Note: Descriptions are shown in the official language in which they were submitted.
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CA 02771628 2012-03-12
TITLE: SYSTEM AND METHOD FOR DETERMINING FLUX)OF ISOTOPOLOGUES
7-
FIELD
[0001] Exemplary embodiments described herein relate to systems and
method
for determining properties of soil efflux.
INTRODUCTION
[0002] The measurement of stable carbon isotopes has become a
critical tool in
elucidating the biological and environmental controls on many of the pathways
by which
CO2 can be produced and emitted from the soil.
SUMMARY
[0003] The embodiments described herein provide in one aspect a method for
determining flux of a component of a gas of interest, the method comprising:
placing a
first chamber having an open bottom sealably in contact with a soil location,
the first
chamber being in communication with the soil via an inlet membrane covering
the open
bottom and being in communication with atmosphere surrounding the first
chamber via
one or more outlet membranes; after allowing gas in the first chamber to reach
equilibrium, measuring a first concentration of a first isotopologue of the
gas of interest
within the first chamber and a first concentration of a second isotopologue of
the gas of
interest within the first chamber; placing a second chamber having a closed
bottom in a
vicinity of the first chamber, the second chamber being in communication with
the
surrounding atmosphere via one or more outlet membranes of the second chamber,
measuring the atmospheric concentration of the first isotopologue within the
second
chamber after allowing gas in the second chamber to reach equilibrium and
measuring
the atmospheric concentration of the second isotopologue within the second
chamber
after allowing gas in the second chamber to reach equilibrium; and determining
a flux of
the first isotopologue through the inlet membrane relative to a flux of the
second
isotopologue through the inlet membrane.
[0004] The embodiments described herein provide in another aspect a
system for
determining flux of a component of a gas of interest comprising:
a first chamber comprising chamber walls and a lid defining a first cavity
having a first
size and shape, the chamber walls also defining an opening for sealably
contacting the
cavity with a soil location, the first chamber walls further defining one or
more first outlet
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openings providing communication between the cavity and atmosphere surrounding
the
first chamber; the first chamber further comprising an inlet membrane covering
the
opening having an inlet membrane diffusivity and one or more first outlet
membranes
covering the one or more first outlet openings having a lower diffusivity than
the inlet
membrane diffusivity;
a second chamber comprising chamber walls and a lid defining a second cavity
having a
height and width substantially equal to the shape and size of the first
cavity, the
chamber walls further defining one or more second outlet openings being shaped
and
sized substantially equal to the one or more first outlet openings, the one or
more
second outlet openings providing communication between the second cavity and
atmosphere surrounding the second chamber, the second chamber further
comprising
one or more second outlet membranes covering the one or more second outlet
openings having a diffusivity substantially equal to the diffusivity of the
one or more first
outlet membranes;
and
one or more measuring devices for measuring a first concentration of a first
isotopologue of the gas of interest within the first cavity, a first
concentration of a second
isotopologue of the gas of interest within the cavity, an atmospheric
concentration of the
first isotopologue of the gas of interest within the second cavity and an
atmospheric
concentration of the second isotopologue of the gas of interest within the
second cavity.
[0005] Further aspects and advantages of the embodiments described
will appear
from the following description taken together with the accompanying drawings.
DRAWINGS
[0006] These and other features of exemplary embodiments will become
more
apparent from the following in which reference is made to the appended
drawings
wherein:
[0007] FIG. 1 is a perspective view of an isotopic forced diffusion
chamber;
[0008] FIG. 2 is a section view of the isotopic forced diffusion
chamber;
[0009] FIG. 3 is a perspective view of an isotopic forced diffusion
chamber
connected to a measuring device;
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[0010] FIG. 4 is a perspective view of an isotopic forced diffusion
chamber
connected to a measuring device;
[0011] FIG. 5 is a perspective view an isotopic forced diffusion
chamber and a
reference chamber connected to a measuring device;
[0013] FIG. 7 is a graph showing the absolute probable error in
calculated
isotopic flux value.
[0014] FIGs. 8a-8d are graphs showing the observe decal in
isotopologues.
[0015] FIG. 9 is a graph of iso-FD measured isotopic flux values.
DESCRIPTION OF VARIOUS EMBODIMENTS
[0016] When measuring soil gas efflux and its stable isotopic
signature, chamber
methods, such as those using static or dynamic chambers, may drive a
potentially large
bias because of non-steady state diffusion processes. These biases are likely
to co-vary
with environmental conditions, thereby confounding the interpretation of
results further.
Such methodological biases have been documented and some solutions have been
offered, such as modification of conventional chamber designs to minimize the
bias, and
model fitting of the chamber to remove bias artifacts. Although each of these
approaches is likely to offer data that is somewhat more reliable, it is
desired to have
ways for measurement that have a lower risk of bias.
herein described for use in the measurement of isotopic fluxes. Steady state
diffusion
based chamber design seeks to address the issue of bias in the measurement of
isotopic fluxes.
[0018] Exemplary embodiments described herein refer to the measurement
of
gas fluxes and isotopic fluxes with reference to CO2, 12CO2 and 13CO2, such
reference is
by way of example only. It will be understood that systems and method
described may
be applied to other gases of interest.
Chamber design
[0019] Referring to Figure 1, therein illustrated is a perspective
view of an
exemplary isotopic forced diffusion chamber 2 (iso-FD chamber). Figure 2
illustrates a
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section view of the exemplary isotopic forced diffusion chamber 2 along the
line I-I. The
iso-FD chamber 2 is shown as placed in a field location upon the soil 4. The
iso-FD
chamber 2 comprises chamber walls 6 which define a cavity 8. The chamber walls
are
formed of a non-permeable or low-permeability material. In some exemplary
embodiments, the chamber walls may be formed of polyvinyl chloride (PVC).
[0020] A top portion of the cavity 8 is sealed by a lid 10 formed of a
non-
permeable or low-permeability material such that communication between the
cavity 8
and the atmosphere surrounding the iso-FD chamber 2 is substantially
restricted or
prevented. In the embodiments where the chamber walls 6 are formed of PVC, the
lid
10 may be a PVC plug. Alternatively the lid 10 may be integrally formed with
the
chamber walls 6.
[0021] In some exemplary embodiments, lid 10 may be selectively opened
to
allow communication of the cavity 8 with the surrounding atmosphere 12 through
the top
portion of iso-FD chamber 2. It may be desirable to open the lid 10 to allow
vegetation
within the cavity 8 to be exposed to elements of nature, such as air, water,
and sunshine
between taking of measurements of isotopic concentrations in the cavity 8.
Thus, a lid-
opening mechanism may be attached to the lid 10 to allow automatic control of
the
opening and closing of lid 10.
[0022] The chamber walls 6 define a bottom opening 14. The iso-FD
chamber 2
may be placed such that the bottom opening 14 is sealably in contact with the
soil.
When so placed, the cavity 8 is in communication with the soil 4 through the
bottom
opening 14.
[0023] In some exemplary embodiments, the bottom opening 14 may be
covered
by an inlet membrane 16 having a known diffusivity for different gases and
various
isotopologues of each of the gases. Accordingly, the cavity 8 is in
communication with
the soil 4 through the inlet membrane 16 such that soil gas permeating through
the inlet
membrane 16 is diffused through the cavity 8. In some embodiments, the inlet
membrane 16 is formed of UV resistant TyvekTm material.
[0024] The chamber walls 6 further define one or more outlet openings
18, which
are each covered by outlet membranes 20 having known diffusivities for
different gases
and various isotopolgues for each of the gases. Accordingly, the cavity 8 is
in
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communication with the surrounding atmosphere 12 through the outlet membranes
20
covering the outlet openings 18.
[0025] After a period of time, as gases permeate from the soil 4 into
the cavity 8
through the inlet membrane 16 being diffused by the inlet membrane 16, and as
they
permeate to and from the cavity 8 and the surrounding atmosphere 12, diffusion
of
gases in the cavity 8 will reach a steady-state, or equilibrium. When in this
diffusive
steady state, the concentration of a particular gas of interest, such as CO2
and its
isotopologues will be measurable. As will be appreciated, the time for
reaching the
steady state is a function of the size, shape and diffusivity of the inlet
membrane 16 and
the size, shape and diffusivity of the outlet membranes 20.
[0026] Described below, a critical factor in the accuracy of
determination of flux of
isotopologues is the ratio between the 12CO2 concentration in the lso-FD
chamber 2,
and concentration in the atmosphere (Chamber/Atmosphere; C2/C1). There is also
a
smaller error that is induced by difference between the isotopic signature in
the iso-FD
chamber 2 and the atmosphere (Chamber/Atmosphere; 62/61), as this difference
becomes small the error is minimized. For example, figure 7 shows the absolute
probable error in the calculated isotopic flux value for C2/C1 and 62-431 with
1% and 5%
uncertainty in measured concentration values and 0.5%0 and 1.0%0 uncertainty
in
measured isotopic signatures (i.e. analytical uncertainty).
[0027] Accordingly, the size, shape, and diffusivity of inlet membrane 16
of the
iso-FD chamber 2 are selected such that concentration of isotopologues of the
gas of
interest in the cavity 8 can be built up to be within a preferred range. To
decrease
uncertainties and obtain accurate determination of isotopic flux, it is
preferable that
when the gases in the cavity 8 reach a diffusive steady state there will be a
high
concentration of the isotopologues of the gas of interest to be measured in
relation to
the concentration of the same isotopologues of the gas of interest in the
atmosphere.
However, care should be taken such that the concentration of isotopologues of
the gas
of interest in the cavity 8 is not so high that isotopologues of the gas of
interest cannot
naturally flow from the soil ground 4 into the cavity 8. For example, too a
high
concentration in the cavity 8 may lead to drastically increased subsurface CO2
concentrations below the chamber and may further cause shifts in the biology
near the
location of the iso-FD chamber 2.
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[0028] Preferably, the diffusivity of the inlet membrane 16 is greater
than the
diffusivity of the outlet membranes 20 to allow buildup of a sufficiently high
concentration
of CO2 within the cavity 8 so that isotopic measurements of different isotopic
concentrations may be made with minimal error. For example, the outlet
membranes
may be formed of GORE-TEXTm, which has a lower diffusivity than TyvekTm.
Similarly, it
is preferable to size and shape the outlet openings 18 to allow a build of a
sufficiently
high concentration of CO2 within the cavity 8.
[0029] For example, according to one embodiment, the cavity 8 defined
by the
chamber walls 6 has a diameter of approximately 5 cm and a length of 8 cm. Two
approximately 10cm2 outlet openings 18 are located in the chamber walls 6 to
oppose
each other. In some embodiments, outlet opening 18 may extend
circumferentially
around the chamber walls 6 at a height above the soil 4, depending on the size
of the
chamber to achieve a preferential concentration of isotopologues of the gas
interest in
the cavity 8.
[0030] A suitable measuring device 30 is used to measure concentration of
various isotopologues of the gas of interest, such as CO2, 12CO2 or 13CO2,
within the
cavity 8 once gases in the cavity 8 have reached a diffusive steady-state.
According to
some exemplary embodiments, the lid 10 may comprise an outlet port 22 that
allows the
drawing of gases found within the cavity 8 for measurement of concentration of
isotopologues of the gas of interest. For example, outlet port 22 may be
attached to an
outlet tube 24 that is connected to an input port 32 of a measuring device 30.
[0031] One type of measuring device suitable for measuring
concentration of
isotopologues of the gas of interest is a cavity ring down spectrometer. In
such cases,
the outlet port 22 may be connected to an input of the cavity ring down
spectrometer.
For example, a PicarroTMs G1101-I CRDSTM analyzer may be used.
[0032] In some exemplary embodiments, the Iso-FD chamber may further
comprise an inlet port 26, which allows for the insertion of air into the
cavity 8. Such
insertion of air may be useful for maintaining a substantially constant
pressure within the
cavity 8 to avoid biases in isotopologues of the gas of interest concentration
measurements caused by fluctuations in pressure.
[0033] Referring to Figure 3, therein illustrated is a perspective
view of the iso-FD
chamber 2 being connected to a measuring device 30 such as a cavity ring down
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spectrometer. In this exemplary embodiment, the outlet port 22 of the iso-FD
chamber 2
is connected to an input 32 of the measuring device 30 by outlet tube 24.
Inlet port 26 of
the iso-FD chamber 2 is further connected to an inlet tube 28. One end of the
inlet tube
28 is open and in communication with the atmosphere 12 surrounding the iso-FD
chamber 2. The inlet tube 28 has a length that such that there is a
significant
concentration gradient over its length. Therefore diffusion of surrounding
atmosphere 12
through the inlet tube 28 does not substantially affect concentration of soil
gas in the
cavity 8. However, when there is a change of pressure in the cavity 8, such as
when
cavity gases are drawn for measurement, the change in pressure in the cavity 8
causes
air from the surrounding atmosphere 12 to be drawn into the cavity 8, thereby
maintaining constant pressure.
[0034] Referring now to Figure 4, therein illustrated is a schematic
view of an
exemplary embodiment the iso-FD chamber 2 having the outlet port 22 being
connected
to an input port 32 of the measuring device 30 via the outlet tube 24. The
inlet port 26 of
the iso-FD chamber 2 is further connected to an output port 33 of the
measuring device
via the inlet tube 28. The measuring device 30, such as a spectrometer, is
configured to
circulate gas drawn from the cavity 8 through the outlet port 22 and input
port 32 back
into the cavity 8 through output port 33 of the measuring device 30 and inlet
port 26.
Preferably the drawing of gas from the cavity 8 and the reinsertion of gas
back into the
cavity 8 should be done continuously or almost continuously such that any gas
drawn
from the cavity 8 by the measuring device 30 is quickly reinserted back into
the cavity 8.
The continuous drawing of gas and reinsertion of gas creates a continuous flow
loop of
gas having a defined flow rate from cavity 8 to the measuring device 30, and
back to the
cavity 8. The continuous drawing and reinsertion of gas restricts disruption
of gas
pressure in the cavity 8 because any gas drawn from the cavity 8 by the
measuring
device 30 is offset by gas reinserted by the measuring device 30. As gas is
drawn from
the cavity 8, and before the gas is reinserted into the cavity 8, the
measuring device $0
can measure the concentration of isotopologues of the gas of interest within
the gas that
is drawn. The flow rate of the continuous flow loop should be sufficiently
high such that
sufficient gas is drawn to obtain accurate measurements of isotopologues of
the gas of
interest, However, the flow rate should not be so high that the measuring
device is
unable to obtain accurate measurements or be above a maximum flow rate for
which
the measuring device 30 is capable of operating.
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[0035]
According to some exemplary embodiments, the iso-FD chamber 2
comprises a sampling port for receiving a sampling canister. Preferably, the
sampling
port is located on the lid 10 of the iso-FD chamber 2. The sampling port may
comprise a
valve to selectively open or close the port depending on whether a sampling
canister is
coupled to it. When a sampling canister, such as a gas canister or vial, is
coupled to the
sampling port, the valve may be opened to allow gas of the cavity 8 to enter
the
sampling canister. The sampling canister may then be brought for measurement
and
analysis. It will be appreciated that use of the canister in this way allows
the sampling
canister to be brought off-site from the location of the iso-FD chamber 2. For
example,
the sampling canister may be brought to a laboratory for in-depth analysis.
According to
such embodiments, either one or both of the inlet port 26 and outlet port 22
may be
omitted from the iso-FD chamber 2. However, alternatively, it is possible for
the iso-FD
chamber 2 to have the sampling port in addition to the inlet port 26 and
outlet port 22
such that an operator may choose between obtaining cavity gas measurements
using a
measuring device connected to the outlet port 22 or obtaining measurements
using
sampling canister attached to the sampling port.
[0036]
According to some exemplary embodiments, the sampling canister may be
a molecular sieve sampling can that is semi-automated using control hardware
such as
a flow measurement and control device. The control hardware may further
include a
pump with concentration sensors. The control hardware provides quality
assurance in
verifying that a sample has been drawn correctly and in large enough volume.
The
control hardware may also set the sample to be drawn at pre-selected time
intervals. As
the sampling port and sampling canister allow the gases in the cavity 8 to
maintain
equilibrium, samples can be repeatedly taken without having to reconfigure the
chamber.
[0037]
While some examples of the measuring device 30 have been provided, it
is contemplated that other measuring devices may be used to measure
concentration of
isotopologues of the gas of interest in the cavity 8. For example, it is
contemplated that
in-cavity measuring devices may become available for easier measurements. Such
devices are intended to be covered by the present description.
[0038] In
some embodiments, the steady-state chamber system comprises at
least one iso-FD chamber 2 and a reference chamber 50. The reference chamber
50 is
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used to measure concentration of isotopologues of the gas of interest in the
atmosphere. This measurement is then used to determine the relative flux of
isotopologues of the gas of interest. The reference chamber 50 is designed to
be similar
to the iso-FD chamber 2. In particular, the reference chamber 50 also
comprises
chamber walls that define a cavity having a diameter and length that is
substantially
equal to the diameter and length of the cavity 8 of the iso-FD chamber 2. The
chamber
walls of the reference chamber 50 are preferably also formed of the same
material as
the chamber walls 6 of the iso-FD chamber 2. A top portion of the cavity of
the reference
chamber 50 is also covered by a lid to seal the top portion of the cavity.
Moreover the
chamber walls of the reference chamber 50 also define one or more outlet
openings that
are each covered by outlet membranes. Preferably, the reference chamber 50 has
the
same number of outlet openings as the iso-FD chamber 2 and each of the outlet
openings have the same shape and size as the outlet openings 18 of the iso-FD
chamber 2.
[0039] Each of the outlet openings of the reference chamber 60 are covered
by
outlet membranes having a diffusivity substantially equal to the diffusivity
of the outlet
membranes 20 of the iso-FD chamber 2. However, importantly, the bottom opening
of
the cavity defined by the chamber walls of the reference chamber 50 is sealed
with a
non-permeable material, such that soil gases do not flow from the soil 4 into
the cavity 8.
By having a reference chamber 50 that has a similar configuration to the iso-
FD
chamber it is possible to closely monitor changes in the concentration of CO2
in the
atmosphere and correct for such changes in the flux determinations. Exemplary
embodiments of the iso-FD chamber 2 described herein may also be applied to
the
reference chamber 50 where appropriate.
[0040] Referring to Figure 6, therein illustrated is a section view of the
iso-FD
chamber 2 and the reference chamber 50 being integrally formed according to
some
exemplary embodiments. Chamber walls 6 may be made to be approximately twice a
high to define an enlarged cavity. The enlarged cavity is divided by lateral
wall 56, which
divides the enlarge cavity into soil sub-cavity 8a and reference sub-cavity
8b. Soil sub-
cavity 8a is in communication with the soil 4 through bottom opening 14 being
covered
by inlet membrane 16. Chamber walls 6 further define outlet openings 18 being
covered
by outlet membranes 20 to provide communication between the soil sub-cavity 8a
with
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the surrounding atmosphere 12. Reference sub-cavity 8b, being closed by
lateral wall
56, is not in communication with soil 4. However, reference sub-cavity 8b is
communication with the surrounding atmosphere 12 via reference sub-cavity
recesses
58 defined on an inside surface of the chamber walls 6. The reference sub-
cavity
recesses connect to bores 60 drilled in the chamber walls 6. The bores 60
extend
downwardly towards soil 4 to connect with recesses 62 defined on an outside
surface of
the chamber walls 6. The recesses 62 are further covered by outlet membranes
20.
Therefore, sub-cavity 8b communicates with the surrounding atmosphere 12 via
reference sub-cavity openings 58, bores 60 and recesses 62. Top openings 64 of
the
bores 60 are plugged to restrict communication of the reference sub-cavity 8b
with the
surrounding atmosphere through the top openings 64. The recesses 62 are
located at a
height above the ground approximately equal to the height of the outlet
openings 18.
Therefore, soil sub-cavity 8a, and reference sub-cavity 8b communicate with
the
surrounding atmosphere 12 at approximately the same points in space, thereby
decreasing errors caused by lateral and vertical deviations in concentrations
of the
isotopologues of the gas of interest in the surrounding atmosphere 12.
Measurements of
concentrations of isotopologues of interest in the soil gas is obtained by
measurement of
concentrations of gas in the soil sub-cavity 8a. Reference concentrations of
isotopologues of interest in the atmosphere is obtained by measurement of
concentrations of gas in the reference sub-cavity 8b.
[0041] Measurements of concentration of isotopic fluxes in the cavity
8 of the
reference chamber 50 may be carried out with the measurement device according
to
any of embodiments provided above with reference to the iso-FD chamber 2. To
obtain
accurate measurements and minimize errors caused by lateral deviations in
concentration of isotopologues in the surrounding atmosphere 12, the reference
chamber is preferably placed as close as possible to the iso-FD chamber 2.
Furthermore, in some embodiments, the iso-FD chamber 2 is connected to a first
measuring device 30 while the reference chamber 50 is connected to a second
measuring device 30 that is separate from the first measuring device. However,
a single
measuring device 30 may be used to measure concentrations of isotopologues of
the
gas of interest in both the iso-FD chamber 2 and the reference chamber 50.
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[0042] Referring now to Figure 5, therein illustrated is an exemplary
embodiment
of the iso-FD chamber system 40 comprising a valving system for taking bulk
measurements of concentration of isotopologues of the gas of interest in a
plurality of
chambers. For example, iso-FD chamber system 40 comprises one iso-FD chamber 2
and a reference chamber 50 positioned in proximity to one another. Cavity 8 of
the iso-
FD chamber 2 sealably contacts the soil 4 such that soil gases permeate into
cavity 8
through inlet membrane 16. Inlet port 26 is connected via inlet tube 28 to a
valving
system 52. Inlet port 64 of reference chamber 50 is connected via inlet tube
56 to the
valving system 52. The valving system 52 is further connected to an input port
32 of the
measuring device 30 via connecting tube 58. The valving system 52 comprises a
plurality of valves and interconnecting tubes.
[0043] The valving system 52 allows measurements of isotopologues of
the gas
of interest found in a plurality of chambers to be taken using a measuring
device 30
having a single input port, such as a spectrometer PicarroTMs G1101-i CRDSTM
analyzer is factory equipped with a single inlet port. The valving system 52
may
comprise eight EV-2M two-way valves connected to a gas tight manifold. Two of
these
valves are dedicated to standard gases, while the other 6 are free to collect
samples.
The valves are fired using a PhidgetInterfaceKit 0/0/8 electronic relay
(Phidgets Inc.,
Calgary, Alberta), which is connected to a PicarroTM G1101-i CRDSTM and is
commanded by a controller. Accordingly, the controller can selectively control
the
valving system to connect the input port 32 of the measuring device 30 with
the outlet
ports of either the iso-FD chamber 2 or the reference chamber 50.
[0044] The controller may also control the lid opening mechanism of
lid 10 to
selectively open the lid after taking a measurement of concentration of
isotopologues
and to selectively close the lid when another measurement of concentration of
isotopologues is to be taken. The controller may further be configured to
receive
measurements of concentrations of isotopologues taken by the measuring device
and to
determine the flux of one isotopologue of the gas of interest relative to the
flux of
another isotopologues of the same gas of interest. Furthermore, the controller
may
further be configured to control actions carried out by the measuring device
30 such as
the taking of measurements of concentrations of isotopologues, the drawing of
gas for
measurement, the reinsertion of gas into the cavity 8, and the control of
sample port
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valve in some embodiments. The controller may further be coupled to a display
unit and
data input device, such as keyboard or mouse, for entering various parameters,
such as
membrane diffusivities.
[0045]
The controller described herein may be implemented in hardware or
software, or a combination of both. It may be implemented on a programmable
processing device, such as a microprocessor or microcontroller, Central
Processing Unit
(CPU), Digital Signal Processor (DSP), Field Programmable Gate Array (FPGA),
general purpose processor, and the like. In some embodiments, the programmable
processing device can be coupled to program memory, which stores instructions
used to
program the programmable processing device to execute the controller. The
program
memory can include non-transitory storage media, both volatile and non-
volatile,
including but not limited to, random access memory (RAM), dynamic random
access
memory (DRAM), static random access memory (SRAM), read-only memory (ROM),
programmable read-only memory (PROM), erasable programmable read-only memory
(EPROM), electrically erasable programmable read-only memory (EEPROM), flash
memory, magnetic media, and optical media. The controller may be implemented
within
the measurement device 30.
[0046]
According to some exemplary embodiments, the controller may be
implemented as a module within the measurement device. Alternatively, the
controller
may be implemented independently of the measuring device 30, but may be in
communication with the one or more measuring devices 30. In such embodiments,
some control steps may be implemented using the independently implemented
controller while other control steps herein described are implemented by the
measuring
device 30.
[0047] In
some exemplary embodiments, the iso-FD chamber system may
comprise any number of iso-FD chambers 2 to take measurements of isotopic soil
flux
at multiple locations. Each outlet port 22 of the iso-FD chambers 2 can be
connected to
a valving system 52, which is further connected to the input ports 32 of one
or more
measuring devices 30. According to such embodiments, it is possible to
automatically
take bulk measurements of concentration of isotopologues of the gas of
interest in each
of the multiple iso-FD chambers 2 and subsequently calculate the relative gas
flux at
multiple soil locations. In some exemplary embodiments, multiple iso-FD
chambers 2
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may be used in order to simultaneously determine isotopic flux of a gas of
interest at
multiple soil locations. It may be sufficient to have one reference chamber 60
to obtain
reference measurements of concentration of isotopologues of the gas of
interest in the
atmosphere. However, where the iso-FD chambers 2 are sufficiently spread out
in area
over various soil locations, multiple reference chambers 50 may be used. Each
iso-FD
chamber 2 is then associated with one of the reference chambers 60 that is in
its
vicinity. It will be understood that more than one iso-FD chambers 2 may be
associated
with a single reference chamber 60. Depending on the total number of chambers,
one or
more measuring devices 30 may be used to measure concentration of
isotopologues of
a gas of interest in each of the chambers. For example, two or more chambers
may be
connected to one measuring device 30 via the valving system 52.
Advantageously, iso-
FD chambers 2 that are associated with the same reference chamber 50 are
connected
via valving system 62 to the same measuring device 30 with the reference
chamber 60.
The controller may be configured to selectively control the valves of the
valving system
62 to allow the measuring device 30 to sequentially measure concentration of
isotopologues of the gas of interest. For example, the measuring device 30 and
the
valving system 52 may be controlled so that measurement of concentration of
isotopologues in one iso-FD chamber 2 and measurement of concentration of
isotopologues in the associated reference chamber 50 are taken within a short
time of
each other. Preferably, measurements should be taken at substantially the same
time.
Alternatively, where an iso-FD chamber 2 and its associated reference chamber
50 are
connected to separate measuring devices, the measuring devices 30 may be
controlled
to take measurements of concentration of isotopologues in the iso-FD chamber 2
and
measurement of concentration of isotopologues in the associated reference
chamber 50
at substantially the same time.
[0048] In some exemplary embodiments, the iso-FD chamber 2 may further
comprise a sensor for measuring concentration of a bulk gas of interest, such
as CO2,
without specifically measuring the isotopologues of that gas. Accordingly, a
measuring
device 30 may still be used to measure the concentration of isotopologues of
the gas of
interest. However, it is contemplated that the same sensor may be used to
measure
both the concentration of the bulk gas as well as isotopologues of that gas.
Measurements of the concentration of the bulk gas may be useful for monitoring
changes in carbon balance of gases in the iso-FD chamber 2. According to such
-13-
CA 02771628 2012-03-12
embodiments, the iso-FD chamber 2 may be configured, for example, by varying
the
size of the cavity walls, size of outlet openings, and outlet membrane
diffusivity, such
that a lower concentration of soil gas is found in the cavity 8 when diffusive
equilibrium
reached. Because more significant errors are introduced for measurements of
concentration of bulk gas when there is a higher concentration of that gas, it
may be
desirable when measuring both bulk gas and isotopologues of that gas to have a
concentration of soil gas that is lower than the concentration of soil gas
when only
concentrations of isotopologues are to be measured. However, the concentration
of the
soil gas is still maintained to be sufficiently high to obtain accurate
measurements of
concentrations of isotopologues.
Theory
[0049] While not being bound in any way, the inventors propose the
following
theoretical basis. The theory presented herein relates to CO2 and its
isotopologues,
however it will be understood that such theory can be applied to any other
gases and
isotopologues of such gases. For the iso-FD chamber 2, the mass balance for
bulk CO2
measurements may be calculated as:
(1)
where V is the cavity 8 volume, C is concentration, t is time, AB is the area
of the inlet
membrane in contact with the soil surface, Fo is the flux into the cavity 8
and AT is the
area of the outlet membranes 20 in contact with the atmosphere, Fout of the
chamber
can be thought of as the diffusive gradient across the membrane from the
concentration
in the chamber C(t) to the concentration in the atmosphere Catm, which is
dependent on
both the path length of diffusion (L) and the diffusivity of the membrane
material (D), as
per Fick's Law. With these substitutions for Four, the equation can be
modified to:
V = A BF,õ - AT -L(C(t)- cilm) (2)
[0050] Since measurements are taken when the diffusion of gases in and
out of
the iso-FD chamber 2 is at equilibrium, it is approximated that diffusion
through the
chamber has reached steady-state. Equation may be reduced to:
AT D
(CFD- cm)(3)
- 14 -
CA 02771628 2012-03-12
[0051] Equation 3 describes the soil flux, where the atmospheric CO2
concentration, Catm is subtracted from the CO2 concentration in the chamber,
CFD. Catm is
measured in the reference chamber 50 having a non-permeable bottom.
[0052] In the case of isotopic flux, each of the carbon isotopologues
of CO2 is
treated as separate diffusing gases, which allows for the writing of similar
equations for
both 12CO2 and 13CO2. By taking the ratio of the fluxes of each isotopologues
the
isotopic composition of soil flux is gained:
AT D''''
(C FD C atmC
AB L )
Fini3
(4)
/F' C AT DI2C 12c I2c
________________________________ (C FD ¨ C aim)
A13 L
which can be simplified based on the understanding that 1) the area variables
will
cancel because the same chamber is used for each isotope, and 2) the path
length (L)
and diffusivity will reduce to the reciprocal of diffusion fractionation
(1.0044) yielding the
final lso-FD solution:
[0053]Fin" c/2 = I/ (C lic lic
FD C) (5)
In c A .0044 (c'F21' _ ca':<;')
[0054] Equation 5 can be converted to del-notation for more convenient
use.
[0055] As described above Isotopologues concentration within the cavity 8
of 'so-
FD chamber 2 may be measured using a measuring device such as a cavity ring
down
spectrometer connected to outlet port 22 of the lso-FD chamber 2 via outlet
tube 24. As
described above, according to some embodiments, the chamber gas can be
continuous
flow loops to allow gas to be recirculated to maintain the steady-state
concentration
within the chamber (as in Equation 2)
[0056] According to some other embodiments, gas can be drawn from the
cavity
8 while volume replacement via the inlet tube 28. According to such
embodiments,
maintenance of pressure in the measurement cell of the measuring device relies
on a
difference between inflow and outflow rates and because of this recirculation
would
cause undesired pressure changes in the Iso-FD chamber and likely lead to
biased
results because of over/under pressurization in the cavity 8. Therefore,
according to
embodiments where air is drawn from the cavity 8 of the iso-FD chamber 2 and
air of
- 15-
CA 02771628 2012-03-12
the surrounding atmosphere 12 flows in to the cavity 8 via an inlet tube 28
and replace
air drawn by the measuring device, the original mass balance equation (Eq. 2)
should
be modified to:
V = A 13Fm
ot ¨ A7 ¨D (C(t)¨ C air,)+F(C ,õõ,¨C(t))
(6)
where F is the measuring device pump draw rate (m3/s). To determine what
effect the
pump draw and atmospheric air dilution will have on the final calculation of
the isotopic
flux Equation 6 is solved analytically assuming that Fin is constant. As the
analytically
modeled chamber air is drawn by the measuring device the concentration decays
until it
reaches an equilibrium value between the incoming atmospheric air (modeled at
380
ppm) and the soil flux rate into the chamber. Further analysis shows that for
a fixed total
pumping time, the difference between the isotopic flux determined using the
steady-
state lso-FD and the pump drawn Iso-FD is always a constant value, regardless
of the
Fin flux rate or the isotopic signature of the flux. This allows for the
determination of an
offset value for a given design of the iso-FD chamber 2 and applies it to the
lso-FD
solution (Eq. 5) to correct it for the pump effects.
Numerical Modeling
[0057] To ensure that the lso-FD chambers do not suffer from any of
the lateral
diffusion artifacts present in other chamber systems, the chambers were
modeled using
a three-dimensional soil-atmosphere-chamber model. This new model has cubic
grid
geometry, making it more flexible to use both for varying soil properties and
varying
chamber sizes and geometries. In brief, the model transports gas between its
six
nearest-neighbor cells using Fick's Law:
AC1,2
FI,2 DI 2 (7)
A(i,/, 4,2
where F is the flux between cells, D is the intercell diffusivity constant, AC
is the
difference in the cell gas concentrations and A(i,j,k) is the three-
dimensional difference
in cell positions. After each time step, the concentrations in each cell are
re-calculated
taking into account relevant fluxes during the last time step. To this end,
steady state
chamber concentrations and isotopic signatures for diffusivities and
production rates
spanning three orders of magnitude (Dõii: 1x10-8 ¨ 1x10-6 m2 s-1; Production
(P): 0.1 ¨
- 16 -
CA 02771628 2012-03-12
mol m-2 s'1) were simulated , as well as several values for the diffusivity of
the Iso-
FD chamber membrane (D term in Equation 3). The modeled and actual chamber had
similar surface areas (Modeled: AT=AB=16 cm2 Actual: AT=20 cm2, A8=19cm2)
although
the volume of the modeled system was about half for computational reasons
(Modeled
5 V=64 cm3 Actual V= 133 cm3); however, this difference in V does not affect
the final
model results. For each combination of parameters (Dsw, P, D) the true
isotopic flux of
the modeled soil was compared to the flux estimated using the modeled Iso-FD
chamber to estimate potential bias under the various conditions. It will be
appreciated
that dimensions of the system presented herein are given as examples only.
Especially,
10 where modeling is used to validate the system design, dimensions used for
the
modeling relate to only one embodiment, but other dimensions are contemplated.
Method
[0058] According to exemplary embodiments of a method for determining
flux of a
component of a soil gas in application of the theory described above, the iso-
FD
chamber 2 is placed at a soil location. When so placed, the bottom opening 14
is
sealably in contact such that the cavity 8 is in communication with the soil 4
via the inlet
membrane 16 covering the bottom opening. A collar having a first end inserted
into the
soil and a second end in contact with the bottom portion of the chamber walls
6 may be
used to aid in the sealing of the bottom opening 14 to the soil 4. When so
placed, the
cavity 8 is further in communication with the surrounding atmosphere 12 via
the one or
more outlet membranes 20 covering the one or more outlet openings defined by
the
chamber walls 6.
[0059] A reference chamber 50 may also be placed at a soil location in
order to
obtain reference measurements of the surrounding atmosphere 12. Preferably,
the
reference chamber 50 is placed in the vicinity of the location of the first
chamber 2 in
order to obtain accurate reference measurements. Placing the reference chamber
50 in
the vicinity of the iso-FD chamber 2 decreases errors that may be introduced
due to
horizontal differences in concentration of isotoplogues. When so placed, the
cavity of
the reference chamber 50 is in communication with the surrounding atmosphere
12 via
one or more outlet membranes of the reference chamber 50 covering the one or
more
outlet openings defined by the chamber walls.
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CA 02771628 2012-03-12
[0060] The iso-FD chamber 2 is placed for a waiting time Ati to allow
soil gases
permeating and diffusing into the cavity 8 and gases 8 diffusing in and out of
the cavity 8
through the one or more outlet membranes 20 to reach steady-state or
equilibrium. For
example it is possible to estimate the time needed for gas to reach
equilibrium based on
the known diffusivity of the outlet membranes 20, the known diffusivity of the
inlet
membranes, or a combination thereof. Modeling and validation in may be further
used to
verify the estimated time to equilibrium.
[0061] The reference chamber 50 may also be placed for a waiting time
At2 to
allow soil gases permeating and diffusing in and out of the cavity 8 through
the one or
more outlet membranes to reach steady-state or equilibrium. Where the outlet
openings
18 of the iso-FD chamber 2 and the outlet openings of the reference chamber
are equal
in quantity and have substantially the same shapes and sizes, and the outlet
membranes covering the openings have the same diffusivity, the waiting time
At2 for the
iso-FD chamber 2 and the waiting time At2 for the reference chamber may be
approximately equal.
[0062] After allowing gas in the iso-FD chamber 2 to reach
equilibrium, a
measurement is taken using the measuring device of the concentration of a
first
isotopologue of the gas of interest in the cavity 8 of the iso-FD chamber 2
and a
measurement is taken of the concentration of a second isotopologue of the gas
of
interest in the cavity 8 of the iso-FD chamber 2. Preferably, the measurement
of the first
isotopologue and the second isotopolgue are measured at the same time, or as
closely
in time to each other as possible to minimize the time elapsed between
measurements
that could affect the accuracy of the formula used for determination of the
relative flux.
In some embodiments where flux of CO2 gas is to be determined, the first
isotopologue
of the gas of interest may be 12002, and the second isotopologue of the gas of
interest
may be 13CO2, however measurements of any other isotopologue of the gas of
interest
may be taken.
[0063] Atmospheric concentrations of the first and second
isotopologues of the
gas of interest may be measured using the reference chamber 50. After allowing
gas in
the reference chamber 50 to reach equilibrium, a measurement is taken using
the
measuring device of the concentration of the first isotopologue of the gas of
interest in
the cavity of the reference chamber and a measurement is taken of the
concentration
-18-
CA 02771628 2012-03-12
the second isotopologue of the gas of interest in the cavity of the reference
chamber 50.
Since the reference chamber 50 has a closed bottom and therefore does not
receive a
substantial amount of soil gases, these measurements represent concentrations
of the
first and second isotopologues of the gas of interest in the atmosphere. The
same first
and second isotopologues of the gas of interest are measured in the iso-FD
chamber 2
and the reference chamber 60.
[0064] Preferably, the measurements of isotopologues of the gas of
interest in the
iso-FD chamber 2 and the measurements of the isotopologues of the gas of
interest in
the reference chamber 50 should be undertaken closely together in time. Doing
this
allows for minimization of inaccuracies that may be introduced by changes in
the
surrounding atmosphere or soil gas over time.
[0065] According to some exemplary embodiments, the iso-FD chamber 2
may
be connected to an input port 32 measuring device 30 via the outlet port 22
and outlet
tube 24. Alternatively, the iso-FD chamber 2 may be connected to the input
port 32 via
the outlet port 22 and a valving system that is connected to the measuring
device 30. In
both cases, replacement of gas drawn by the measuring device 30 in order to
maintain
substantially constant pressure in the cavity 8 is provided by an inlet port
26 and inlet
tube 28 in communication with the surrounding atmosphere. Measurement of the
first
and second isotopologues in the cavity 8 is conducted by drawing gas from the
cavity 8.
The drawing of gas from the cavity 8 may be for a specified time interval such
that a
high concentration of soil gas may be analyzed. However, it will be understood
that as
gas is drawn from the cavity 8 by the measuring device 30, air from the
surrounding
atmosphere 12 is inserted via the inlet port 26 to replace the gas that is
withdrawn.
Therefore the specified time interval is sufficiently short such that only an
amount of gas
is drawn that would not lead to a significant drop in soil gas concentration
due to the
insertion of air from the surrounding atmosphere 12 through inlet port 26.
According to
some embodiments, the measuring device 30 may repeatedly and periodically draw
gas
from the cavity 8 to measure isotopologues of the gas of interest within the
cavity 8.
Preferably, the measuring device 30 is configured to wait a time interval
between
drawing of gas for measurements where the length of the time interval is
sufficiently long
for soil gas concentrations within the cavity 8 to build up again to a
sufficiently high
concentration.
-19-
CA 02771628 2012-03-12
[0066] Where the reference chamber 50 is configured similarly to the
iso-FD
chamber 2, a similar method may be used for measuring the concentrations of
isotopologues in the cavity of the reference chamber 50. Moreover, such
measurements
may be taken using the same measuring device 30 used for the iso-FD chamber 2,
where both chambers are connected by a valving system 52. In such cases, the
valves
of the valving systems 52 are controlled such that the measurements of
isotopologues in
the iso-FD chamber 2 and the measurements of isotopologues within reference
chamber 50 are taken as close in time as possible. Alternatively, each of the
iso-FD
chamber 2 and the reference chamber 50 are connected to separate measuring
devices
30. In this case, the measuring devices are controlled to obtain measurements
of
isotopologues in the iso-FD chamber 2 and the reference chamber 50 at
substantially
the same time.
[0067] According to some exemplary embodiments, the iso-FD chamber 2
may
be connected to an input port 32 of measuring device 30 via the outlet port 22
and
further have inlet port 26 connected to an output port 33 of the measuring
device.
Accordingly, the connection of outlet port 22 and inlet port 26 of the iso-FD
chamber
creates a continuous loop through the input port 32 and output 35 of the
measuring
device. In this case, the measuring device 30 continuously draws gas from the
cavity 8
of the iso-FD chamber via the outlet port 22 and input port 32. The
continuously drawn
gas spends a short amount of time in the measuring device 30 before the
measuring
device 30 ejects the gas through the output port 33 causing it to be
reinserted into the
cavity 8 through the inlet port 32. Therefore, as the measuring device 30 is
continuously
drawing gas from the cavity 8, it is also continuously reinserting gas into
the cavity 8,
thereby allowing maintenance of pressure within the cavity 8. The measuring
device 30
may measure the first and second isotopologues of gas that pass through it
when the
gas is between the input port 32 and output port 33 of the measuring device 30
in the
continuous flow of gas. According to some embodiments, the measuring device 30
may
periodically and repeatedly measure the concentration of isotopologues of gas
continuously passing through it. Where the reference chamber 50 is configured
similarly
to the iso-FD chamber 2, a similar method may be used for measuring the
concentrations of isotopologues in the cavity of the reference chamber.
- 20 -
CA 02771628 2012-03-12
[0068]
According to some exemplary embodiments, the iso-FD chamber 2
comprises a sampling port for receiving a sampling canister and a valve to
selectively
open or close the port. Accordingly, when a measurement is to be taken, a
sampling
canister is attached to the sampling port and the valve is controlled to be
opened to
allow gas in cavity 8 to enter the sampling canister. When the sampling
canister is filled,
the valve is controlled to be closed. The concentrations of isotopologues of a
gas in the
sampling canister may be automatically measured on-site or measured off-site,
for
example in a laboratory. Where concentrations of isotopologues in the sampling
canister
are measured automatically, such measurements may be repeatedly and
periodically
taken. Where the reference chamber 50 is configured similarly to the iso-FD
chamber 2,
a similar method may be used for measuring the concentrations of isotopologues
in the
cavity of the reference chamber 50. Moreover, the valve of the sampling port
the iso-FD
chamber 2 and the valve of the sampling port of the reference may be
controlled to be
opened at substantially same time to obtain samples of gas contained in both
chambers.
[0069]
According to some exemplary embodiments, the iso-FD chamber 2
comprises a lid that may be selectively opened and closed. Accordingly, after
measuring
the isotopologues of the gas of interest within the cavity 8, the lid 10 is
opened to
expose cavity 8 to the surrounding atmosphere 12 through the top opening 12.
The lid
10 is left open for a time interval that is sufficiently long for vegetation
in the cavity 8 to
be exposed to the surrounding atmosphere 12 and elements of nature such as
air,
sunshine, rain, and other natural elements. To take a further measurement, the
lid 10 is
closed and a waiting time is allowed to pass such that the soil gases
permeating into the
cavity 8 and gas permeating in and out of the cavity 8 through the outlet
membranes 20
are allowed to reach equilibrium. A further measurement of concentration of
isotopologues of the gas of interest within the cavity 8 can then be taken.
According to
embodiments where measurements are repeatedly and periodically taken, the
steps of
taking a measurement, opening the lid, waiting a time interval, closing the
lid, allowing
gases to reach equilibrium are repeated for each measurement. Where the
reference
chamber 50 is configured similarly to the iso-FD chamber 2, a similar method
may be
used for measuring the concentrations of isotopologues in the cavity of the
reference
chamber.
- 21 -
CA 02771628 2012-03-12
[0070] According to some embodiments where the iso-FD chamber 2
further
comprises a sensor for measuring concentrations of bulk gas, measurements of
isotopologues of a gas and measurements of the bulk gas may be taken
independently
of each other.
[0071] After taking the measurements, the flux of the first isotopologues
of the
gas of interest from the soil ground 4 relative to the flux of the second
isotopologues of
the gas of interest from the soil ground 4 may be determined from the measured
concentration of the first isotopologues of the gas of interest in the cavity
8, the
measured concentration of the second isotopologues of the gas of interest in
the cavity
8, the measured concentration of the first isotopologues of the gas of
interest in the
cavity of the reference chamber 50 and the measured concentration of the
second
isotopologues of the gas of interest in the cavity of the reference chamber
50. This
determination of the relative flux is further based on the diffusivity of the
first
isotopologues of the gas of interest through the one or more outlet membranes
20 of the
iso-FD chamber 2 and through the one or more outlet membranes of the reference
chamber and on the diffusivity of the second isotopologues of the gas of
interest through
the one or more outlet membranes of the iso-FD chamber 2 and through the one
or
more outlet membranes of the reference chamber 50.
[0072] Where the outlet membranes 20 of the iso-FD chamber 2 and the
outlet
membranes of the reference chamber 50 have the same diffusivities, the flux
may be
determined according to the following equation, which is equivalent to the
equation (5):
Dgl(Chl ¨ Cagtim)
F'õi = D92(cfp2 _ cagt2m)
wherein Frei is the flux of the first isotopologues of the gas of interest
through the inlet
membrane of the first chamber relative to a flux of the second isotopologues
of the gas
of interest through the inlet membrane of the second chamber, D91 is the
diffusivity of the
first isotopologues of the gas of interest through the one or more outlet
membranes, Dg2
is the diffusivity of the second isotopologues of the gas of interest through
the one or
more outlet membranes, Cis the measured first concentration of the first
isotopologue
of the gas of interest within the iso-FD chamber 2, Cis the measured second
concentration of the first isotopologue of the gas of interest within the iso-
FD chamber 2,
- 22 -
CA 02771628 2012-03-12
Ch2i is the measured first concentration of the second isotopologue of the gas
of interest
within the reference chamber 60, and Cis the measured second concentration of
the
second isotopologue of the gas of interest within the reference chamber 50.
[0073] Determination of the relative flux may be made by the measuring
device
30 where one device is used to measure concentration of the first and second
isotopologues in both the cavity 8 of the iso-FD chamber 2 and the cavity of
the
reference chamber 50. Alternatively, a controller may be coupled to receive
measurements from the measuring device, wherein the controller also performs
determination of the relative flux. In some embodiments where multiple
measuring
devices are used to measure concentrations of first and second isotopologues
in
multiple iso-FD chambers 2 or reference chambers 50, the controller may
receive
measurements from each of the measuring devices and perform a determination of
relative fluxes for each of the iso-FD chamber locations. Validation
[0074] According to one exemplary embodiment of the system and method
described herein a custom built Flux Generator (FG) was provided to test the
design of
the system and method. The FG is functionally similar to that of Martin and
Bolstadl,
using most of the same operational parameters and mass balance equations for
calculating flux. Within a 234.23 litre gas reservoir, a fan circulates
injected gases at a
fixed speed, mixing the whole volume in approximately 15 seconds. A 0.324m2
tray on
top of the reservoir contains a homogenized synthetic "soil" of glass beads
(22 cm
deep). Concentrations of CO2 in the gas reservoir are monitored continuously
using a
LiCor LI-820 infrared gas analyzer (IRGA). A four-port exhaust manifold and
fan is
situated over the tray to maintain the soil surface concentration near ambient
levels, as
described in Martin and Bolstad. A custom-designed LabVIEW interface and
National
Instruments Data Acquisition device automated the function of the FG
(including CO2
injections), performs calculations, and records data.
[0075] Within the FG glass bead soil two filtered sampling tubes were
inserted,
one near the top of the soil (about 2-3 cm deep) and one near the bottom of
the soil (
about17-18 cm deep). These sampling tubes allowed for calculation of the true
isotopic
1 Martin, J.G., Bolstad, P.V., 2003. "A carbon dioxide flux generator for
testing infrared gas analyzer-
based soil respiration systems." Soil Society of America Journal 68, at 514-
518.
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CA 02771628 2012-03-12
flux leaving the FG instrument so that the iso-FD chambers can be calibrated
for the
pump offset and also validate their ability to measure isotopic flux.
Calculation of flux
from these profile tubes uses the diffusion corrected two point Keeling plot
approach.
[0076] An
Iso-FD chamber and a modified lso-FD chamber for atmospheric
measurement (bottom surface sealed) were situated on the surface of the glass
bead
synthetic soil. The two soil profile tubes, two chambers, two atmospheric
tubes, and two
standards were all sampled for 15 minutes duration. The atmospheric tubes (in
this
case simply open to lab air) were sampled between soil profile and chamber
measurements to ensure the sampling pathway was purged of any residual gases
from
the previous measurements.
[0077]
Two separate laboratory trials of the lso-FD method were performed on
consecutive days. The FG was injected with CO2 until the reservoir
concentration
reached 6000 ppm, after which time the gas was allowed to diffuse freely
through the
glass bead soil and into the lab atmosphere, with each run lasting
approximately 15
hours.
Field Trial
[0078]
According to one exemplary embodiment of the system and method
described herein, iso-FD chambers were placed in a about 20 year old
plantation of red
pine (Pinus resinosa) located in Heatherton, Nova Scotia (N 45 33' 54", W 61
46'
20"). Annual average rainfall for the region is 1100 mm/year with average
monthly
rainfalls in August of 92 mm and 101 mm in September. Annual average
temperature for
the region is 7 C and average temperatures in August and September are 18.9
and
15.3, respectively.
[0079]
For a period of approximately 3 days, the PicarroTm 01101-i spectrometer
was used to sample the two chambers (one iso-FD chamber and one reference
chamber) as well was three horizontal soil gas well (about 4,13,26 cm depth)
that were
installed at the site in May 2011. The gas well was constructed using 50 cm
long
sections of 1.3 cm inside diameter PVC tubing. Holes (1.0 cm diameter) were
drilled on
opposing sides along the length of the pipe at -4.5 cm intervals. The outsides
of the
wells were wrapped in Tyvek building material to exclude water from entering.
An
- 24 -
CA 02771628 2012-03-12
approximately 10m long section of vinyl tubing was connected to the well via a
barbed
fitting to allow for sampling by the PicarroTM G1101-I spectrometer.
[0080] In a manner similar to the lab validation, described above the
isotopic flux
was calculated for the lso-FD chambers (Equation 5) and compared to both the
isotopic
flux calculated via a two point Keeling plot approach using the shallowest
subsurface
gas well and the atmospheric CO2 concentration and the isotopic signature of a
4-point
Keeling plot that includes all three subsurface wells and the atmosphere.
[0081] Simulations of the Iso-FD method produced concentration and
isotopic
plumes directly below the chamber similar to those found using both static and
dynamic
chambers. However, in all simulations the predicted isotopic signature of flux
using the
lso-FD method was very near the true value (True-Predicted; Mean Deviation
<O.01%0).
This quality may be attributed to the diffusive nature of the exchange of CO2
with the
surroundings. This allows the Iso-FD chamber to attain a new diffusive steady
state
during the measurement period that reflects the natural diffusive steady state
and
therefore allows the method to predict the true steady state value of flux,
rather than a
biased value.
[0082] Two separate laboratory trials of one embodiment of the the
lso-FD
method were carried out on consecutive days. The FG was injected with CO2
until the
reservoir concentration reached 6000 ppm, after which time the gas was allowed
to
diffuse freely through the glass bead soil and into the lab atmosphere, with
each run
lasting approximately 15 hours. Figure 8a shows the observed decay in 12CO2
concentrations in the glass bead soil, as measured by the soil profile tubes.
To the right,
in Figure 87b is the trajectory of soil profile isotopic composition during
the same time
period. Figure 8c shows the concurrent changes in the Iso-FD and atmospheric
chamber 12CO2 concentrations, with the isotopic signature of both chambers
shown in
Figure 8d. Good correlation was observed between the "true" isotopic flux,
calculated
using the soil profile tubes, and the Iso-FD measured isotopic flux values,
presented in
Figure 9. Linear regression results yielded a slope of 0.956 (S.E.=0.0575) and
y-
intercept of -1.958 (S.E.=1.848) with an r2 value of 0.9322. This suggests
that the
desired offset for this particular lso-FD chamber design (and measurement
length) being
1.958%0, however the regression standard error is quite high leading to a
large amount
of uncertainty in the estimate. This large spread in the potential intercept
value (-3.806
- 25 -
CA 02771628 2012-03-12
to -0.110) is due in part to the variability in the data and the large
distance to extrapolate
the curve to the y-axis. This may be constrained better by using injection
gases with
several different isotopic signatures (around 0%0 or heavier), although since
the offset
value is constant through time for a the same pump rate it will not affect the
isotopic
variability measured by the Iso-FD approach.
[0083] During a field trial, data from the lso-FD tracked well with
data from both
the two-point and multi-point subsurface Keeling plots. In most cases,
departures from
the relatively stable Iso-FD signatures (for example around day 265) are well
correlated
with sudden spikes in CO2 flux, as measured by a LiCOR LI-8100 located near
the Iso-
FD chamber (data not shown). It is also important to consider here, that the
subsurface
methods are measuring a more stable, time-integrated (because of diffusive
processes)
signal and therefore deviations seen in the Iso-FD data may in face be high
frequency
changes in microbial/root processes near the surface which do no last for a
sufficient
period of time to express themselves in the soil gas concentrations. These
field data are
not shifted to take into account the offset caused by drawing air from the
chamber,
largely because of the uncertainty associated with the offset calculated
during the Flux
Generator testing. Assuming, however, the offset is similar to the estimated
1.958%0 the
isotopic signature measured by the probes would fall between the root respired
isotopic
composition from the site (-27%0 1.6%o, unpublished incubation data) and the
fluxes
measured using the subsurface Keeling plot which will tend to be biased toward
deeper
soil respiration rather than the very near surface where the bulk of the fine
root mass is
at this site (-60% of fine root mass within the first 15 cm of soil is in the
top 0-5 cm
depth increment).
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