In this study we present gas-exchange measurements conducted in a large-scale
wind–wave tank. Fourteen chemical species spanning a wide range of solubility
(dimensionless solubility,
The world's oceans are key sources and sinks in the global budgets of
numerous atmospherically important trace gases, in particular CO
The principles behind gas exchange at the air–sea interface have been
reported in detail within previous reviews
(
The transfer velocity,
Gas transfer velocities have been determined in both field studies (using
mass balance, eddy correlation or controlled flux techniques) and laboratory
experiments described in previous gas-exchange reviews
(
Mass balance methods have been applied in the field using geochemical tracers
(O
In this study, gas-exchange experiments were performed in a state-of-the-art
large-scale annular wind–wave tank. An experimental approach based on mass
balance has been developed, whereby air- and water-side concentrations of
various tracers are monitored using instrumentation capable of on-line
measurement. For the first time, parallel measurements of total air and
water-side transfer velocities for 14 individual gases within a wide range of
solubility, have been achieved. Wind speed conditions (reported at 10 metres
height,
In this study, total transfer velocities for low as well as medium to highly soluble tracers were determined using a mass balance approach. The wind–wave tank is interpreted in terms of a box model.
The basic idea of the box model method is the development of a direct
correlation between the air- and water-phase concentrations,
Figure
Mass balances for the air and water side. Naming convention is as
follows:
The first term on the right hand side of each equation represents the
exchange of a tracer from one phase to the other due to a concentration
gradient. The second term stands for possible tracer input
(
In the following sections, two different box model solutions, as used in this
work for the low solubility tracers (water-side controlled:
Sect.
Simulated concentration time series for a water (
The following approach was used for tracers with relatively low solubility
(
Figure
The ambient tracer concentration in the air entering the air space through
leaks or during flushing can be safely assumed as negligible in comparison to
the levels used for all examined tracers. Omitting parameter
Applying Eq. (
In this approach, tracers with relatively high solubility (
In Fig.
At an equilibrium point, the concentration time derivative
After SS
The total transfer velocities in the wind–wave tank box are calculated from
In most wind–wave facilities, small air leaks are inevitable. The amount of
tracer escaping the air space of the facility needs to be monitored and
corrected for, as described in Sects.
An aerial illustration of the Aeolotron tank and its main
features. The numbers denote the segments. The axial fans producing the wind
can be seen in the roof of segments 4 and 12. The air pipes supplying fresh
air and removing waste air are shown in grey; figure adapted from
The air–water gas-exchange experiments were conducted in the large-scale
annular Aeolotron wind–wave tank at the University of Heidelberg, Germany
(Fig.
In the facility, several ambient parameters are monitored. Temperature measurements are provided by two temperature sensors (PT-100) installed in the water and air phase of segment 15 (at heights of 0.5 and 2.3 m, respectively). On the ceiling of the same segment a fan anemometer (STS 020 by Greisinger electronic GmbH) installed in the centre line, determines the wind velocity. Two humidity sensors are mounted in segments 2 and 13. An optical ruler provides the water height using the principle of communicating vessels. Segments 1 and 11 contain the tracer inlets for the air and water phase, respectively. The leak test gas is introduced in segment 11.
The annular geometry of the wind–wave tank, contrary to a linear geometry, permits homogeneous wave fields and unlimited fetch. The well-mixed air space (at few centimetres height above the surface) ensures no concentration gradients and therefore concentration measurements independent of the sampling height. On the other hand, the restricted size of the facility which leads to waves reflecting off the walls, results with a different wave field to that found in the open ocean.
A series of 14 tracers covering a wide solubility (
All tracers were monitored on-line in both the air and the water phase. The
VOC measurements were performed using proton reaction mass spectrometry
(PTR-MS) from Ionicon Analytik GmbH (Innsbruck, Austria), while for the
halocarbons and N
Molecular masses (
For the surfactant experiments, the soluble substance Triton X-100,
C
The operation and sampling conditions for both air and water phases are briefly described below. Additional instrumentation for substantial supplementary measurements follows.
In the water phase a PTR-Quadrupole-MS (PTRQ-MS) (water inlet in segment 3) and a FT-IR
spectrometer (water inlet in segment 6) were used to measure the
concentration levels of the VOCs and the halocarbons and N
The membrane equilibrator device includes a thin gas permeable membrane capable of separating the gas from the liquid phase (commercially available and often used in medicine as a human lung replacement to oxygenate blood). Water from the Aeolotron is constantly pumped through the membrane device. Inside the equilibrator gas exchange occurs, due to the partial pressure difference of the gases involved, until equilibrium between air and water is achieved (Henry's law at constant temperature).
Schematic time series of the wind speed, flushing periods and air/water tracer inputs.
A detailed configuration of the membrane set-up in conjunction with the
PTR-MS is shown in Fig.
A similar set-up using a second membrane equilibrator was connected to the
FT-IR instrument. The water flow was kept at a rate of about
3 L min
The time constant of the membrane equilibrator was evaluated as described in
Air-side concentrations obtained for example water-side
The PTR-MS detection technique has been described in detail elsewhere
(
Water-phase calibrations were performed in conjunction with the membrane
equilibrator set-up (see Fig.
The key aspects of FT-IR spectroscopy are described in detail in
In the air phase, a PTR-time of flight (ToF)-MS (inlet in segment 3) with a
time resolution of 10 s provided very fast on-line measurements for the VOCs
while a FT-IR (inlet in segment 2) with a time resolution of 30 s was in
parallel monitoring the halocarbons and N
The ionisation principle of the PTR-ToF-MS is the same as the PTRQ-MS;
however, here a time-of-flight mass spectrometer is used. Throughout the
measurements, the PTR-ToF-MS was configured in the standard V mode with a
mass resolution of approximately 3700 m
Calibrations in the air phase were conducted under high humidity conditions equivalent to the sampling conditions during the experiments (85–90 % RH). The desired mixing ratios (1–600 ppbv) were obtained by appropriate dilution of the multi-component VOC gas standard with synthetic air. Linear response was established for all examined tracers.
A second Thermo Scientific Nicolet iS10 FT-IR spectrometer was used to
measure the air-side concentrations. The measuring cell with a folded light
path of a total length of 2 m, was kept at a constant temperature of
35
The individual total transfer velocity uncertainties were calculated applying
the propagation of error for uncertainties independent from each other to
Eq. (
The concentration uncertainties for the PTR-MS measurements were calculated
using the background noise and the calibration uncertainty of each examined
tracer. Relatively low uncertainties were obtained for the air-phase
concentration levels SS
The uncertainty of the concentration measurement with the FT-IR spectrometers was found to be concentration dependent. All concentration uncertainties lie below 4 % for the typical concentrations measured in the described experiments.
The individual uncertainties for the leak and flush rates of all conditions were of the order of 0.5 and 1 %, respectively. Based on the geometrical parameters of the facility the surface area uncertainty was calculated to be approximately 2 %, while a maximum of 3 % is estimated for the volume uncertainty. For the solubility values provided by literature, accurate uncertainty estimations are difficult. Here we assume a maximum uncertainty of 10 % for all literature sources.
The overall estimated total transfer velocity uncertainties therefore ranged
between 6–12 and 6–20 %, respectively, for the
Membrane equilibrator – PTRQ-MS set-up schematic. The dark blue and orange lines represent the water and air loops of the system, accordingly.
Supplementary measurements of wind driven, surface associated, physical parameters, such as the mean square slope and the water-sided friction velocity, were additionally made in the Aeolotron wind–wave tank to enable further investigations of the physical mechanisms of air–water gas exchange.
The mean square slope measurements, reflecting the surface roughness
conditions, were performed in parallel with the gas-exchange measurements
using a colour imaging slope gauge (CISG) installed in segment 13. The CISG
device uses the refraction properties of light at the air–water boundary. A
colour coded light source was placed below the water while a camera observed
the water surface from above. Using lenses to achieve a telecentric set-up, a
relationship between surface slope and the registered colour can be
determined. Errors are calculated from the statistical fluctuations of the
individually measured mean square slope values. A more detailed description
can be found in
The water-sided friction velocity,
The Aeolotron facility was filled to 1 m height (
Individual gas-washing bottles containing highly solubility tracers in liquid
form were purged with a controlled flow of clean air that swept the
air-tracer gas mixture into the air phase of the facility. The bottles were
kept in a thermostatic bath at 20
At the beginning of each experiment, the first wind speed condition was
applied while the flushing of the air space was turned on (open air space) in
order to achieve a background point for all tracers. Thereafter, the flushing
was turned off (closed air space) and the tracer concentration (air and
water-side controlled) started to increase (see more in
Sect.
The wind speed varied from very low values
(
Reference velocities,
The experimental procedure described above was repeated four times at clean
surface conditions for all tracers listed in Table 1. Three further
repetitions were accomplished with a surfactant (Triton X-100) covered water
surface. The surfactant concentration in the fifth repetition was
0.033 mg L
Despite the well-reproduced experimental conditions, small variations between
the repetitions were observed. Table 2 displays a mean value of the main
measured parameters along with the standard deviation, expressing the extent
of variability between the repetitions, of each case. For the highest wind
speed condition of the clean case, only three repetitions were performed.
Also in repetition two, the
In this work, total transfer velocities of two contrasting tracers at
opposite ends of the solubility spectrum, N
In Figs.
Total transfer velocity of N
As indicated in Fig.
Total transfer velocity of CH
The air-sided transfer velocities
Overall, the observed trends and transfer velocity magnitudes of both
After obtaining clear, reproducible transfer velocity trends for a clean
water surface, the effect of a surfactant was evaluated using two different
surfactant (Triton X-100) concentrations. As expected, the surfactant
suppressed the transfer velocity as well as the friction velocity,
As indicated in Fig.
Effect of the two different surfactant concentrations on the total
transfer velocities of
Reduced surface stress and roughness change the hydrodynamic properties of
the water surface and consequently affect the gas transfer. As given in
Table
Comparison between the N
The gas transfer velocities of weakly soluble tracers has been extensively
studied over the previous years. Numerous
Looking at the lower wind speed range (0.7 to 4 m s
The transfer velocities obtained in this study, show a closer agreement with
the
In contrast to the weakly soluble gases, high solubility tracers have
received much less attention. In Fig.
Comparison between the CH
As indicated in Fig. 11, our results agree very well with the previous
laboratory parameterisations lying nearer to
The transfer velocity values provided by model and field studies are about 1.5 to 2 times lower than the ones derived from the laboratory measurements. This is to be expected as model and field studies include an extra turbulent resistance in the air space at 10 m height.
This study has demonstrated that the Aeolotron wind–wave tank in
combination with the adopted box model methodology, experimental procedure
and instrumentation are capable of generating reliable and reproducible gas
transfer velocities for species spanning a wide range of solubilities. The
molecules nitrous oxide and methanol have been used to exemplify the behaviour
of sparingly soluble and highly soluble species. These represent cases of
a water-side and an air-side layer control, as described in
Sect.
Particularly interesting are the effects on the gas transfer velocity induced
by the addition of a surfactant. Despite the surface micro-layer being
commonly present on the ocean, its effect on air–sea gas transfer is poorly
understood and there is a paucity of data both from the laboratory and the
field. The impact of the surfactant is markedly different on the two tracers
shown here. A strong reducing effect (up to a factor of 3) was observed
for the water-side controlled tracer, N
We maintain that it is important to monitor the transfer process in both the
water-phase (using water-side controlled tracers) and the air-phase layer (using
air-side controlled tracers) in order to develop a true enduring and
generally applicable model for air–sea gas transfer. The results produced
here correspond reasonably well with previous expressions for
This study, based on data from the world's largest operational annular wind–wave facility, derived from advanced analytical technology which has been set-up to monitor the gas concentration changes in both the air and the water phase simultaneously at unprecedented measurement frequency, has proven to produce high quality transfer velocity measurements. On the basis of our results, we recommend the proposed methodology for future air–water gas-exchange measurements.
The experimentally calculated water-sided total transfer velocities,
Air-sided friction velocities can be converted to water-sided friction velocities by
The wind speed at a height of 10 m,
We own a special thank to J. Auld and T. Klüpfel
for their valuable assistance and support during the gas exchage experiments.
The mean square slope values were kindly provided by R. Rocholz. R. Sander is
thanked for the helpful and insightful discussions on solubility matters
considering this manuscript. Furthermore, we thank all members of
B. Jähne's group for their understanding and support during the
measurements. We acknowledge the financial support of the BMBF Verbundprojekt
SOPRAN (