236
facilities and funding available to the investigator.
In the following sections, we outline some of the
most commonly used methods for estimation of
trace gas exchange, including enclosure and micrometeorological approaches. We devote most of our
attention to issues surrounding enclosure approaches, given that micrometeorological approaches are addressed in other chapters of this
book. Our discussion of these approaches draws
heavily on material published in Biogenic Trace
Gases: Measuring Emissions from Soil and Water
(Matson and Harriss 1995), and we direct interested
readers to that volume and other literature cited
here for more detailed information.
Enclosure Methods
Gas Exchange at the Soil-Air Interface
Gas transport across the soil-air interface is primarily a function of advection (mass flow) and diffusion (Nazaroff 1992; Livingston and Hutchinson
1995). Molecular diffusion across the interface is
controlled by the trace gas concentration gradient
and the diffusivity of the gas in the soil-water matrix. Rates of gas transport in soils and sediments
are typically much reduced compared with those in
air, because the physical structure (e.g., soil particles) increases the pathlength that gas molecules
must travel to the atmosphere (tortuosity) and because soil water obstructs many of the diffusive
pathways. Not surprisingly, then, soil textural differences affect the exchange of gases. Likewise, the
presence of water in soils and sediments reduces
the volume of air-filled pore spaces and decreases
diffusivity, reducing advective transport and diffusional transport in the soils. Variation in these transport processes, together with differences in production and consumption of gases (Firestone and
Davidson 1989), account for the large spatial and
temporal variation for which trace gas fluxes are
notorious. For complete discussion of the gas transport processes at the soil-atmosphere interface, see
Livingston and Hutchinson (1995).
Enclosures cover the surface of soils or sediments in order to restrict the volume of air available
for exchange, so that any net flux between the enclosed air and soil can be measured as a change in
head-space gas concentration. Enclosures (also
known as chambers) are typically low in cost and
Pamela Matson and Allen Goldstein
technology requirements and relatively easy to use.
They are especially useful for addressing research
questions on the biological, chemical, or physical
controls of surface gas exchange. Because chambers are inexpensive and highly mobile, measurements at many different sites are possible, including
sites remote from electricity and security. In addition, collection of gases from enclosures may be
the only option for some gases for which fieldsuitable analytical sensors are not available or are
prohibitively expensive for routine use.
Numerous terms have been used to describe different enclosure designs; we adopt the terminology
used by Livingston and Hutchinson (1995), in
which they describe systems in terms of being
steady state or non-steady state, flow-through or
non-flow-through, and vented or nonvented (Fig.
15.1). In steady state systems, the trace gas concentration gradient that controls molecular diffusion across the air-soil interface is held constant,
typically by using an open-path circulation system
to sweep the enclosed volume with a constant flow
of external air; thus, the trace gas concentration is
similar inside and outside the enclosure. In contrast,
in a non-steady state system, the concentration
within the enclosure gradually increases such that
the concentration gradient is continuously diminishing. Thus, in non-steady state systems, a negative feedback on diffusion is possible, and the dimensions of the chambers and the time they are
deployed have to be carefully selected to minimize
that potential. Non-steady state systems may be
most useful in air-soil systems with very low exchange rates (especially when ambient values are
below the analytical system's detection limit), because they allow concentrations to increase over
time. Steady state systems must have very high analytical precision because small concentration differences are multiplied by larger air flows. On the
other hand, steady state systems cause less perturbation of the concentration gradient and may be
more useful for high-frequency repeat measurements (Denmead 1979).
Enclosures are typically fit with a small-diameter
vent designed to communicate atmospheric pressure changes to the enclosed volume and thus minimize the effects of the chamber on mass flow
across the interface; in nonvented systems, advective transport is suppressed. In addition, venting
also allows compensation for air sample with-
facilities and funding available to the investigator.
In the following sections, we outline some of the
most commonly used methods for estimation of
trace gas exchange, including enclosure and micrometeorological approaches. We devote most of our
attention to issues surrounding enclosure approaches, given that micrometeorological approaches are addressed in other chapters of this
book. Our discussion of these approaches draws
heavily on material published in Biogenic Trace
Gases: Measuring Emissions from Soil and Water
(Matson and Harriss 1995), and we direct interested
readers to that volume and other literature cited
here for more detailed information.
Enclosure Methods
Gas Exchange at the Soil-Air Interface
Gas transport across the soil-air interface is primarily a function of advection (mass flow) and diffusion (Nazaroff 1992; Livingston and Hutchinson
1995). Molecular diffusion across the interface is
controlled by the trace gas concentration gradient
and the diffusivity of the gas in the soil-water matrix. Rates of gas transport in soils and sediments
are typically much reduced compared with those in
air, because the physical structure (e.g., soil particles) increases the pathlength that gas molecules
must travel to the atmosphere (tortuosity) and because soil water obstructs many of the diffusive
pathways. Not surprisingly, then, soil textural differences affect the exchange of gases. Likewise, the
presence of water in soils and sediments reduces
the volume of air-filled pore spaces and decreases
diffusivity, reducing advective transport and diffusional transport in the soils. Variation in these transport processes, together with differences in production and consumption of gases (Firestone and
Davidson 1989), account for the large spatial and
temporal variation for which trace gas fluxes are
notorious. For complete discussion of the gas transport processes at the soil-atmosphere interface, see
Livingston and Hutchinson (1995).
Enclosures cover the surface of soils or sediments in order to restrict the volume of air available
for exchange, so that any net flux between the enclosed air and soil can be measured as a change in
head-space gas concentration. Enclosures (also
known as chambers) are typically low in cost and
Pamela Matson and Allen Goldstein
technology requirements and relatively easy to use.
They are especially useful for addressing research
questions on the biological, chemical, or physical
controls of surface gas exchange. Because chambers are inexpensive and highly mobile, measurements at many different sites are possible, including
sites remote from electricity and security. In addition, collection of gases from enclosures may be
the only option for some gases for which fieldsuitable analytical sensors are not available or are
prohibitively expensive for routine use.
Numerous terms have been used to describe different enclosure designs; we adopt the terminology
used by Livingston and Hutchinson (1995), in
which they describe systems in terms of being
steady state or non-steady state, flow-through or
non-flow-through, and vented or nonvented (Fig.
15.1). In steady state systems, the trace gas concentration gradient that controls molecular diffusion across the air-soil interface is held constant,
typically by using an open-path circulation system
to sweep the enclosed volume with a constant flow
of external air; thus, the trace gas concentration is
similar inside and outside the enclosure. In contrast,
in a non-steady state system, the concentration
within the enclosure gradually increases such that
the concentration gradient is continuously diminishing. Thus, in non-steady state systems, a negative feedback on diffusion is possible, and the dimensions of the chambers and the time they are
deployed have to be carefully selected to minimize
that potential. Non-steady state systems may be
most useful in air-soil systems with very low exchange rates (especially when ambient values are
below the analytical system's detection limit), because they allow concentrations to increase over
time. Steady state systems must have very high analytical precision because small concentration differences are multiplied by larger air flows. On the
other hand, steady state systems cause less perturbation of the concentration gradient and may be
more useful for high-frequency repeat measurements (Denmead 1979).
Enclosures are typically fit with a small-diameter
vent designed to communicate atmospheric pressure changes to the enclosed volume and thus minimize the effects of the chamber on mass flow
across the interface; in nonvented systems, advective transport is suppressed. In addition, venting
also allows compensation for air sample with-