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176 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION
5.1.3 Time resolved Measurements of Air Water Gas Exchange
Participating scientist Kai Degreif
Abstract Laboratory measurements are performed to determine the influence of short wind waves
on air-sea gas exchange. UV-spectroscopy is introduced as a powerful method for tracking the fate
of gaseous and volatile tracers simultaneously both in the gas and water phase. The “controlled
leakage technique”, a new measurement technique that permits the calculation of time resolved flux
rates exclusively from the air-side concentration, could be validated within the scope of the UVmeasurements.
Reliable gas flux measurements of dissolved trace gases for a broad range of species
without the need for extraction are now feasible.
a b
d e
wind speed [m/s]
relative concentration
5
4
3
2
1
0
0 1 2 3
time[h]
4 5 6
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
measured concentration
calculated concentration
0
0 1 2 3
time [h]
4 5 6
c
f
relative concentration
transfer velocity k [cm/h]
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0 1 2 3
time [h]
4 5 6
25
20
15
10
5
0
0 1 2 3
time [h]
4 5 6
Figure 5.3: Time resolved gas exchange measurements at the small Heidelberg Wind Wave Facility. a
Experiments are performed in a small circular water channel of 1.2 m diameter. b Rotating windpaddles
produce controlled wind speeds. c The investigated trace gases are dissolved in the water phase.
The air-volume being constantly flushed with fresh air, the air-side concentration of the released tracers
is proportional to the gas transfer and the water concentration. d Concentration measurements of
volatile aromatics are performed using UV-spectroscopy simultaneously in the air and water phase. e
The calculated water concentration is in good agreement with the measured water concentration. f
Gas transfer velocities are achieved with a high time resolution.
Background The oceans can act as a source
or sink for climate relevant gases such as carbon
dioxide, methane or dimethylsulphide. As hydrodynamics
at the free oceans surface is not well
understood yet, the estimation of total flux rates
of those gases has to rely on empiric parameterizations.
Precise and systematic laboratory measurements
can help us to get a better understanding
of the gas transfer rates dependence on tracer
parameters like Schmidt number and solubility or
hydrodynamic parameters like wind stress, waviness
of the water surface and turbulence intensity.
Methods and results Conventional gas exchange
measurements rely on the knowledge of
concentration gradients across the water surface.
Therefore the information about the concentration
of the dissolved species is indispensable. UVspectroscopy
provides the possibility for direct
and fast measurement of multiple tracer concentrations
both in the water and the atmosphere.
Volatile aromatic hydrocarbons such as benzene,
thiophene, pyridine and derivates, and some other
species can be measured.
The “controlled leakage technique” permits the reconstruction
of the water concentration from gases
that are released by the water body only by measuring
the air concentration. Therefore flux rates
of tracers with a wide range of physicochemical
parameters, i.e. Schmidt number and solubility
can be measured simultaneously so that a detailed
experimental study of the influence of these parameters
on air-water gas exchange has now become
feasible and will be presented within the
work.
Funding Project funded by the DFG
Main publication Degreif, Kai, Doktorarbeit,
Institut für Umweltphysik und Interdisziplinäres
Zentrum für Wissenschaftliches Rechnen, Universität
Heidelberg, 01/2006