170
nique, there are two other main sets of corrections
that need to be applied:
1. The concentration of a trace gas may require
correction if its partial pressure is measured
rather than mixing ratio, as changes to air density arise from the simultaneous transfer of sensible and latent heat.
2. Corrections that apply to the whole system arise
from the unavoidable fact that any measuring
system has some influence on the measurement
itself and no sampling system can be regarded
as responding perfectly to all flux-carrying eddies. Although eddy covariance is the most direct micrometeorological method for measuring
surface fluxes, the limitations of sensor response
and data acquisition hardware introduce some
uncertainty in the flux measurements.
We can summarize the required corrections as:
1. corrections to constituent density because of simultaneous transfer of sensible heat (to be discussed below);
2. the sensor may not be capable of responding
quickly enough to all of the flux-carrying eddies
moving past it;
3. the sensor for w may be too far from the scalar
sensor and flux loss can occur (by reducing the
covariance between the signals);
4. the entity in question must be averaged to some
extent by the finite sampling volume of the sensor, for example, typically 20 cm between sonic
transducers or between the source and detector
of infrared radiation in an open-path analyzer;
5. the data acquisition hardware may not sample
rapidly enough to capture high-frequency eddies; and
6. in the special case of using a closed-path analyzer, when the air is brought to the analyzer
through a sampling tube, some meters long,
some of the turbulent structure will be lost,
particularly the higher frequencies and, unless
accounted for, this will result in flux underestimation.
Corrections for Changes in Air Density
A paper by Webb, Pearman, and Leuning (1980),
hereafter WPL, drew attention to the need to consider corrections to the measured flux because of
changes to air density. The simultaneous transfer of
John B. Moncrieff, Paul G. Jarvis, and Ricardo Valentini
sensible and latent heat causes fluctuations in air
density that can be erroneously attributed to fluctuations in CO 2 and latent heat in sensors that measure the partial density of CO 2 or H 2 0 in air. A
number of papers have explored the consequences
of the WPL corrections; for example, Leuning et
al. (1982) provided experimental proof of the WPL
corrections, Leuning and Moncrieff (1990) developed the arguments for a closed-path system, and
Leuning and King (1992) applied the equations to
both open- and closed-path systems. For a closedpath system, the relationship between the CO 2 density in air (Pc) and that measured inside the optical
bench of the infrared gas analyzer (IRGA) is
r PIi ]
Pc = [AT Pci
(11.20)
where the subscript i refers to conditions inside the
optical bench. The final working equation for CO2
measured by a closed-path analyzer is
F - [PTi][-I-I
{ii 1- - , - ,
c - Pit w Pci + flV'c'Pa)W Pvi] (11.21)
This equation assumes that both water vapor and
CO 2 are brought to a common temperature and
pressure within the optical bench. This is true for
the Li-Cor 6262, an IRGA commonly used in
closed-path systems; if the latent heat flux is measured by an open-path instrument, the latent heat
flux has to be WPL corrected also before it can be
used in Equation 11.8 (Leuning and Moncrieff,
1990). In a closed-path system, evaporation can be
found from:
[ PTi ]
-
E = pl (1 + fla)w'p~i (11.22)
where E is water vapour flux; fl = m/mv, the ratio
of the molecular masses of dry air to water vapor;
a = pj Pa, the ratio of mean water vapor density
to that of dry air; Tis Kelvin temperature; the overbars indicate time averages; and the primes denote
fluctuations.
Instrumentation and Site Requirements
for Eddy Covariance
Sensors
The primary requirement for sensors that are to be
used in eddy covariance is the ability to respond to
the full range of flux-carrying eddies. If we refer to
nique, there are two other main sets of corrections
that need to be applied:
1. The concentration of a trace gas may require
correction if its partial pressure is measured
rather than mixing ratio, as changes to air density arise from the simultaneous transfer of sensible and latent heat.
2. Corrections that apply to the whole system arise
from the unavoidable fact that any measuring
system has some influence on the measurement
itself and no sampling system can be regarded
as responding perfectly to all flux-carrying eddies. Although eddy covariance is the most direct micrometeorological method for measuring
surface fluxes, the limitations of sensor response
and data acquisition hardware introduce some
uncertainty in the flux measurements.
We can summarize the required corrections as:
1. corrections to constituent density because of simultaneous transfer of sensible heat (to be discussed below);
2. the sensor may not be capable of responding
quickly enough to all of the flux-carrying eddies
moving past it;
3. the sensor for w may be too far from the scalar
sensor and flux loss can occur (by reducing the
covariance between the signals);
4. the entity in question must be averaged to some
extent by the finite sampling volume of the sensor, for example, typically 20 cm between sonic
transducers or between the source and detector
of infrared radiation in an open-path analyzer;
5. the data acquisition hardware may not sample
rapidly enough to capture high-frequency eddies; and
6. in the special case of using a closed-path analyzer, when the air is brought to the analyzer
through a sampling tube, some meters long,
some of the turbulent structure will be lost,
particularly the higher frequencies and, unless
accounted for, this will result in flux underestimation.
Corrections for Changes in Air Density
A paper by Webb, Pearman, and Leuning (1980),
hereafter WPL, drew attention to the need to consider corrections to the measured flux because of
changes to air density. The simultaneous transfer of
John B. Moncrieff, Paul G. Jarvis, and Ricardo Valentini
sensible and latent heat causes fluctuations in air
density that can be erroneously attributed to fluctuations in CO 2 and latent heat in sensors that measure the partial density of CO 2 or H 2 0 in air. A
number of papers have explored the consequences
of the WPL corrections; for example, Leuning et
al. (1982) provided experimental proof of the WPL
corrections, Leuning and Moncrieff (1990) developed the arguments for a closed-path system, and
Leuning and King (1992) applied the equations to
both open- and closed-path systems. For a closedpath system, the relationship between the CO 2 density in air (Pc) and that measured inside the optical
bench of the infrared gas analyzer (IRGA) is
r PIi ]
Pc = [AT Pci
(11.20)
where the subscript i refers to conditions inside the
optical bench. The final working equation for CO2
measured by a closed-path analyzer is
F - [PTi][-I-I
{ii 1- - , - ,
c - Pit w Pci + flV'c'Pa)W Pvi] (11.21)
This equation assumes that both water vapor and
CO 2 are brought to a common temperature and
pressure within the optical bench. This is true for
the Li-Cor 6262, an IRGA commonly used in
closed-path systems; if the latent heat flux is measured by an open-path instrument, the latent heat
flux has to be WPL corrected also before it can be
used in Equation 11.8 (Leuning and Moncrieff,
1990). In a closed-path system, evaporation can be
found from:
[ PTi ]
-
E = pl (1 + fla)w'p~i (11.22)
where E is water vapour flux; fl = m/mv, the ratio
of the molecular masses of dry air to water vapor;
a = pj Pa, the ratio of mean water vapor density
to that of dry air; Tis Kelvin temperature; the overbars indicate time averages; and the primes denote
fluctuations.
Instrumentation and Site Requirements
for Eddy Covariance
Sensors
The primary requirement for sensors that are to be
used in eddy covariance is the ability to respond to
the full range of flux-carrying eddies. If we refer to