measuring high co2 flux rates with the li-8100a … · however, at high flux sites such as those...

IntroductionDepending on soil organic matter content,temperature, moisture, nutrients, pH, etc., thetypical rates of CO2 emissions due to biogenicprocesses can range from about 0.5 to 10μmol CO2 m-2s-1. At geologically active sites(e.g. geothermal degassing sites) the rates ofCO2 emissions can be much higher - hundredsor even thousands of times higher. Similarly, atCO2 sequestration sites where CO2 is beingstored at high pressure deep below ground,large leaks to the atmosphere are a potentialproblem if geologic conditions are inappropri-ate for storage. In this note we discuss how touse the LI-8100A System to measure high ratesof soil CO2 emissions that might be encoun-tered at such locations, and describe a simplecollar adapter for increasing the maximumCO2 flux rates that can be measured with aclosed chamber system.

BasicsIn a closed chamber system such as theLI-8100A Automated Soil CO2 Flux System (fordetails about chamber measurements, seeWelles, et. al., 2001 or LI-COR ApplicationNote #124) soil CO2 flux is given by

where Fc is the soil CO2 flux rate (μmol m-2 s-1),V is volume (cm3), P0 is the initial pressure(kPa), S is soil surface area (cm2), T0 is initial airtemperature (°C), R is the Universal GasConstant, and dC/dt is the initial rate of changein CO2 mole fraction (μmol mol-1 s-1). In thisequation the dilution effect of water vapor onCO2 concentration are being ignored. For amore detailed discussion, refer to the LI-8100AAutomated Soil CO2 Flux System & LI-8150Multiplexer Instruction Manual, available onLI-COR’s website.

From this equation, it can be seen that for agiven soil CO2 flux, the initial rate of increaseof chamber CO2 concentration (dC/dt) dependson the ratio of chamber volume to soil area

Technical Note #136

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(V/S). A larger V/S ratio will lead to a slowerrise in chamber CO2 concentration. At veryhigh rates of ground CO2 emissions, CO2

concentrations in the chamber can quicklyover-range the CO2 gas analyzer. With theintroduction of the LI-8100A, the gas analyzeroperating range increased from a maximum of3,000 ppm to about 20,000 ppm CO2. The useof a double rectangular hyperbolic calibrationfunction enables the LI-8100A to retain thesame high sensitivity at ambient concentrationsfor measuring low biogenic rates of emissionwhile maintaining good accuracy at the highconcentrations (± 1.5% of reading). Underbiogenic conditions, and to avoid perturbingthe soil CO2 profile excessively, the chamberCO2 concentration during measurement istypically allowed to increase only slightlyabove ambient (~380 ppm). However, at highflux sites such as those mentioned above,chamber CO2 concentration can rise thousandsof ppm above ambient in less than a minute,and the higher operating range of the LI-8100Asystem will be required. From the flux equationabove, it can be estimated that with the olderLI-8100 System, which uses a polynomialcalibration function and has a CO2 measure-ment range of up to around 3,000 ppm, it ispossible to measure maximum CO2 fluxes ofabout 300 μmol m-2 s-1. Note that it is possibleto upgrade the LI-8100 analyzer to measure inthe same range as the LI-8100A (contactLI-COR for details).

Using the LI-8100A at a High CO2 FluxSiteFigure 1 shows a regression of chamber CO2concentration versus time. This measurementwas taken at The University of Montana ZeroEmission Research and Technology (ZERT) sitein July of 2010, immediately over a leak point,and two days after starting to inject CO2 into aperforated underground pipe. The ZERT sitewas established for testing leak detectiontechniques for use at CO2 sequestration sites –(see ZERT website for details:

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http://www.montana.edu/zert/). This measurement wastaken with the LI-8100A system, using the 8100-102 10cm diameter soil CO2 flux chamber. It can be seen thatchamber CO2 concentration exceeded the 20,000 ppmcalibration range of the gas analyzer after only about 50seconds from the chamber closing down over the in-stalled soil collar. Using the chamber CO2 concentrationdata between 20 to 50 seconds after closing, ground CO2flux was computed to be just over 1,900 μmol m-2s-1.Using non-linear regression, the soil CO2 concentrationnear the surface (Cx) was estimated to be about 140,000ppm (see LI-8100A Instruction Manual for more explana-tion). Pure CO2 was being injected through the perfo-rated test pipe fixture about 2 m below the surface.During this measurement, the atmospheric CO2 concen-tration (Co) a few centimeters above the collar was over2,000 ppm.

Figure 1. Example of time series of CO2 concentration in theLI-8100A System chamber headspace during measurementat a high leak rate location at the ZERT test site taken in July2010.

In this measurement we allowed an initial 20 secondDeadband for chamber volume mixing after closure overthe collar (see LI-8100A Instruction Manual) and another30 seconds of data collection (30 data points) for curvefitting to determine the initial rate of increase of chamberCO2 concentration (dC/dt) for calculating the soil CO2flux. For this analysis we did not include CO2 concentra-tions above 20,000 ppm, as the gas analyzer is notcalibrated above this point. Also, because of the veryhigh chamber concentrations involved, it is important toinstall the collars deep enough into the soil to preventCO2 from diffusing out around the edges of the collarwhen the chamber concentration increases above theCO2 concentration in the surface layer of the soil. A soilCO2 flux of about 2,000 μmol m-2s-1 is about the highest

rate of emission that can be reliably measured using astandard LI-8100A System, since a minimum of about 50seconds of data is required to allow for chamber volumemixing and collecting and an adequate number of datapoints for curve fitting. Fluxes higher than this wouldcause chamber CO2 concentrations to rise beyond theoperating range of the system CO2 gas analyzer before anadequate number of readings can be taken. In makingthis statement, we assume that there is an approximatelylinear increase in chamber CO2 concentration with time.As a first order approximation this is true, since the soilCO2 concentration (Cx) at high efflux sites is likely to bemuch higher than the maximum operating range of theLI-8100A gas analyzer.

Below we present a simple way to modify the chambervolume to soil area ratio for measuring CO2 fluxes thatare up to about 8,000 μmol m-2s-1; or about a thousandtimes larger than typical biogenic fluxes.

Modifying an LI-8100A System for MeasuringHigh CO2 Flux RatesUsing the closed chamber flux equation given above, wediscussed earlier how for a given soil CO2 flux, increas-ing the chamber volume or decreasing the soil areacould lower the rate of increase of chamber CO2 concen-tration (dC/dt) to provide more time for a flux measure-ment to be completed before the gas analyzer over-ranges. However, since the air flow available for mixingthe chamber volume is limited, we cannot simply in-crease the chamber volume or there will be inadequatechamber volume mixing. The air flow available from theLI-8100A Analyzer Control Unit is about 1.5 LPM. Withthe CO2 flux chamber attached directly to the LI-8100AAnalyzer Control Unit, employing a chamber with avolume much larger than the 20 cm diameter chamber(volume ~5,000 cm3) could result in inadequate mixing.Thus, without additional measures such as installing amixing fan inside the chamber, the chamber volumeshould not be increased significantly. The chamber soilinterface area, however, can be reduced. The photo inFigure 2 shows an adapter which makes it possible to usethe 8100-103 20 cm Survey Chamber on a 10 cmdiameter collar. This adapter reduces the soil area beingmeasured from about 315 cm2 down to about 80 cm2

providing a 4-fold increase in the chamber volume-to-area ratio and a corresponding 4-fold increase in themaximum CO2 flux that can be measured with theLI-8100A System. If an additional mixing fan were to beinstalled inside the chamber, or if using the LI-8150Multiplexer (which has a larger capacity pump forproviding an air flow of ~3 LPM), the chamber volumecould be increased further for measuring even higherground CO2 fluxes.

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Figure 2. Photo of the ZERT site at Montana State University,July 2010. The LI-8100A System with a 20 cm diameter soilCO2 flux chamber is being used with an adapter to fit on a10 cm diameter soil collar.

The collar adapter shown above in Figure 2 was madeusing a PVC pipe ‘reducer’ fitting found at hardware orhome improvement stores. This reducer fitting had aninternal diameter of 16 cm at the large end and a 10.7cm internal diameter at the smaller end. A short, 2.5 cmlength of the 20 cm diameter collar was glued onto thelarge end of the reducer using a circular plate cut out of asheet of plexiglass. A ring of foam gasket material wasglued inside the reducer funnel at the top end of the 10.7cm diameter section (below, and top right).

The total volume of this funnel-shaped adapter was justover 3000 cm3. This is about the same volume as a 20cm diameter collar sticking out 10 cm above the soil. Sothe overall volume is not larger than if using the 20 cmchamber on a 10 cm tall collar. The mixing in this adapter/chamber combination volume was barely sufficient. Ashort piece of tubing extending from the inlet port insidethe chamber dome, down towards the soil collar wouldhave improved the chamber volume mixing.

Figure 3 below shows what the CO2 concentration vs.time plot might look like if there is insufficient mixing ofthe chamber volume.

Figure 3. An example of inadequate chamber volumemixing; the time series of chamber CO2 concentration doesnot rise smoothly.

ConclusionsUsing the LI-8100A System (or an upgraded LI-8100) it ispossible to measure soil CO2 fluxes of up to about 2,000μmol m-2s-1. The total measurement time including a 20second Deadband should be kept to less than about 60seconds for measuring such a rate. The measurementchamber volume to soil area ratio can be modified tofurther increase the maximum CO2 flux that can bemeasured to about 8,000 μmol m-2s-1. Installing the soilcollars to adequate depth is important for preventingunder-estimation of the ground CO2 flux which can becaused by CO2 diffusing out of the chamber/collarsystem as the chamber CO2 concentration rises to highlevels.

ReferenceJ. M. Welles, T. H. Demetriades-Shah, D. K. McDermitt.Considerations for measuring ground CO2 effluxes withchambers. Chemical Geology 177 (2001) 3-13.

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In this top view, thefoam gasket is visible,and is glued under thestop-lip at the top of thenarrow end of thereducer. The plexiglassring is glued to the topof the reducer, and the2.5 cm length of the 20cm soil collar is gluedon top of the plexiglassring.

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LI-COR® is a registered trademark of LI-COR, Inc. The LI-8100A product line is covered by U.S. and foreign patents pending, andU.S. patents including U.S. 7,509,836; 7,568,374; 7,7448,253; and 7,856,899. Copyright 2012, LI-COR, Inc. Printed in the U.S.A.