Field measurements of tracer gas transport by barometric pumping

Gas fluxes between soil and atmosphere play an important role for the global greenhouse gas budgets. Several methods are available to determine soil gas fluxes. Besides the commonly used chamber methods the gradient method becomes more and more important. Chamber methods have the disadvantage that the microclimate can be influenced by the chamber which can affect gas fluxes. This problem does not occur with the gradient method. Furthermore the gradient method has the advantage that it can provide information about the depth profile of gas production and consumption in the soil.The concept of the gradient method is to calculate gas fluxes by the vertical concentration gradient of a gas in the soil. For the calculation of the flux the effective diffusivity coefficient of the soil is needed. This can be approximated by models or by lab measurements. However, both of these approaches often fail in explaining site specific characteristics and spatial variability. Another way to determine soil gas diffusivity is to apply the gradient method using a tracer gas. By the injection of a tracer gas with known flux soil gas diffusivity can be measured in-situ.We developed an innovative sampling set-up to apply an improved gradient method including the possibility to determine soil gas diffusivity in situ. We designed a sampler with build-in CO2 sensors in multiple depths that can easily be installed into the soil. With this sampler CO2 concentrations can be measured continuously in several depths. This enables the identification of short-time effects such as the influence of wind-induced pressure pumping on gas transport. The sampler allows tracer gas injection into the soil for in-situ diffusivity measurement. We decided for CO2 as a tracer gas because it can be measured with small sensors which keep the set-up simple. To account for the natural CO2 production in the soil we developed a differential gas profile approach. Using an additional reference sampler allows measuring the natural CO2 gradient without the tracer signal, and thus subtracting the tracer CO2 signal from the natural CO2 signal.The sampler consists of one 3D print segment per depth each containing one CO2 sensor. These parts can be combined to a sampler with flexible amount of measurement depths. The construction with individual segments allows a better maintenance in case of sensor defects. For the installation of the sampler a hole has to be drilled, into which the sampler is inserted. To prevent gas bypassing along the wall of the drill hole we equipped each segment with an inflatable gasket between the measurement locations.In a next step we will evaluate the sampler and test it in the lab and under different environmental conditions. We expect that with this sampler we will be able to run gas transport experiments in the field with a high temporal resolution and relatively low effort.AcknowledgementsWe thank Alfred Baer and Sven Kolbe for the technical support.

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The importance of barometric pumping for subsurface gas transport at U20az : Barnwell

10.2172/1417807 ◽

2018 ◽

Author(s):

Suzanne Michelle Bourret ◽

Edward Michael Kwicklis ◽

Dylan Robert Harp ◽

Philip H. Stauffer

Keyword(s):

Gas Transport ◽

Barometric Pumping

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Air change efficiency of room ventilation units

E3S Web of Conferences ◽

10.1051/e3sconf/201911101017 ◽

2019 ◽

Vol 111 ◽

pp. 01017 ◽

Cited By ~ 1

Author(s):

Alo Mikola ◽

Juhan Rehand ◽

Jarek Kurnitski

Keyword(s):

Field Measurements ◽

Airflow Rate ◽

Test Results ◽

Heating Period ◽

Tracer Gas ◽

Three Solutions ◽

Air Jet ◽

Room Ventilation ◽

Air Temperatures ◽

Gas Measurements

The purpose of this study is to investigate the air change efficiency of commonly used residential room ventilation units with tracer gas concentration decay method. Carbon dioxide was used as a tracer gas in both laboratory and field measurements. The performance of room ventilation units was compared to the conventional mixing ventilation. Therefore, the laboratory measurements were conducted with horizontal supply air jet from the overhead air diffuser on the fixed supply airflow rate with various supply air temperatures. The test results showed that nearly fully mixing ventilation was achieved. Furthermore, lower supply air temperature increased the air change efficiency. In next step, the air change efficiency of three room ventilation systems were measured in the test room. Tested systems included pair-wise units, monoblock unit and ventilation radiator, which was combined with mechanical extract ventilation. The measurements were carried out both during the heating period and outside the heating period. The results confirmed that all three solutions were capable of producing fully mixed ventilation. The air change efficiency was not affected by the variation of the air flow rate. Finally, tracer gas measurements were carried out in naturally ventilated apartments and the air change efficiency of these measurements were compared with the results of rooms based ventilation units.

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