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152 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION
5.1.8 Spectroscopic Techniques for Gas-Exchange Measurements
Felix Vogel
Abstract To evaluate the applicability of different spectroscopic techniques for temporally resolved
gas-exchange measurements a Raman spectometer was built and existing UV/Vis systems were improved.
Laboratory experiments yielded that Raman spectroscopy is not suitable for dynamic gasexchange
measurements due to its insufficient temporal resolution while the applicability of UV/Vis
spectroscopy was confirmed.
Figure 5.8: Simultaneous measurement of the concentration of anisole in the air and water-bulk during
an invasion experiment
Background Studying gas-exchange processes
contributes to the understanding of our climate
system as well as it is fundamental research of
turbulent processes in fluids.
Accounting for small scale processes to yield an
accurate parametrisation of the transfer rate, temporally
resolved in-situ measurements of the concentration
change have to be performed.
Besides classical methods measuring dissolved
gases, recently temporally highly resolved spectroscopic
techniques with volatile aromatics have
been introduced as a new tool for the investigation
of the transfer processes [Degreif2006].
Funding None
Methods and results Experiments with pure
and dissolved aromatics yielded that the newly
built Raman setup is not suitable for systemanic
gas-exchange measurements. To conduct experiments
at the circular wind-wave flume of the University
of Heidelberg and the linear wind-wave
flume of the University of Hamburg, one waterphase
and one air-phase setup was developed. The
performance of both UV/Vis spectrometers was
evaluated at the Aeolotron laboratory. With an
absorption path length of up to 10 metres the
temporal resolution of the air-phase spectrometer
is of the order of deciseconds. Invasion experiments
yielded that the in-situ concentration
can be monitored continuously in the range of
30 ppm−0.3 ppm for example for benzaldehyde.
For the UV/Vis setup to measure the water-phase
concentration an absorption length of one metre
and an integration time of 40 ms turned out to be
adequate.
Outlook/Future work To reduce the uncertainties
of the calculated piston velocities, the solubilities
of the applied substances should be determined
in laboratory experiments, as the values
given in the available literature vary largly. The
acquistion of concentration gauged spectra is also
needed to determine the piston velocities of moderately
soluble substances. As their transfer rate
is not dominated by the transfer rate of one compartment,
both air and water-phase concentration
have to be taken into account.
Main publications [Vogel, 2006]