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4.1.3 Tracer
Experiments
In order to measure air movements and air
transport routes, it is possible to enrich the air with a tracer
and follow and measure its motion. The simplest method uses
smoke canisters. The smoke, typically colored, mixes with the
flowing air and is transported so that e.g. it can be
photographically documented and evaluated. Smoke canisters are
usually employed to mark cold air flows, since these move
relatively slowly and exhibit only minor vertical mixing. Figure
4/3a and Figure 4/3b shows the application of a smoke
canister to make a cold air flow visible.
More complex is the application of tracer
gases such as sulfur hexafluoride (SF6). The tracer
gas is released continually at a defined location and its
resulting concentration is measured at many sites throughout the
city (Figure 4/4). In addition, air probes are collected
at the measurement sites at regular intervals of time and are
later evaluated with the means of a gas chromatograph. Suitable
tracer gases are non-poisonous, not highly chemically reactive,
not widely present in the atmosphere, and can be detected even
in small concentrations.
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Fig. 4/3a:
Cold air flow, marked by artificial smoke
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Fig. 4/3b: Cold
air flow, marked by artificial smoke
(KUTTLER
u. DÜTEMEYER, 2003) |
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Fig. 4/4: The distribution
of the tracer gas SF6 as an average over the total
duration of measurement (ppb) during the night of 9-10 August
1996 for determining the cold air flow in Stuttgart; black
line: cold air trajectory calculated with KALM (s. a. Chapter
4.3.3) (BAUMBACH et al., 1999)
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