Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 10 docx

Measurement of Trace Gases, I: Gas Analysis, Chamber Methods, and Related Procedures Keith A.. Improved methods of measurement of the fluxes of these gases have become of major environmen

Measurement of Trace Gases, I:

Gas Analysis, Chamber Methods,

and Related Procedures

Keith A Smith and Franz Conen

The University of Edinburgh, Edinburgh, Scotland

The exchange of gases between the biosphere and the atmosphere has had a profound effect on the development of the Earth environment Globally, vegetation (principally forests) releases and absorbs about 60 billion tonnes

of carbon dioxide, CO2, annually and it is the perturbation of this exchange

by additional anthropogenic emissions of about a tenth of this quantity that

is the principal contribution to global warming—the ‘‘greenhouse effect.’’ Emissions of methane from natural wetlands, rice fields, and landfills, and of nitrous oxide from fertilized agricultural soils and the soils of tropical rainforests, add to global warming A further contribution comes from tropospheric ozone, produced by reactions involving volatile organic compounds from natural vegetation, and NOx, which comes variously from combustion sources and from soils Soil surfaces can, on the other hand, act as sinks for many pollutant gases in the atmosphere, through both physicochemical sorption and microbial oxidation Improved methods

of measurement of the fluxes of these gases have become of major environmental importance, both to determine flux levels and in order to improve understanding of the processes involved, to aid the prediction of future trends.

This chapter covers the principles of current instrumental techniques for the determination of trace gases, and the experimental procedures used

to make flux measurements in the field on a small scale: from 0.1 m 2 of land surface to 10–100 m 2 Examples of recent applications are included Chapter

11 contains complementary information on micrometeorological methods for the determination of fluxes at larger scales Throughout, the commonly used concentration units ppm and ppb will be used for convenience, instead

of the more rigorous notation in volumetric units (mL L
1

and nL L
1

), or partial pressure units (1 ppm ffi 0:1 Pa and 1 ppb ffi 10
4 Pa) unless the context dictates otherwise.

Gas chromatography, GC, is the principal analytical method used for the measurement of trace gases in soil air and in the atmosphere above the soil.

In particular, it has provided most of the data on the fluxes of the greenhouse gases methane and nitrous oxide and is also used in some studies for measuring carbon dioxide released by soil respiration.

Chromatography is essentially the separation of the components of a mixture resulting from differences in their partition between a stationary phase with a large active surface area and a moving phase that flows over the stationary phase Depending on the state of the moving phase, a distinction is made between liquid and gas chromatography Separations that would be very difficult to achieve by any other means are possible by chromatographic methods, because even small differences between the components in their partition between the phases are multiplied many times during their passage through the chromatographic system.

GC may be conveniently classified into two types: gas–liquid and gas– solid chromatography In the former, the stationary phase is an involatile liquid coated either on an inert support material in a packed column, or on the internal wall of an open tubular (capillary) column In the latter type, the column is packed with an adsorbent of small particle size, or the inner surface of a capillary column is coated with a thin adsorbent layer For separations of gaseous mixtures, solid adsorbents (see below) have been used more widely than liquid stationary phases.

At constant temperature, pressure, and carrier gas velocity, the rate at which a component travels along the column is related to the partition coefficient K:

where Cs and Cg are the concentrations in the stationary and gaseous phases, respectively, The more strongly bound components stay for a longer time in the stationary phase and thus travel along the column more slowly than the more weakly bound components The zone occupied by a component broadens as it moves along, due to the diffusion of molecules

in the gaseous phase.

The separation of the components in a sample depends on the relative values of the partition coefficients The closer the values are to each other, the longer is the column required to give an adequate separation Reducing the temperature has the effect of increasing the separation In practice, the variables that are most commonly exploited to achieve satisfactory separations are

1 The material used as stationary phase

2 The length and diameter of the column

3 The column temperature

4 The flow rate of the moving phase (the carrier gas)

The essential parts of all gas chromatographs consist of

1 A carrier gas system, generally in the form of a cylinder supply at pressures up to 20 MPa (200 bar), with a two-stage regulator to reduce the pressure and a flow-rate regulating valve

2 A device for introduction of the sample into the carrier gas stream

3 A column to separate the components of the sample

4 A detector to indicate the elution of each component and produce

a response proportional to the quantity of the component present

5 A recording system to provide a permanent record of the detector response

The column and detector (and sometimes the injection port) are normally enclosed in separate thermostats (‘‘ovens’’) that can be controlled individually This is because the optimum column temperature for achieving

a desired separation is usually different from the optimum detector temperature.

in Table 1.

beads, marketed under such names as Porapak and HayeSep, are commonly used for separations of permanent gases These materials are available in different grades that vary in their retention characteristics for particular substances Porapak Q and its equivalents have been used widely for separating CH 4 , CO 2 , and/or N 2 O from nitrogen and oxygen in air samples Columns 1–2 m in length, 1/8–1/400 (3.2–6.4 mm) in diameter, packed with these materials, give satisfactory separations of these gases at modest temperatures in the range 30–70C.

alternative to porous polymer beads for separating permanent gases Separations of air (oxygen plus nitrogen), carbon monoxide, methane, CO2, and/or C2 hydrocarbons can be readily achieved with a 2 m  1/800 packed column of carbon molecular sieve, depending on the column temperature (Fig 1).

Table 1 Some Manufacturers of Gas Chromatographs

Palo Alto, CA 94303, USA

www.agilent.com Formerly Hewlett-Packard

CT 06855, USA

www.bucksci.com Small transportable instruments

suitable for field use

Milan, Italy

www.ceinstruments.it Formerly Carlo Erba/Fisons.

Now part of Thermoquest GOW-MAC Instruments 277 Brodhead Rd, Bethlehem,

PA 18017-8600, USA

www.gow-mac.com PerkinElmer Instruments 710 Bridgeport Ave., Shelton,

CT 06484-4794, USA

www.perkinelmer.com Shimadzu Scientific

Instruments

1 Noshinokyo Kuwabarocho, Kyoto 604-8511, Japan and

7102 Riverwood Dr.

Columbia, MD 21046, USA

www.shimadzu.co.jp www.shimadzu.com

Torrance, CA 90503, USA

transportable instruments suitable for field use Unicam Chromatography Viking Way, Bar Hill

Cambridge, CB3 8EL, UK

www.unicam.co.uk Formerly Pye-Unicam Now

part of Onix Process Analysis Varian Instruments 2700 Mitchell Dr., Walnut Creek,

Capillary Columns and Open Tubular Columns Complex mixtures of volatile organic compounds (VOCs), emitted to the atmosphere from vegetation, are analyzed using long capillary or open tubular columns Examples are a Megabore Carbowax 20 M column, 60 m long, with a film thickness of 1.2 mm, and a 50  0.22 mm BP1 column, with a film thickness

of 0.12 mm (Kesselmeier et al., 1996) (see Sec IV below).

detector, which responds to combustible compounds (in effect, organic compounds) and, to some extent, to oxygen, but which is insensitive to the

Figure 1 Effect of temperature on separation of permanent gases on Carbosphere carbon molecular sieve (Reproduced by permission of Alltech Associated Inc., Deerfield, IL, USA.)

presence of inorganic gases and water vapor Methane must, therefore, be separated satisfactorily from oxygen and other organic compounds, and this can be done readily on Porapak Q and similar materials The limit of detection for this gas is of the order of 1 ppb, so there is no problem of determining concentrations at ambient level (1.7 ppm) and above In some studies, in fact, dilution of samples becomes necessary to avoid nonlinear detector response (see below).

selective electron capture detector (ECD: see below), operated at high temperature (340–390C) The ECD responds primarily to gases with a high affinity for electrons, e.g., oxygen, water vapor, and halogenated compounds Thus effective separation of N2O from such gases is very important This is achieved easily on Porapak Q or HayeSep Q Analysis times can be reduced by using backflushing techniques to remove the more slowly eluted substances such as water vapor (e.g., Arah et al., 1994) ECDs also have some sensitivity for CO2, and separation from the latter gas can also be achieved on the same column packings, if the temperature

is low enough Alternatively, the sample can be passed through a precolumn of soda lime to remove the CO2 before it enters the analytical column.

than with an IRGA, the most common procedure is separation from oxygen/nitrogen on Porapak Q and detection with a thermal conductivity detector (TCD) Aternatively an ECD can be used, operated at a lower temperature (ca 270C) than is commonly employed for N2O; however, if the relative concentrations of CO2 and N2O are in a suitable range, these two gases can be determined simultaneously by this procedure (Thomson

et al., 1997).

They play an important part in atmospheric chemistry by, for example, contributing to tropospheric ozone production (through photochemical reactions involving NOx) Separation requires the use of a capillary or open tubular column (see above) Temperature programming, in which the column oven increases in temperature at a predetermined rate during the analysis, is also employed, to speed up the elution of the more involatile components of the mixture Detection of VOCs is normally by FID, but the mass spectrometer has become an increasingly attractive alternative, because of its high sensitivity and its capability for distinguishing between compounds with similar retention times by their fragmentation patterns (Parrish and Fehsenfeld, 2000).

Simultaneous Measurement of Two or More Gases The separation and quantification of a family of related compounds, such as the VOC separation discussed in the previous paragraph, may require a long column and take many minutes for a single analysis, but it normally only requires one column and one detector Sometimes, however, there is a need to determine simultaneously in the same sample different gases that require different columns and/or different detectors Some earlier multicolumn and multidetector systems for this purpose were described in the previous edition (Smith and Arah, 1991), and more recent examples are given in Sec IV below.

The concentration of the target gas in a sample is conveniently determined

by comparisons of peak height (or peak area) with those obtained with gas standards of known concentration, having first established the relationship between peak height (or area) and detector response Most modern detector/ amplifier systems have a linear response over a concentration range covering several orders of magnitude If the response becomes nonlinear but is not saturated, the system can still be used, provided several standards of different concentrations are analyzed in order to construct a response curve The alternative is to make dilutions, e.g., with air or nitrogen, before sample injection Better reproducibility of results is usually obtained with a sampling valve than by direct injection with a syringe.

When large numbers of samples have to be analyzed, a degree of automation can greatly reduce operator time Commercial headspace samplers are available that consist of a conveyor/sample changer system carrying 10–50 glass vials sealed with serum caps, and an automatically operated gas syringe The syringe needle punctures the cap of each vial in turn, a gas sample is withdrawn, the syringe moves to a position above the GC injection port, and the sample is injected through the septum The main problem is expense—a headspace sampler commonly costs more than the GC to which

it is attached.

Automated systems for the injection of gas samples collected in the field in Tedlar bags, syringes, or other containers are not commercially available Either the samples have to be transferred to evacuated vials, and thence to a headspace sampler, or a ‘‘do-it-yourself’’ automatic injection system has to be constructed In one such system used by our group, the loop of a motor-driven gas sampling valve is evacuated by a pump and then coupled, by the operation of a rotary switching valve, to each of a series of

sample containers (syringes, bags, or even small incubation vessels) in turn Each container is connected to the valve via luer-lock fittings The gas in the sample vessel expands into the loop, the contents of which are then injected (Fig 2).

Data acquisition and processing is nowadays almost always performed electronically The chromatographic data are recorded either with a digital integrator or via a computer interface, using specialized software Statistical evaluation of replicate analyses can take place, and results can be stored for subsequent retrieval, without the manual transfer of data at any stage This has the benefit of eliminating errors, as well as saving operator time.

Gaseous compounds such as CO2, N2O, CH4, and H2O vapor absorb electromagnetic radiation in the infrared (IR) part of the spectrum, due to the particular vibrational/rotational energies of their interatomic bonds This is, of course, the characteristic that makes these compounds greenhouse gases and thus the focus of much environmental research, but

it also provides the basis for sensitive methods of analysis The main

Figure 2 Schematic diagram of automatic GC injection system for gaseous samples (From Arah et al., 1994.)

constituents of the atmosphere, N2 and O2, being symmetrical diatomic molecules, do not absorb in the IR region; thus analyzers based on IR absorption can be used to determine trace concentrations of CO2, N2O, CH4, and H2O vapor in air without interference from N2 or O2 These instruments contain three main components: an infrared source, a gas cell through which the radiation passes, and an IR detector Absorption of

IR radiation at any particular wavelength takes place according to the Beer–Lambert law:

where I and I0 are the intensities of the transmitted and incident radiation, respectively, k is the molar extinction coefficient for the particular wavelength, c is the concentration of the absorbing gas, and d is the path length through the cell.

Infrared gas analyzers (IRGAs) fall into two main categories: dispersive (DIR) and nondispersive (NDIR) In the former, an IR beam from a source filament passes through a monochromator, and radiation of a selected wavelength that is strongly absorbed by the target gas passes through a gas cell and then to a detector By changing the wavelength passing through the cell, other gases may be targeted, and if a scanning system is used, an absorption spectrum will be obtained However, for study

of, for example, CO2 fluxes during photosynthesis or respiration, a simpler approach, NDIR analysis, is generally used Here there is no attempt to select particular wavelengths Instead, the broad band of radiation from the source is employed, and the total absorption of all the lines in the 4.26 mm major absorption band of CO2 is measured (Long and Ha¨llgren, 1993).

Current IRGA system include

The range of instruments made by LI-COR (Lincoln, Nebraska, USA), such as the LI-6252 CO2 Analyzer and the LI-6262 CO2/H2O Analyzer

The Rosemount Analytical (Orrville, Ohio, USA) BINOS series of instruments

The PP Systems Inc (Haverhill, Mass., USA) CIRAS-2 portable photosynthesis system

The ADC Bioscientific Ltd (Hoddesdon, Herts., UK) Mini-IRGA These analyzers are compact and portable and can be used in the field under battery power, as well as in the laboratory on a mains AC power

supply The most sensitive instruments, such as the LI-5262, are capable of determining a change of < 1 ppm of CO2 against a background ambient concentration of about 360 ppm For dedicated CO 2 analysis, an optical filter only allows radiation within the 4.26 mm band to pass, thus preventing interference from other IR-absorbing gases, particularly water vapor Gas concentration is measured by detecting the difference in IR absorption in two parallel optical cells (Fig 3) The system can be operated

in two different ways In absolute mode, the reference cell contains CO2-free air and the sample cell contains an air sample with unknown CO2 concentration; in differential mode, the reference cell contains a known concentration of CO2 Infrared radiation is alternately transmitted through each cell, via a chopping shutter, and the resulting detector output V is proportional to the difference between the signals generated by the detector when it sees the sample cell (s) and when it sees the reference cell (r):

The constant K then scales the infrared absorption, represented by

1
(s/r) in Eq (4), to analyzer output V (Eckles et al., 1993) The relationships between s/r and gas concentration, and concentration and V, respectively, are shown in Fig 4.

Figure 3 Schematic diagram of infrared absorption gas analyzer (IRGA) This version (LI-COR 6262) has two detectors, for measurement of both CO 2 and water vapor (Courtesy of LI-COR Corp., Lincoln, NB, USA.)

Infrared Source This is a vacuum-sealed long-life filament, heated to

a temperature between 1000 and 1250 K, that emits radiation over a wide range of wavelengths Typically, the filament temperature is monitored by a light-sensing diode that is part of a feedback circuit that controls the power input to the source, thus maintaining a constant output and ensuring a stable radiation source (Eckles et al., 1993).

instruments) and gold-plated to enhance IR reflection The radiation is collimated by CaF2 lenses and focused on the detector Air from the sample source (e.g., a photosynthesis or soil respiration chamber, see Sec III below)

is pumped through the sample cell, while a CO2 standard of known concentration flows through the reference cell.

temperature-controlled solid-state devices, made of lead selenide, cooled to

5C by a thermoelectric Peltier cooler The LI-6252 analyzer has a single detector for CO2, while the LI-6262 analyzer (Fig 3) incorporates a beam splitter and two detectors, one for CO2 and the other for H2O The filter for the H2O detector is centered at 2.59 mm.

Instruments vary considerably in the frequency with which they require calibration, and it is advisable initially to recalibrate daily If no significant change in settings is found, then a longer interval may be employed (Long and Ha¨llgren, 1993) Calibration gas mixtures may be purchased from commercial sources, or prepared using either gas mixing pumps or high- precision mass flow controllers In the field, a gas syringe may be used to mix

a small quantity of pure CO2 with CO2-free air.

Figure 4 Relationships between (A) signal ratio (IRGA cell/reference cell) and

CO 2 concentration, and (B) CO 2 concentration and IRGA output voltage (Courtesy

of LI-COR Corp., Lincoln, NB, USA.)

2 Response and Averaging Time

Response times of NDIR analyzers are very rapid, typically 1–15 s They measure concentrations in a flowing gas stream, in contrast with GC systems, which analyze discrete small samples, and in normal operation the signal is averaged over a period of 1–30 s; the noise decreases as this period lengthens Figure 5 shows the output of an IRGA (LI-6250) logged at 1 s intervals.

In photoacoustic spectroscopy, the gas to be measured is irradiated by infrared radiation of a preselected wavelength, in an optical cell Some of the radiation is absorbed, which results in heating and therefore expansion and

a rise in pressure Interrupting the infrared beam by a chopper causes the pressure alternately to increase and decrease, and an acoustic signal is generated, which is detected by two microphones This principle is the basis

of the 1300 series of trace gas analyzers (TGAs) produced by Innova Airtech Instruments (Ballerup, Denmark) The TGAs (shown schematically in Fig 6) are fitted with up to five optical filters, each of which is suitable for a different trace gas, and one for water vapor The filters are mounted in a carousel and used in turn to determine the concentrations of the different gases Gas samples are pumped from up to 12 flux chambers in turn, via automatic switching valves, and each measurement of all the target gases can be completed in about 2 min (Ambus and Robertson, 1998) The analyzers are portable and can be used in the field, but they require 110 or 220–240 V electric power from the mains or an AC generator.

Figure 5 CO 2 concentration readout from an IRGA logged at 1 s intervals (Courtesy of LI-COR Corp., Lincoln, NB, USA.)

The system corrects for interference from water vapor, which affects the determination of most gases, and also for other interferences such as CO2 during the measurement of N2O The limit of detection for N2O is

30 ppb (one-tenth of ambient) and for methane 0.1 ppm (about 6% of ambient), using 5 s integration times The response is linear over a wide range of concentrations Longer times up to 50 s may be used, which will improve accuracy Ambus and Robertson (1998) compared the results obtained for N2O and CO2 chamber concentrations with those obtained by electron-capture GC and NDIR methods, respectively The agreement was good (Fig 7).

Laser (TDL) Absorption Spectrometry

Figure 6 Schematic diagram of Innova Model 1312 photoacoustic infrared spectrometry system used for trace gas analysis (Courtesy of Innova Airtech Instruments A/S, Ballerup, Denmark.)

beam provides the spectrum of the input beam and the absorption by any sample inserted in the beam (Griffith and Galle, 2000) For analysis of trace gases, air is drawn continuously from the sampling point through Teflon tubing and then into two 25-L optical cells (White cells) Each part of the split IR beam is reflected backwards and forwards 70 times through one of the White cells, to achieve a long absorption path length (70 m), for maximum sensitivity and then, after recombination, is focused on an InSb

or MCT detector (Griffith and Galle, 2000) The spectra are analyzed by a specialized computer program that employs a library of reference spectra (Griffith, 1996).

The FTIR system has been used both in micrometeorological flux measurements by the flux gradient method (see Chap 11) and in combination with large or ‘‘mega’’ chambers (Galle et al., 1994; see Sec V.C below).

TDL absorption spectroscopy combines very high sensitivity, comparable with that of FTIR analyzers, with fast response and great selectivity The tunable diode laser source is usually a crystal of a lead salt semiconductor of the general formula Pb1
xSnxSe (Werle, 1999) It is mounted together with a small heater on a copper bar in a liquid nitrogen–filled dewar Precise control of a current through the heater can adjust the laser temperature by a minute fraction of a degree, and this produces fine adjustment—tuning—of

Figure 7 Relationships between (A) CO 2 concentrations measured by acoustic trace gas analyzer (TGA) and by IRGA, and (B) N 2 O concentrations measured by TGA and by ECD-GC (Reproduced from Ambus and Robertson,

photo-1998, by permission of the Soil Science Society of America.)

the emission wavelength of the laser (in the near-infrared) A particular laser diode may only be used for one gas, though two may be possible if suitable

IR absorption lines are close enough and the system can be tuned from one equivalent emission wavelength to the other Available commercial systems include the Campbell TGAI00 (Campbell Scientific, Logan, Utah, U.S.A.), which uses only a single pass of the radiation through a 1.5 m absorption cell, and the Aerodyne (Billerica, Mass., U.S.A) models, which employ much longer path lengths, achieved by multiple reflections through the cell Like the FTIR, TDL analyzers can be used in micrometeorological applications (see next chapter) and are fast enough to be used for eddy covariance as well as in the flux gradient mode; however, because of their sensitivity they are also very useful for determining very small changes from normal ambient concentrations in discrete gas samples (usually a few liters

in size collected in Tedlar bags), which are pumped through the optical cell

of the analyzer Such samples may be taken from chambers, or collected from the atmosphere, e.g., during aircraft flights to determine average emissions over a wide area (Beswick et al., 1998) More local trace gas accumulation, e.g., under a nocturnal inversion layer, may be measured directly by raising the inlet end of a sampling tube to the desired height with

a balloon, and pumping the air directly through the optical cell (Beswick

et al., 1998).

The chemiluminescence technique is now well established as a reliable way

of determining nitric oxide, NO, and nitrogen dioxide, NO2, in atmospheric samples (Parrish and Fehsenfeld, 2000) The technique for NO is based on the chemical reaction with added ozone, and the detection of the chemiluminescence from the excited NO2 reaction product by a photo- multiplier (Fontjin et al., 1970; Ridley and Howlett, 1974) Commercially available chemiluminescence analyzers also routinely measure NO2 For NO determination, the sample airstream is switched directly by a solenoid valve

to the reaction chamber, whereas for NO2 determination the stream passes first through a catalytic converter (molybdenum oxide) where the NO2 is reduced to NO (‘‘NOx mode’’ in Fig 8) This mode will give an output equivalent to the sum of the NO and NO2 concentrations, and analyzers such as the Thermo Environmental (Franklin, Mass., U.S.A.) Model 42C automatically cycle between the NO and NOx modes and calculate the NO, NO2, and NOx concentrations The system is extremely sensitive: the limit of detection for NO, using a 60 s averaging time, is 0.4 ppb.

To measure soil emissions of NO, air is passed at between 50 and

150 L min
1 through a charcoal filter, to remove any ozone, then through a

chamber covering the soil (see Sec V below) and finally through the chemiluminescence analyzer (e.g., Skiba et al., 1992) This procedure is necessary to prevent reaction between the NO and ozone in the flux chamber.

Ammonia, released from land surfaces following the application of organic manures or urea fertilizer, or emitted from livestock housing or manure stores, is a major pollutant and can contribute greatly to N deposition on sensitive ecosystems (Wyers et al., 1993) The method of choice to determine the atmospheric concentration of ammonia is the denuder tube Ferm (1979) devised diffusion denuder tubes in which gaseous ammonia in an airstream passing through the tubes diffused to an acid-coated surface where it was trapped, while ammonium in solution in water droplets or in the form of aerosol particles passed straight through Several automated denuder systems have been described (e.g., Keuken et al., 1989; Langford et al., 1989; Wyers et al., 1993) In the sensitive system developed by Wyers et al (1993), ambient air is pumped at 30 L min
1 through an annular denuder rotating at 30 rev min
1 The ammonia is collected in 3.6 mM NaHSO4

Figure 8 Schematic diagram of chemiluminescence analyzer (Thermo mental Model 42C) used for measurement of NO and NO 2 (Courtesy of Thermo Environmental Instruments Inc., Franklin, MA, USA.)

Environ-solution covering the walls of the annular space in the denuder Two peristaltic pumps continuously pump the solution in and out, in the opposite direction to the airflow The solution leaving the denuder is mixed with 0.5 M NaOH to release the ammonia, part of which passes through a semipermeable membrane into deionized water and is determined con- ductometrically The limit of detection in air is 6 ng NH3 m
3

The system can be employed in micrometeorological flux gradient systems for field-scale flux measurement (see Chap 11), as well as in atmospheric monitoring stations.

The most commonly used methods for measuring the exchange of trace gases such as CH4, N2O, and NO between soils and the atmosphere have involved enclosure, or chamber, methods Much of the current investigation

of total CO2 exchange between terrestrial ecosystems and the atmosphere is conducted on a landscape scale by micrometeorological techniques (see Chap 11); however, here too there is a need for chambers, to determine individual fluxes such as soil or leaf respiration rates, or the rate of photosynthesis, that contribute to the overall CO2 flux.

Chamber methods involve what is basically a very simple technique:

an inverted box of known dimensions is placed over the soil surface (or water surface, in wetlands and rice paddies), and the change in concentra- tion with time of a gas emanating from the soil (or being absorbed by the soil from the air in the box) is measured by one of the instrumental techniques discussed in the preceding sections This section describes various versions of the technique and discusses the advantages and disadvantages that they offer.

Methods involving closed chambers are the simplest variant of this approach to gas flux measurement In essence, a container of known dimensions is sealed over the soil surface (or water surface for studies in wetlands, rice paddies, lakes, and rivers), for periods usually of the order of 20–60 min When operated in the static mode, gas samples are taken from the chamber at intervals during the closure period (or only at the end—see below) and analyzed off-line for the target gas: most commonly CH4, N2O,

or CO2 A variety of types of static chamber is shown in Fig 9.

Figure 9a shows an example of a simple shallow box-shaped chamber, sealed into the soil to a depth of 5 cm or so, and remaining in place for an

extended period When a flux measurement is to be made, a lid is placed on the box, fitting into a water-filled channel that makes an effective gas seal (Fig 9b).

Figure 9c shows another common variant, in which a ‘‘collar’’ or chamber base (which may be circular, square, or rectangular) remains permanently installed in the ground and protrudes only a few cm above the surface The chamber is sealed to the collar when a measurement is to be made, and then removed This approach has advantages when it is necessary

to avoid shading of growing plants between measurements, and readily allows chambers of different height to be used on the same bases.

Figure 9d shows a movable chamber made from a metal beaker that can be easily inserted into a new sampling point prior to making the measurement.

Figure 9e shows a more elaborate system, appropriate for use in measuring methane emissions from rice fields or natural wetlands.

Figure 9 Various designs of closed static chamber: (a), (b) Square sheet-metal chamber with water-sealed lid (After IAEA, 1992.) (c) Removable chamber on permanent soil collar (d) Removable metal chamber with vent (From Hutchinson and Mosier, 1981, by permission of the Soil Science Society of America.) (e) Transparent acrylic chamber with mixing fan and pneumatic actuators for opening and closing the lid, on steel mounting frame, used in rice paddy (From Butterbach-Bahl, 1993, by permission of Wissenshafts-Verlag Dr Wigbert Maraun Frankfurt/M, Germany.)

Pneumatic actuators are used to open and close the lid of a transparent plastic box, which encloses the plants as well as an area of the water surface.

An electric fan is used to mix the air within the chamber during closure Some other designs can be found in the companion volume to this one (Smith and Mullins, 2000), and in IAEA (1992).

Closed chambers may also be operated in the dynamic mode, in which the air in the chamber is circulated through a gas analyzer (e.g., an IRGA) (Fig 10), and the increase in concentration with time is determined In the procedure described by Longdoz et al (2000), before making a measure- ment, the airstream is diverted through a soda lime scrubber to deplete the chamber CO2 concentration, and then the air concentration is measured about every 3 s from 15 ppm below to 15 ppm above atmospheric concentration The soil efflux is deduced from the slope of the linear increase

in concentration with time.

Rochette et al (1992) reported that soil CO2 fluxes obtained with dynamic closed chambers were consistently higher than those obtained with static chambers incorporating NaOH traps for the CO2 Norman et al (1997) came to a similar conclusion, when they compared two dynamic closed chamber/IRGA systems and two closed static systems combined with

GC analysis However, the different methods were strongly correlated The size of the discrepancy between methods depends on soil conditions, and results obtained by both methods for a given site can be compared provided intercalibration exercises are carried out under the conditions prevailing at that site.

A dynamic chamber system has been developed for measuring CO2 exchange by tree branches in situ Between measurements, the ventilation fan blows air through the chamber (or ‘‘branch bag’’) at a high rate

Figure 10 Schematic diagram of system for soil CO 2 flux measurement, using a closed dynamic chamber and IRGA.

(40 L min
1 ), and air is circulated between the bag and a box containing an IRGA at 5 L min
1 Then at measurement time the bag is closed off by flap valves (Fig 11a) and a fraction of the circulating air (0.2 L min
1

) is diverted through the IRGA (Fig 11b).

Figure 11 Dynamic chamber (‘‘branch bag’’) system for measuring CO 2 exchange

by tree branches in situ (a) Branch bag with ventilation and mixing fans and flap valves (From Rayment and Jarvis, 1999, by permission of NRC Research Press, Canada.) (b) Gas circulation system between branch bag and IRGA located near ground level.

Advantages and Disadvantages of Closed Chambers The advantages are that they are capable of measuring very small fluxes, the source of the flux is well defined, the method can be applied in conventional experimental designs involving replicated field plots, and most of the chamber designs are cheap and easily fabricated in a laboratory workshop The main disadvantage is the potential to influence the magnitude of the flux that is

to be measured; covering the surface with a chamber, and circulating or not circulating the air within it, can affect gas transport between soils and the atmosphere Evidence for this is provided by the comparison between dynamic and static systems discussed above Pressure changes can accelerate gas exchange by inducing some mass flow close to the surface in very coarse and porous media (Kimball and Lemon, 1971) This is prevented when a sealed chamber is used; an improved design with a vent tube (Fig 9d), which transfers these pressure fluctuations to the inside of the chamber, was introduced by Hutchinson and Mosier (1981) However, wind passing over this vent tube can cause a continued depressurization of the chamber (Venturi effect), leading to a continuous mass flow of soil gas into the chamber (Conen and Smith, 1998) This can result in much larger measurement errors than could be expected from the exclusion of pressure fluctuations.

A chamber can also induce a rise in temperature inside it, increasing the rate of production or consumption of the target gas Using insulated and/or reflective materials can help to minimize this effect (e.g., Matthias et al., 1980) Keeping closure times to a minimum also helps Closed chambers also affect gas concentration profiles within the soil The accumulation of gas inside the chamber is accompanied by gas accumulation within the soil profile below the chamber Thus the air-filled pore space below the chamber may be regarded as an extension of the chamber volume This can be corrected for (Conen and Smith, 2000) or, more practically, minimized by increasing the chamber height, especially on soils with a large air-filled porosity.

Gas sampling usually involves withdrawing a few mL of air from the chamber headspace through a septum or a sampling port fitted into the chamber top or wall at known times after closure Gas-tight syringes, evacuated vials, Vacutainers or metal tubes, or Tedlar (polyvinyl fluoride) gas sample bags may be used as sample containers The samples are then analyzed (usually by GC methods) off-line in the laboratory Acceptable sample storage times depend on the containers used Samples in syringes should preferably be analyzed within a few hours of collection, whereas