Chemistry Reference
In-Depth Information
small eddy model, see Fig. 1) differ in the prediction of concentration pro-
files, but not in the dependence on environmental parameters such as wind
speed.
In order to gain insight into the transport mechanisms, new techniques
for the quantitative investigation of gas exchange have been developed,
e.g., boundary layer visualization with laser induced fluorescence imaging
(Münsterer and Jähne 1998), particle tracking velocimetry (Siddiqui et al.
2004) and infrared imaging (Schimpf et al. 1999), but only lately pro-
gressed to a state where quantitative measurements of relevant turbulent
properties are feasible. By using novel visualization and image processing
techniques, it is possible for the first time to get an insight into the mecha-
nism of these dynamic processes (Haußecker et al. 1998) and to investigate
turbulent processes that underlie the transport mechanisms across the
aqueous boundary layer. Despite experimental and theoretical effort, there
is still a lack of understanding how other processes, e.g. rain, wave break-
ing, and intermittent meteorological conditions affect the transport proc-
esses.
In the following discussion, the
Controlled Flux Techniqu
e (Jähne et al.,
1989) is presented, using heat as a proxy tracer for gases. The high spatial
and temporal resolution of this unique technique allows the study of key
questions about transfer processes. Detailed laboratory measurements were
carried out in the Heidelberg wind wave facility and during the 1997
Coastal Ocean Processes Study in the Atlantic Ocean. Experimental evi-
dence shows that a surface film decreases the gas exchange rate and modi-
fies the length scales of near surface turbulence in comparison to a clean
interface.
2 Heat as a proxy tracer for gases
Typical mass balance methods to measure the air-sea gas transfer have one
major drawback: the response time is of the order of hours to days, making
a parameterisation with parameters such as wind forcing, wave field, or
surface chemical enrichments nearly impossible. The controlled flux tech-
nique uses heat as a proxy tracer for gases to measure the air-sea gas trans-
fer rate locally and with a temporal resolution of less than a minute. This
method offers an entirely new approach to measure the air-sea gas fluxes
in conjunction with investigation of the wave field, surface chemical en-
richments and the surface micro turbulence at the water surface. The prin-
ciple of this technique is very simple: a heat flux is forced onto the water
surface and the skin-bulk temperature difference across the thermal
sublayer is measured.
