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| Titel |
Influence of wind-induced air pressure fluctuations on topsoil gas
concentrations within a Scots pine forest |
| VerfasserIn |
Manuel Mohr, Thomas Laemmel, Martin Maier, Dirk Schindler |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250151597
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| Publikation (Nr.) |
 EGU/EGU2017-16285.pdf |
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| Zusammenfassung |
| Commonly it is assumed that soil gas transport is dominated by molecular diffusion. Few
recent studies indicate that the atmosphere above the soil triggers non-diffusive gas
transport processes in the soil, which can enhance soil gas transport and therefore soil
gas efflux significantly. During high wind speed conditions, the so called pressure
pumping effect has been observed: the enhancement of soil gas transport through
dynamic changes in the air pressure field above the soil. However, the amplitudes and
frequencies of the air pressure fluctuations responsible for pressure pumping are still
uncertain. Moreover, an in situ observation of the pressure pumping effect is still
missing.
To investigate the pressure pumping effect, airflow measurements above and below the
canopy of a Scots pine forest and high-precision relative air pressure measurements were
conducted in the below-canopy space and in the soil over a measurement period of 16 weeks.
To monitor the soil gas transport, a newly developed gas measurement system was used. The
gas measurement system continuously injects helium as a tracer gas into the soil until a
diffusive steady state is reached. With the steady state concentration profile of the tracer gas,
it is possible to inversely model the gas diffusion coefficient profile of the soil. If
the gas diffusion coefficient profile differed from steady state, we deduced that
the soil gas transport is not only diffusive, but also influenced by non-diffusive
processes.
Results show that the occurrence of small air pressure fluctuations is strongly dependent
on the mean above-canopy wind speed. The wind-induced air pressure fluctuations have
mean amplitudes up to 10 Pa and lie in the frequency range 0.01-0.1 Hz. To describe the
pumping motion of the air pressure field, the pressure pumping coefficient (PPC) was defined
as the mean change in pressure per second. The PPC shows a clear quadratic dependence on
mean above-canopy wind speed. Empirical modelling of the measured topsoil helium
concentration demonstrated that the PPC is the most important predictor for changes in the
topsoil helium concentration.
Comparison of time periods with high PPC and periods of low PPC showed that the soil
gas diffusion coefficient in depths between 5-10 cm increased up to 30% during periods of
high PPC compared to steady state. Thus, the air pressure fluctuations observed in the
atmosphere and described by the PPC penetrate into the soil and influence the topsoil gas
transport. |
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