Trace Gases in Soil

The movement of gases across the soil-atmosphere interface provides an important link to below-ground processes. The efflux of CO2 from soil to the atmosphere, for example, has been linked to biological processes in the soil, action of water on carbonates and the extent of contamination in soils polluted with petroleum hydrocarbons. One commonly used approach to measure gas fluxes across the soil-atmosphere interface is chamber based measurements, in particular dynamic or transient-closed chambers. This measurement technique relies on the change in a gas species' concentration over time in a known measurement volume. Typically, a sealed chamber is placed over an area of the soil surface and the gas in the chamber head space is circulated between an analyzer and the chamber. The rate of change of a particular gas species in the chamber is measured and then used to compute a flux.

This method has marked advantages over other flux measurement techniques, in that it provides a direct measurement of the flux, relies on simple assumptions, and has the ability to resolve extremely small fluxes. However, because the fluxes being measured are generally diffusive in nature, the chamber head space concentration can have an effect on the gradient driving the flux. Also, because soil is a porous medium, any perturbations in the chamber pressure relative to that of the soil or surrounding atmosphere may induce a bulk flow of gas. Ultimately, alteration of the gas concentration gradient and bulk flow of the measured species will result in errors in the measurement. A carefully designed chamber can avoid these problems.

The LI-8100A Automated Soil CO2 Flux System uses specially designed chambers that use automation to limit the amount of time the chamber is closed. This ensures that gas concentration gradients in the soil are minimally affected by the measurement, and can return to normal between measurements. It also ensures that at the start of each measurement the chamber head space is at ambient concentration. Additionally, these soil flux chambers employ a patented pressure vent which allows the pressure in the chamber head space to track ambient pressure. This vent is unique in that it eliminates the effects of wind seen when traditional pressure vent designs are used.

The 8100-664 Trace Gas Sampling Kit can be used to manually sub sample the chamber head space at various times throughout a flux measurement.

When used with the LI-8100A Analyzer Control Unit, these chambers allow for fast, precise measurements of soil CO2 flux. For other gas species (e.g. CH4, NOx ...), a septum can be placed in-line between the chamber and the LI-8100A Analyzer Control Unit, and used to manually sub sample the chamber head space at various times throughout a flux measurement (see 8100-664 Trace Gas Sampling Kit). Typically, samples are drawn into a gas-tight syringe and analyzed using a gas chromatograph after the fact. While this method is straightforward and allows for simple flux measurement of many gas species, it can be labor intensive and precludes unattended long term flux measurements.

The 8100-104 Long Term Chamber can be used to make unattended flux measurements.

The 8100-104 and 8100-104C long term chambers are designed to be left in the field for extended lengths of time to make unattended flux measurements. While originally designed to be used with the LI-8100A, these chambers can be operated independently, using a simple Transistor-transistor logic (TTL) controller and a 12 VDC supply. Used independently, these chambers can be combined with various gas analyzers that make real time measurements of many trace gases, allowing for long term unattended monitoring of trace gas fluxes. The integrated chamber auxiliary sensor interfaces (typically used for soil temperature and moisture probes), provide dedicated 0 to 5 VDC inputs for simple integration into custom trace gas flux systems.

8100-401 Chamber Control Kit

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LI-COR offers the 8100-401 Chamber Control Kit that allows users to operate the 8100-104 and 8100-104C chambers independent of the LI-8100A for measuring trace gases. This kit includes an interface cable, soil temperature and moisture sensors, tubing and the necessary connectors for integration with other gas analyzers.

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