242
through the chamber and the increase in concentration of the gas is monitored, often after collection
on filter or foil collectors (MacTaggart et al. 1987,
Lamb et al. 1987).
Microrneteorological Approaches
While chambers and enclosures have advantages in
that they are portable and inexpensive, micrometeorological approaches using towers or aircraft
have other important advantages. With them, it is
possible to measure fluxes without disturbance of
the soil, plant, or water surface. Moreover, micrometeorological approaches inherently average over
a surface area that increases with height of the measurements over the surface, and so represent integrated fluxes from a larger proportion of the ecosystem rather than from small plots within it.
Micrometeorological approaches allow the examination of fluxes over continuous time scales from
minutes to years. Finally, in the case of measurements over vegetation canopies, micrometeorological approaches allow the examination of vertical
transport through the entire canopy, not just at a
single interface. For gases such as CO 2 and NO y
that can be emitted or taken up at various points in
the soil-canopy system, micrometeorological approaches are the only way to evaluate actual exchange between ecosystems and the atmospheric
boundary layer. The concept and theories of micrometeorological approaches are described in detail
in a number of books and chapters, including those
by Lenshow (1995), Kaimal and Finnigan (1992),
Baldocchi (1991), Baldocchi et al. (1988), and Lenshow and Hicks (1989). In this book, Chapters 11
and 13 present in-depth discussions of selected micrometeorological techniques and their use in water, energy, and CO 2 exchange. Thus, our discussion of micrometeorological approaches will be
brief.
There are a variety of micrometeorological techniques for measuring trace gas fluxes. As with the
enclosure approach, no single technique is best for
all gases or all situations. For a discussion of eddy
correlation, eddy accumulation, gradient and difference techniques, and mass balance and Bowen
ratio techniques, we recommend the thorough descriptions and explanations of Lenshow (1995) and
Baldocchi (1991) as well as the other chapters in
this book. Micrometeorological approaches were
Pamela Matson and Allen Goldstein
originally designed to be used for flux measurements over horizontally homogeneous surfaces
with long fetch and flat terrain. However, ecological questions are most often addressed in landscapes that are anything but homogeneous. Variations in vegetation type and cover, topography and
parent material, soil type and fertility, soil moisture,
to name only a few, all contribute to the enormous
variation we see in ecological processes and trace
gas flux. While this relatively fine-scale heterogeneity has been one of the factors impeding the use
of micrometeorological approaches in ecology, new
approaches are now being developed to evaluate
the use of these techniques in the heterogeneous
conditions of many natural ecosystems.
One way that surface heterogeneity is being addressed is through estimation of a flux footprint,
which is the contribution of each distinct element
of the surface area in the upwind source region to
the vertical flux measured at a point above the surface. Flux footprints are typically estimated by analytic dispersion models (Horst and Weil 1992),
and Langrangian stochastic dispersion models
(Schuepp et al. 1990). Integrating the flux footprint
to determine the effective source area for the sensor
is fundamentally difficult for quantities that are carried by turbulent diffusion because the footprint is
a constantly varying quantity that depends on characteristics of the air flow, including thermal stability and crosswind turbulence (Schmid 1994, 1997).
The effective source area of a flux measurement
increases strongly with increasing measurement
height, and less strongly with increasing stability
and crosswind turbulence. Tools for estimating the
characteristic dimensions of the area responsible
for the majority of the measured surface fluxes in
heterogeneous environments have been developed
(Schmid 1997). A conservative estimate of an appropriate fetch area during the day is 100 times the
measurement height, with a larger fetch area at
night.
Analytical Methods
for Trace Gases
There is a wide array of analytical approaches
available for the measurement of trace gas concentrations. Some of the instruments and analytical
through the chamber and the increase in concentration of the gas is monitored, often after collection
on filter or foil collectors (MacTaggart et al. 1987,
Lamb et al. 1987).
Microrneteorological Approaches
While chambers and enclosures have advantages in
that they are portable and inexpensive, micrometeorological approaches using towers or aircraft
have other important advantages. With them, it is
possible to measure fluxes without disturbance of
the soil, plant, or water surface. Moreover, micrometeorological approaches inherently average over
a surface area that increases with height of the measurements over the surface, and so represent integrated fluxes from a larger proportion of the ecosystem rather than from small plots within it.
Micrometeorological approaches allow the examination of fluxes over continuous time scales from
minutes to years. Finally, in the case of measurements over vegetation canopies, micrometeorological approaches allow the examination of vertical
transport through the entire canopy, not just at a
single interface. For gases such as CO 2 and NO y
that can be emitted or taken up at various points in
the soil-canopy system, micrometeorological approaches are the only way to evaluate actual exchange between ecosystems and the atmospheric
boundary layer. The concept and theories of micrometeorological approaches are described in detail
in a number of books and chapters, including those
by Lenshow (1995), Kaimal and Finnigan (1992),
Baldocchi (1991), Baldocchi et al. (1988), and Lenshow and Hicks (1989). In this book, Chapters 11
and 13 present in-depth discussions of selected micrometeorological techniques and their use in water, energy, and CO 2 exchange. Thus, our discussion of micrometeorological approaches will be
brief.
There are a variety of micrometeorological techniques for measuring trace gas fluxes. As with the
enclosure approach, no single technique is best for
all gases or all situations. For a discussion of eddy
correlation, eddy accumulation, gradient and difference techniques, and mass balance and Bowen
ratio techniques, we recommend the thorough descriptions and explanations of Lenshow (1995) and
Baldocchi (1991) as well as the other chapters in
this book. Micrometeorological approaches were
Pamela Matson and Allen Goldstein
originally designed to be used for flux measurements over horizontally homogeneous surfaces
with long fetch and flat terrain. However, ecological questions are most often addressed in landscapes that are anything but homogeneous. Variations in vegetation type and cover, topography and
parent material, soil type and fertility, soil moisture,
to name only a few, all contribute to the enormous
variation we see in ecological processes and trace
gas flux. While this relatively fine-scale heterogeneity has been one of the factors impeding the use
of micrometeorological approaches in ecology, new
approaches are now being developed to evaluate
the use of these techniques in the heterogeneous
conditions of many natural ecosystems.
One way that surface heterogeneity is being addressed is through estimation of a flux footprint,
which is the contribution of each distinct element
of the surface area in the upwind source region to
the vertical flux measured at a point above the surface. Flux footprints are typically estimated by analytic dispersion models (Horst and Weil 1992),
and Langrangian stochastic dispersion models
(Schuepp et al. 1990). Integrating the flux footprint
to determine the effective source area for the sensor
is fundamentally difficult for quantities that are carried by turbulent diffusion because the footprint is
a constantly varying quantity that depends on characteristics of the air flow, including thermal stability and crosswind turbulence (Schmid 1994, 1997).
The effective source area of a flux measurement
increases strongly with increasing measurement
height, and less strongly with increasing stability
and crosswind turbulence. Tools for estimating the
characteristic dimensions of the area responsible
for the majority of the measured surface fluxes in
heterogeneous environments have been developed
(Schmid 1997). A conservative estimate of an appropriate fetch area during the day is 100 times the
measurement height, with a larger fetch area at
night.
Analytical Methods
for Trace Gases
There is a wide array of analytical approaches
available for the measurement of trace gas concentrations. Some of the instruments and analytical