterization for each patch, by minimizing the rRMSE.
2.10. One‐Dimensional Numerical Model
[22] The one‐dimensional generalized ocean turbulence
model (GOTM) was forced with surface fluxes estimated
from shipboard meteorological measurements and was
nudged to the observations of temperature and salinity from
CTD casts and velocity from the shipboard ADCP. Further
details of the GOTM implementation can be found in the
work of Ho et al. [2011]. Separate runs were carried out for
the time periods of the two tracer patches. The vertical
diffusivity field (a function of time and depth) from these
runs was saved every hour and was used to solve the vertical
tracer evolution equation.
[23] For each patch, tracer concentrations were initialized to be vertically uniform over the model density‐
based mixed layer (the portion of the water column where
r (z)−r (5 dbar) ≤ 0.01 kg m−3) at the time of the first CTD
cast after the tracer injection. The time interval between the
end of tracer injection and the first CTD cast was approximately 39 h for Patch 1 and 15 h for Patch 2. Model 3He and
SF6 concentrations were initialized to their mixed‐layer
values from water samples taken during the first postinjection CTD cast.
[24] Surface gas fluxes in the model were parameterized
using a gas transfer velocity that was a function of u10N,
assuming zero atmospheric gas concentrations. We tested
two functional forms (quadratic and cubic) for the 3He gas
transfer velocity as in the analytical model. k(600) was derived
from k3He using a Sc−1/2 dependency (equation (1)). The optimal coefficients, a2 or a3 for (k = a2u210N) and (k = a3u310N), were
obtained by fitting the model to the observations by minimizing
the cost function:
F¼
N n
X
Robs Rnmod 2
;
n
Robs
n¼1
ð6Þ
where the summation is over all observations within the
model mixed layer (determined as the depth z at which
r (z)−r (5 dbar) > 0.01 kg m−3). Note that the cost function
defined by equation (6) is formally equivalent to the definition in equation (5) (i.e., F = N × rRMSE2). As discussed
previously, the uncertainties in the measured gas concentrations are expressed as relative errors, so that the
uncertainty in the observed tracer ratio is proportional to
its magnitude. Therefore, normalization of the differences
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Table 1. Summary of Ship Wind Speeds and k(600) From SO GasEx
Schmidt
Numbers
Wind Speeds, u10N (m s−1)
Stations
Date Range
(Yearday)
Mean
Range
sa
u210 /u10 2
Numberb
Temperature
Salinity
2–10
16–28
28–51
70.20–74.09
82.12–86.56
86.56–96.03
8.9
11.2
8.2
3.9–14.3
3.1–16.4
1.5–14.6
1.7
2.8
2.8
1.04
1.06
1.11
555
638
1365
5.6
4.8
4.7
33.8
33.7
33.7
Gas Transfer Velocities,
k(600) (cm h−1)
He
SF6
Meanc
Enhancement
Corrected
Error
270.8
281.5
282.9
2182.6
2284.6
2298.5
25.8
30.1
21.0
25.0
28.3
18.9
2.9
3.0
1.3
3
a
Standard deviation.
Number of 10 min averaged measurements used to calculate the wind speed statistics in each interval.
c
Integrated k(600) over the entire period.
b
in the numerator of equation (6) by the observed ratio is
equivalent to weighting by the inverse of the observational
errors, known as chi‐square fitting [Press et al., 1992]. The
cost function (equation (6)) was minimized using a simplex
direct search method (Nelder‐Mead) [Press et al., 1992].
3. Results and Discussion
3.1. Wind Speed
[25] Ship wind speed during Patch 1, which consists of
555 measurements of 10 min averaged winds, ranged from
3.9 to 14.3 m s−1, with a mean of 8.9 m s−1 and a standard
deviation of 1.7 m s−1. Wind speeds during Patch 2,
consisting of 2003 measurements, ranged from 1.5 to
16.4 m s−1, with a mean of 9.2 m s−1 and a standard deviation of 3.1 m s−1 (Table 1 and Figure 1). The corresponding
QuikSCAT wind speeds for Patch 1 from 502 measurements
ranged from 2.4 to 14.5 m s−1, and had a mean of 8.7 m s−1
and a standard deviation of 2.1 m s−1. Patch 2 ranged from
1.4 to 20.8 m s−1, with a mean of 9.4 m s−1 and a standard
deviation of 3.7 m s−1. The QuikSCAT and ship wind
speeds averaged between CTD stations (ca. every 12 h)
compared well (R2 = 0.84), with QuikSCAT winds being
higher than ship winds by up to 2 m s−1 at u10 > 12 m s−1
(Figure 2). However, considering that the QuikSCAT winds
cover a larger region and are based on 2–4 overpasses per
day rather than continuous measurements, and because the
ship winds are averaged over ca. 12 h, this should be considered a good agreement. The ship winds are thus assumed
to be representative over the entire patch.
3.2. SF6 Patch
[26] The center of the tracer patch at the completion of
the first injection on 9 March 2008 (yearday 69) was at
50.6042°S, 38.6308°W. SF6 concentrations as high as
440 fmol L−1 were measured during the initial survey following tracer injection. The final CTD performed before
leaving the study area for the vicinity of South Georgia
Island was Station 10 on 14 March 2008 (yearday 74) at
50.8620°S, 38.2394°W, and the surface SF6 concentration at
this location was 29 fmol L−1. The tracer patch advected
approximately 40 km over the six days of the survey [see
Ho et al., 2011, Figures 16 and 17].
[27] After 4 days at South Georgia Island, the ship
returned to survey in the vicinity of the MAPCO2 buoy that
was deployed in the tracer patch. Low SF6 concentrations of
10 fmol L−1 were detected near the buoy. A CTD station
was conducted at the approximate center of the residual
patch at 51.0401°S, 37.6988°W before moving on to locate
a site for a second tracer injection. Because there were no
high temporal resolution wind speed measurements made at
the patch location when the ship was at South Georgia
Island, the change in 3He/SF6 during this period was not
used in the analysis here.
Figure 1. Histograms of wind speeds encountered during SO GasEx for (a) Patch 1 and (b) Patch 2.
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approximately 78 km over the 15 days after the second
injection.
3.3. The 3He and SF6
[29] For the CTD casts in the tracer patches, the SF6
concentrations were mostly uniform throughout the mixed
layer (Figure 3). For a few of the casts, the surface concentrations were lower than the mixed‐layer average, but
still well above background SF6 concentration. Over the
10.5 days during which CTD casts were done in the first
tracer patch (which includes time away at South Georgia
Island), the SF6 in the mixed layer in the center of the
patch decreased to about 6% of the initial concentration
but remained over two times higher than background
concentrations.
[30] The initial SF6 concentrations in the second patch
were more than 4 times greater than the first patch. Over the
14 days CTD casts were done on the second tracer patch, the
mixed‐layer SF6 concentrations decreased to less than 2% of
the initial concentration but remained over almost three
times higher than background concentration.
Figure 2. Comparison between QuikSCAT and ship wind
speeds for Patches 1 and 2.
[28] The second tracer patch, created on 21 March 2008
(yearday 81) was centered on 51.1442°S, 38.4042°W. The
maximum SF6 concentration following the injection was
999 fmol L−1. The final CTD performed in Patch 2 was
on 5 April 2008 (yearday 96) and located at 51.4650°S,
37.4072°W. The surface SF6 concentrations located at this
location were ∼6 fmol L−1. The second patch advected
3.4. SF6 and 3He/SF6 Depth Profiles
[31] The large number of samples and duration of the
study offers a unique opportunity to further investigate the
basic assumptions of the dual gaseous deliberate tracer
approach. In particular, we assess the issues of mixed‐layer
homogeneity and mixed‐layer depth, which are both critical for calculating k3He. In some previous studies [e.g.,
Wanninkhof et al., 2004], the mixed‐layer depth for gas
transfer is assumed to be a classically defined mixed layer
based on temperature or density criteria. Here we show that
in SO GasEx, the layer exchanging gas with the atmosphere
Figure 3. Profiles of normalized SF6, 3He/SF6, and temperature for (a) Station 5 (Patch 1) and
(b) Station 47 (Patch 2). Normalization of SF6 (solid squares) and 3He/SF6 (open squares) is to mixed‐layer
concentrations, and that for temperature (thin black line) is to the difference of the mixed‐layer average and
the temperature at depth where the measured [SF6] ≈ 0. The red line is the mixed‐layer based on density
criteria, where r (z)−r (5 dbar) ≤ 0.01 kg m−3, while the blue line is based on a ventilation criteria of
0.5 × [SF6]20m, where [SF6]20m is the average concentration in the upper 20 m of the water column.
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Figure 4. Comparison between 3He/SF6 ratios measured during SO GasEx and those derived using the
analytical model described by equation (4) for (a) Patch 1 and (b) Patch 2. Six commonly used parameterizations between wind speed and gas transfer velocity are used in the analytical model.
as determined from a constant 3He/SF6 ratio is often deeper.
This is attributed to the longer characteristic time scales of
gas transfer, expressed as h/k, of order of a week, compared
to the short time scale (<1 day) that it takes the mixed layer
to homogenize based on the modeled mixing rates in the
GOTM [Ho et al., 2011]. Thermal mixed‐layer depth variations can be as short as hours for internal wave motions
to several days owing to changes in wind stress and associated turbulence. Figure 3 shows profiles of normalized
SF6, 3He/SF6 and temperature for Stations 5 (Patch 1) and
31 (Patch 2). Normalization of SF6 and 3He/SF6 are to mixed‐
layer concentration and temperature to the difference of the
mixed‐layer average and the temperature at depth where the
measured [SF6] ≈ 0. Inspections of all profiles suggest that
the depth where SF6 is half that of the average concentration
in the upper 20 m (i.e., [SF6] = 0.5 × [SF6]20m) is a reasonable approximation of the depth that exchanges with the
atmosphere as this is the depth where 3He/SF6 rapidly
decreases. Since the time scale of gas transfer is on the order
of a week, the average [SF6] derived mixing depths are used
for each patch.
3.5. Temporal 3He/SF6 Trends
[32] A puzzling aspect of the 3He/SF6 trends in several
open ocean studies including SO GasEx is that while the
ratio shows a strong decrease over the duration of the study,
the ratios between some individual sampling points either
decrease precipitously or even increase or remain steady
(see Figure 4). These changes are greater than can be
attributed to instrumental uncertainty and cannot be solely
explained by local mixed‐layer dynamics. For instance, the
observed increases are not always caused by stratification of
the mixed layer and outgassing, with a decrease in 3He/SF6,
followed by reestablishment of a deeper mixed layer causing
the isolated waters with high 3He/SF6 to increase the 3He/SF6
such as shown in Figure 3b. We attribute the nonsystematic
changes in 3He/SF6 to two factors both related to mixed‐
layer depth variations.
[33] Several instances show a sharp decrease in 3He/SF6
in a transition from light winds to higher winds and deeper
mixed layers. This is attributed to enhanced gas transfer due
to higher winds preceding mixed‐layer deepening such that
the gas loss is over a smaller volume.
[34] Another cause for the occasional increases of
3
He/SF6 is mixed‐layer heterogeneity, in particular the
variations in depth of water exchanging 3He and SF6 with
the atmosphere over the tracer patch. The tracers were
released in an eddy‐like feature that likely has systematic
spatial differences in depth. Figure 3b shows an extreme
example of variations in SF6 and 3He/ SF6 encountered in
the near surface layer.
3.6. Gas Transfer Velocities
[35] Because of the variability in 3He/SF6 described
above, and to decrease some of the experimental uncertainty
in order to obtain more accurate measurements of k(600)
from 3He/SF6 [e.g., Asher, 2009], we have chosen to derive
k(600) for longer time intervals and use three segments
of the experiment, one for Patch 1 and two for Patch 2
(Table 1). Patch 2 was divided on the basis of the change in
slope of d(3He/SF6)/dt at yearday 86.56 (Figure 4). The
k(600) results shown in Figure 5 have been corrected for
wind speed enhancement assuming a quadratic function
with wind speed (i.e., " = u210 /u10 2 ) [Wanninkhof et al.,
2004] by dividing k(600) by " (Table 1).
3.7. Analytical Model
[36] The two patches were considered separately, and the
analytical model was initialized with the second mean
3
He/SF6 profile for each to ensure that the patch was relatively homogeneous. The same was done with the first
profile with similar results. Because the wind speed was in
the range where many of these wind speed/gas exchange
parameterizations are similar, many of them were able to
produce the 3He/SF6 from SO GasEx to a large extent
(Table 1). For example, for Patch 1, the parameterizations of
Wanninkhof [1992] and Wanninkhof and McGillis [1999]
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Figure 5. Results of 3He/SF6 dual‐tracer experiments conducted in coastal and shelf areas, as well as the
open ocean. Solid symbols are for open ocean experiments; open symbols are for coastal and shelf experiments. The North Sea data are from Nightingale et al. [2000b], Florida Shelf Lagrangian Experiment
(FSLE) data are from Wanninkhof et al. [1997], GasEx‐98 are from McGillis et al. [2001a], Georges Bank
data are from Wanninkhof et al. [1993] and reanalyzed by Asher and Wanninkhof [1998], IronEx II data
are from Nightingale et al. [2000a], Southern Ocean Iron Experiment (SOFeX) data are from Wanninkhof
et al. [2004], and SOLAS Air‐Sea Gas Exchange Experiment (SAGE) data are from Ho et al. [2006].
Also shown are wind speed and gas transfer parameterizations of Wanninkhof [1992], Nightingale et al.
[2000b], Ho et al. [2006], and Wanninkhof et al. [2009]. The SAGE, SOFeX, and SO GasEx data have
been corrected for wind speed enhancement, assuming a quadratic relationship. The two anomalous data
points from SOFeX not used in the analysis are crossed out. The gray shaded area represents best fit to all
the available 3He/SF6 data points, including the 95% confidence interval (k(600) = (0.262 ± 0.022)u210).
Note that the best fit overlaps with the parameterizations of Nightingale et al. [2000b], Ho et al. [2006], and
Wanninkhof et al. [2009] over the wind speed range that contains most of the 3He/SF6 data (5–17 m s−1).
were both able to predict the 3He/SF6 well (Figure 4a), even
though they have different functional forms and diverge at
high wind speeds. For Patch 2, the parameterizations of
Nightingale et al. [2000b], Ho et al. [2006], and Wanninkhof
et al. [2009] were all similar in terms of performance
(Figure 4b). This is not surprising given that in the wind speed
range encountered, these parameterizations share many features (Figure 5). Table 2 also shows the optimized coefficients for quadratic and cubic parameterizations. For Patch 1,
the optimal quadratic coefficient is ca. 8% greater than that
proposed by Ho et al. [2006], and the optimal cubic coefficient is identical to that proposed by Wanninkhof and
McGillis [1999], with the resulting rRMSE being similar
between the optimization and the existing parameterizations.
For Patch 2, the parameterizations of Nightingale et al.
[2000b], Ho et al. [2006], and Wanninkhof et al. [2009]
share similar statistics as the optimization.
Table 2. Statistics of Comparison Between 3He/SF6 Measured
During SO GasEx and 3He/SF6 Predicted by an Analytical Model
Using Various Wind Speed/Gas Exchange Parameterizationsa
Patch 1
Parameterizations
Patch 2
rRMSE
rRMSE
(%)
Coefficients
(%)
Coefficients
Liss and Merlivat [1986]
Wanninkhof [1992]
Wanninkhof and McGillis
[1999]
Nightingale et al. [2000b]
Ho et al. [2006]
Wanninkhof et al. [2009]
6.3
5.5
4.7
12.4
20.8
24.3
5.2
4.9
6.8
7.7
8.6
8.8
Optimization (a2)
Optimization (a3)
4.6
4.7
0.286
0.0298
7.7
8.8
0.254
0.0225
a
Also included are optimizations using quadratic and cubic fits. The
coefficients are for Sc = 600.
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Table 3. The 1‐D General Ocean Turbulence Model, GOTM, Fitting Resultsa
Data Set
Function
a2
Patch 1
quadratic
cubic
quadratic
cubic
quadratic
cubic
0.310 ± 0.046
Patch 2
All
a3
0.0331 ± 0.0042
0.277 ± 0.011
0.0241 ± 0.0010
0.277 ± 0.010
0.0241 ± 0.0010
N
(F/N)1/2
R2
23
23
61
61
84
84
0.060
0.055
0.080
0.088
0.076
0.086
0.837
0.838
0.959
0.950
0.975
0.970
a
The values a2 and a3 are the coefficients of the quadratic and cubic forms, respectively, of k(600) as a function of u10N. The
uncertainties in the best fit coefficients were determined using a bootstrap methodology and represent estimates at the 95%
confidence level.
3.8. Numerical Model
[37] The 3He/SF6 results from the 1‐D GOTM were fitted
to the observations by varying the coefficients, a2 or a3
(described above). The model fitting procedure was performed using observations from each tracer patch separately
as well as for both patches combined. The optimized coefficients and estimates of their uncertainty are given in Table 3.
The uncertainties were estimated using a bootstrap technique [Efron and Gong, 1983], whereby a large number
(500) of simulated data sets were drawn from the actual data
set and each subjected to the model fitting procedure. The
uncertainties in Table 3 are the 95% confidence levels of the
estimated coefficients.
[38] Using data from both patches, the fit of the model to
the observations (the sixth and seventh columns of Table 3)
is quite similar for the quadratic and cubic dependencies.
RMS normalized differences (sixth column, Table 3) are
7.6% and 8.6% for the quadratic and cubic cases, respectively, and the corresponding R2 values are 0.975 and 0.970.
The best fit quadratic and cubic coefficients (for Sc = 600)
are 0.277 ± 0.010 and 0.0241 ± 0.0010, respectively, where
the former corresponds reasonably well to the coefficient of
0.266 proposed by Ho et al. [2006]. Performing the optimization using data from Patch 1 gives somewhat higher
values, 0.310 ± 0.046 for the quadratic and 0.0331 ± 0.0042
for the cubic coefficients, respectively. Owing to the large
estimated uncertainty in the optimized coefficient, the Patch 1
quadratic coefficient is not significantly different from the
value estimated using data from both patches. The values
estimated for Patch 2 are essentially the same as those
derived from both patches, as the major part of the combined data set is composed of Patch 2 data.
[39] Figure 6 shows a comparison of the 3He/SF6 ratios
from the model and observations for the case of quadratic
wind speed dependency. Note that although the model‐data
differences in Figure 6 are shown for all depths, the model
was fit to the data using only observations within the model
mixed layer based on density criteria. The modeled tracer
ratio below the mixed layer is systematically higher than in
the observations. This is likely due to the fact that the
model, lacking a realistic internal wavefield, underestimates
vertical mixing in the pycnocline below the mixed layer.
The individual tracer concentrations in the model below the
mixed‐layer base are very small, thus the model ratios are
not meaningful there. Above the mixed‐layer base, model
ratios in the Patch 1 simulation are higher than observed
in the early portion of the run (i.e., the modeled gas transfer
velocity is too low), and lower near the end (Figure 6d).
In the Patch 2 simulation, agreement is quite good above
the mixed‐layer base except for the period of mixed‐layer
shoaling around yearday 86 when model tracer ratios are
lower than observed. This is partly due to the increase in the
observed tracer ratio at station 26 (yearday 85.62) compared
to the values at station 25 (yearday 85.15), an event that is
not consistent with a 1‐D model. During this period, however, the model mixed‐layer shoals somewhat earlier than
the observed mixed layer. This results in a more rapid
decrease in the near‐surface tracer ratio as the more volatile
tracer (3He) is drawn down within the shallow mixed layer
and contributes to the model‐data misfit near the surface.
The fit of the model with gas transfer velocity as a function
of the cube of wind speed (not shown) is very similar to the
quadratic case shown in Figure 6.
[40] The best fit coefficients a2 and a3 derived with the
numerical model are systematically higher (7 to 10%) than
the values estimated using the analytical model. These differences could be due to the fact that the analytical model
considers the tracer mixed‐layer average 3He/SF6 from each
station, whereas the numerical model uses each individual
3
He/SF6 measurement within the model mixed layer. Gas
exchange in the numerical model is computed as the product
of the gas transfer velocities and the surface tracer concentrations, whereas in the analytical model the exchange is
based on the mixed‐layer average concentrations. Thus, for
equal gas exchange in the two models, lower ratios at the
surface compared to the mixed‐layer average mixed would
lead to an anomalously low gas transfer velocity in the
analytical model (and the coefficients in the wind speed
parameterizations). In the numerical model, the mean value
of the ratio of surface 3He concentration to (tracer) mixed‐
layer 3He concentration is approximately 0.85, and for SF6,
the corresponding ratio is 0.96. This reduction of near‐
surface tracer concentrations relative to the mixed‐layer
values in the model is qualitatively consistent with the
higher gas transfer velocities estimated from the numerical
model. In the data, most of the time, this surface versus
mixed‐layer difference is small as, for example, the SF6
measured at 5 m is typically within 1–2% of its mixed‐layer
average. However, there is one occasion during Patch 1
where this difference is almost 20%, and several occasions
during Patch 2 where the surface concentration is 13–46%
lower (see Figure 3b, for example).
3.9. Relationship Between Wind Speed and Gas
Exchange
[41] Within the errors of the optimization (Table 3), the
analytical and numerical models show consistent results
in their estimates of a2 and a3 for Patches 1 and 2. One
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Figure 6. Model‐data comparison for numerical model simulation of dual‐tracer gas transfer using a
quadratic function for the dependence of gas transfer velocity with wind speed. (a) Wind speed measured
aboard the ship, adjusted to neutral conditions. (b) Model tracer ratio. (c) Observed tracer ratio from the
shipboard water sampling. (d) Difference between observed and modeled tracer ratio. The optimal coefficient from the fit to both patches together is used. Note that only those observations above the model
mixed‐layer depth, based on a density criterion and shown by the dashed lines in Figures 6b–6d, were
used in the model optimization.
interesting aspect of the 3He/SF6 data that shows up in both
the analytical and numerical models is the difference
between Patches 1 and 2 (Tables 2 and 3). For instance, in
the analytical model, while all the parameterizations were
similar in being able to reproduce 3He/SF6 from Patch 1
(rRMSE between 4.7 and 6.8%; Table 2), Wanninkhof
[1992] and Wanninkhof and McGillis [1999] fail to simulate the data from Patch 2 (rRMSE of 20.8% and 24.3%,
respectively; Table 2). Part of the explanation lies in the
wind speeds encountered during the two patches (Figure 1).
For Patch 1, the wind speeds were mostly in the range
between 6 and 12 m s−1, where the various parameterizations overlap. For Patch 2, the wind speeds were between
2 and 16 m s−1, where there is considerable divergence
between the parameterizations. At these higher wind speeds,
Wanninkhof [1992] and Wanninkhof and McGillis [1999]
overestimate k(600), whereas Liss and Merlivat [1986]
underestimate k(600).
[42] The difference between the two patches could also
indicate that something other than direct wind effects
influences air‐sea gas exchange in the Southern Ocean, but
these effects are small relative to the dominant effect of
wind. Both existing parameterizations and the optimizations
yield similar results in terms of rRMSE (i.e., 5%–9%),
indicating wind forcing is the major driver of gas exchange
for slightly soluble gases in the ocean and that other known
impacts are either intrinsically related to wind or have a
minor effect on gas exchange time scales of the order of
days to weeks.
[43] The analytical and numerical models show that to a
large extent, previously proposed parameterizations reasonably describe the relationship between wind speed and
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gas transfer velocity during SO GasEx, and the results from
SO GasEx are in accord with global constraints based on
ocean bomb 14C inventories [Naegler et al., 2006; Sweeney
et al., 2007]. Furthermore, with the exception of two
anomalous points from SOFeX (Figure 5), the parameterizations of Ho et al. [2006], Nightingale et al. [2000b],
and Wanninkhof et al. [2009] explain 83%, 82%, and 80%
of the variance in all 3He/SF6 dual‐tracer studies conducted
in the coastal and open ocean, including SO GasEx. These
parameterizations have different functional forms: that of
Ho et al. [2006] is quadratic; that of Nightingale et al. [2000b]
is a polynomial that includes a linear term and a quadratic
term; and that of Wanninkhof et al. [2009] is a polynomial
that includes a constant, a linear term, a quadratic term, and
a cubic term.
[44] There are three important implications of these findings: (1) Relationships between wind speed and gas
exchange determined in one ocean locale can be applied to
other regions. In fact, Asher [2009] determined that up to
half of the variability in the 3He/SF6 data sets could be due
to experimental variability, so the differences can largely be
explained by experimental uncertainties. (2) The functional
form of wind speed/gas exchange parameterizations (i.e.,
quadratic, cubic, or hybrid) cannot be unequivocally
determined from field experiments, including SO GasEx.
(3) Although it is recognized that wind speed might not be
the only correlate for gas exchange, it is a sufficient one.
The results suggest that wind forcing is the major driver of
gas exchange for slightly soluble gases in the ocean and that
other known impacts are either intrinsically related to wind
or have a minor effect on gas exchange time scales of the
order of days to weeks. There are other parameters to predict
gas exchange, such as turbulence dissipation [e.g., Zappa
et al., 2007], but those parameters are not routinely measured and are difficult to implement in global biogeochemical models.
[45] The parameterizations of Liss and Merlivat [1986],
Wanninkhof [1992], and Wanninkhof and McGillis [1999]
can reasonably describe the relationship between wind
speed and gas exchange in the 5 to 11 m s−1 wind speed
range. However, compared to the entire 3He/SF6 data set
(Figure 5), they are only able to explain 53%, 64%, and 2%
of the variance, respectively. The cubic relationship of
Wanninkhof and McGillis [1999] deviates significantly from
the dual‐tracer results at high wind speeds (>12 m s−1). If
the comparison is limited to the wind speed range below
12 m s−1, then Wanninkhof and McGillis [1999] are able to
explain about 37% of the variance. Like that of Wanninkhof
and McGillis [1999], another cubic parameterization derived
from eddy covariance CO2 measurements during GasEx 98
is that proposed by McGillis et al. [2001a]. This parameterization was derived from the same data set but assumed a
nonzero intercept and predicts k(600) that are 10–15% lower
than that of Wanninkhof and McGillis [1999] between u10 of
12 and 20 m s−1. The result is that it is able to explain 48%
of the variance in the 3He/SF6 data set over the entire wind
speed range.
3.10. The Future of 3He/SF6 Dual‐Tracer Experiments
[46] One of the reasons that short time scale measurements like eddy covariance measurements of CO2 often
produce these higher‐order dependencies (i.e., cubic versus
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quadratic) that are not reflected in the 3He/SF6 data might be
because on these time scales (i.e., minutes to hours), the
fluxes are indeed very high at the near surface. However,
because vertical mixing cannot keep pace with the fluxes,
the full water column is not impacted and therefore
the higher‐order dependence is not reflected in techniques
that utilize full water column mass balances. However, the
3
He/SF6 is an integrative approach that yields robust estimates of total gas exchanged in the mixed layer over periods
ranging from days to weeks. This is often the relevant
parameter in biogeochemical cycling whereas the micrometeorological techniques are discrete, and sometimes discontinuous, measurements with an appreciable uncertainty
in each discrete value. The deviation between 3He/SF6 dual‐
tracer and CO2 eddy covariance measurements is most
pronounced at higher wind speeds (i.e., > 12 m s−1) (e.g.,
Edson et al., submitted manuscript, 2011) where the eddy
covariance values are higher. Simultaneous 3He/SF6 dual‐
tracer and CO2 eddy covariance measurements at high wind
speeds, along with natural tracers of gas exchange and
surface water residence time such as and 222Rn and 7Be
[Kadko and Olson, 1996], and detailed physical mixed‐layer
measurements are expected to help us resolve the conflict
between these two leading techniques.
[47] The 3He/SF6 tracer release experiments conducted in
the ocean have demonstrated the power of deliberate tracers
to quantify gas exchange over a period of several days or
more, and spatial scales of a few tens of kilometers. However, because of the global 3He shortage [Cho, 2009], future
3
He/SF6 experiments could be jeopardized. At the moment,
no alternative tracer with the properties that make 3He the
perfect companion to be used with SF6 (i.e., inert, low
background, similar solubility, very different diffusivity) has
emerged. A concerted effort should be made to obtain 3He
for these experiments, and/or to find alternative deliberate
tracers to quantify air‐sea gas exchange. Since there is no
other volatile tracer with a Schmidt number as low as 3He
and as large a dynamic range over which it can be measured,
the likely candidate is a nonvolatile tracer. The innovative
application of bacteria spores used by Nightingale et al.
[2000b] is an option but has a limited dynamic range for
large‐scale open ocean experiments. Dyes such as Rhodamine WT measured by HPLC with fluorescence detection
[e.g., Hofstraat et al., 1991; Suijlen et al., 1994] hold some
promise, as demonstrated by Upstill‐Goddard et al. [2001],
but losses due to absorption and photobleaching need to be
properly quantified [Smart and Laidlaw, 1977].
4. Conclusions
[48] During the Southern Ocean Gas Exchange Experiment (SO GasEx), two 3He/SF6 injections were made to
quantify gas transfer velocities. The experiment had the
largest number of 3He samples collected during a 3He/SF6
dual‐tracer experiment, and concurrent eddy covariance
measurements of CO2 and DMS were made [see Yang et al.,
2011; Edson et al., submitted manuscript, 2011].
[49] The 3He/SF6 results from SO GasEx show that the
recently proposed parameterizations based on dual‐tracer or
other water column mass balance approaches reasonably
describe the relationship between wind speed and gas
transfer velocity, and that results from SO GasEx are similar
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to those from other ocean basins. The important implications
of these results are as follows.
[50] 1. Within 20%, the wind speed/gas exchange parameterizations obtained in one location can be applied to
another, so, for example, the measurements made during
SAGE or SO GasEx should be applicable to other parts of the
Southern Ocean.
[51] 2. Field experiments, like SO GasEx, can produce
valuable data for determining the relationship between wind
speed and gas exchange and allow us to verify existing
parameterizations. However, because of their similarities in
results they cannot distinguish the functional form of these
parameterizations, since these relationships are empirical
and within a certain wind speed range, different functional
forms will share similar features.
[52] 3. Wind speed, while not the direct mechanism
responsible for enhancing gas exchange, is generally recognized to be a major forcing and is perhaps the best correlate to use, considering its wide spread measurement and
ease of implementation in both simple calculations and in
global biogeochemical models. Furthermore, while the
parameterizations of Liss and Merlivat [1986], Wanninkhof
[1992], and Wanninkhof and McGillis [1999] can describe
the relationship between wind speed and gas exchange in
certain wind speed ranges, they fail to predict gas transfer
velocities derived from the dual‐tracer method over the
entire range from 0 to 16 m s−1.
[53] Acknowledgments. We acknowledge M. Reid and P. Schmieder
for assistance with tracer injection, sampling, and measurement; S. Archer,
G. Lebon, S. Purkey, and M. Rebozo for assistance during tracer injection;
C. McNally for 3 He extraction; R. Friedrich for 3 He measurements;
J. Edson for providing the ship wind speed data; H. Czerski, S. Eggleston,
and L. Larsen for help with data processing; and J. Triñanes for providing
the QuikSCAT data from http://podaac.jpl.nasa.gov/PRODUCTS/p286.
html. Funding was provided by the National Oceanic and Atmospheric
Administration through NA07OAR4310113 and NA08OAR4310890
(D.T.H. and P.S.), GC07–136 (R.W. and K.F.S.), and NA07OAR4310105
(D.H. and D.S.U.). This is LDEO contribution 7466.
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