174
flux, no such closure scheme is possible. Consequently, one tries to test either individual components of the system or the system as a whole by
comparing its behavior against some parameter that
is based on well-established and, hopefully, rigorous theory. For example, the adequacy of the frequency response of the eddy covariance system can
be tested by spectral analysis of the scalar fluxes.
In particular, because of the attenuation of fluctuations through the tube in a ducted system, the frequency response for CO 2 and H 2 0 fluxes is significantly reduced. One way to evaluate the magnitude
of this problem is to compare the sensible heat flux
density co-spectra, which is measured at the entrance to the tube, with the H20 and CO2 flux density co-spectra measured in the analyzer. The difference, usually found at the high-frequency end of
the power spectra, can be used to account for the
loss of fluxes, determined by frequency losses
within the system (Leuning and Moncrieff 1990).
Conditional Sampling
Conditional sampling is a conceptually simple micrometeorological technique in which air is sampled into one of two sampling reservoirs (or sampling lines for on-line analysis), according to
whether an up- or down-draught of air is measured
by a sonic anemometer. After a suitable interval of
time, say 30 to 60 minutes, the net vertical flux of
the trace gas species of interest is proportional to
the difference in gas concentration between the
sampling reservoirs. The method is attractive as it
can be used for trace gases or pollutants for which
no suitable fast-response sensor is available and
hence the alternative technique of eddy covariance
is unsuitable (e.g., Majewski et al. 1993). The requirement for a fast sensor to measure vertical
windspeed remains, but there is considerable relaxation in the speed requirement for the chemical analyzer. A further advantage is that by accumulating
gas into reservoirs, the difference between the samples is enhanced and it is then possible to use highprecision gas analyses in the laboratory to determine the differences (Businger and Delaney 1990)
The method was first suggested by Desjardins
(1977) for sensible heat flux and given the name
eddy accumulation. Although attractive in principle, early studies using this method sampled air at
a rate proportional to the magnitude of the vertical
John B. Moncrieff, Paul G. Jarvis, and Ricardo Valentini
windspeed and this was technically difficult to
achieve with the required accuracy. Hicks and
McMillen (1984) suggested, almost as an aside,
that a simplified method might work which would
eliminate some of the practical difficulties. The
simple method would be to sample air at a constant
rate into either bag rather than at a rate proportional
to vertical windspeed. One bag will then contain air
collected in updraughts and the concentration of
CO 2 , say, would be c +; the downdraught bag will
have a CO 2 concentration of c - . The idea was taken
up by Businger and Oncley (1990) who wrote the
flux (Fc) for a gas with concentration c, as
Fc = {Jaw(c+ - c-)
(11.32)
where {J is an empirical coefficient usually determined by experiment, O"w is the standard deviation
of the vertical windspeed, and (c + - c -) is the gas
concentration difference between the two sampling
bags at the end of the sampling period. As Businger
and Oncley point out, the measurement of (c +
- c -) is a direct difference of concentration not
weighted by the magnitude of vertical wind speed
in the original method. Thus in this method they
relax the conditions for the original eddy accumulation and the method is based simply on sampling
air into different reservoirs on the condition of either upward- or downward-moving air. The technique is known equally as conditional sampling or
relaxed eddy accumulation (CSIREA). Figure 11.6
shows a schematic of a typical conditional sampling system in which fluxes of methane (CH4)
were routinely monitored on-line within a CSIREA
system. In general, the CSIREA method has been
well validated over the past few years although for
fluxes of carbon and water, eddy covariance remains the more appropriate choice (pattey et al.
1992; Oncley et al. 1993). Conditional sampling
has a role in measuring fluxes of biogenic compounds and other trace gases for which no suitable
eddy covariance sensors exist, for example, nitrous
oxide (N 2 0), non-methane hydrocarbons, and CH 4
(Moncrieff et al. 1998).
Errors in Long-Term Measurements
of Fluxes of Carbon and Water
There are a number of sources of uncertainty associated with flux measurements made above the
canopy by the micrometeorological methods de-
flux, no such closure scheme is possible. Consequently, one tries to test either individual components of the system or the system as a whole by
comparing its behavior against some parameter that
is based on well-established and, hopefully, rigorous theory. For example, the adequacy of the frequency response of the eddy covariance system can
be tested by spectral analysis of the scalar fluxes.
In particular, because of the attenuation of fluctuations through the tube in a ducted system, the frequency response for CO 2 and H 2 0 fluxes is significantly reduced. One way to evaluate the magnitude
of this problem is to compare the sensible heat flux
density co-spectra, which is measured at the entrance to the tube, with the H20 and CO2 flux density co-spectra measured in the analyzer. The difference, usually found at the high-frequency end of
the power spectra, can be used to account for the
loss of fluxes, determined by frequency losses
within the system (Leuning and Moncrieff 1990).
Conditional Sampling
Conditional sampling is a conceptually simple micrometeorological technique in which air is sampled into one of two sampling reservoirs (or sampling lines for on-line analysis), according to
whether an up- or down-draught of air is measured
by a sonic anemometer. After a suitable interval of
time, say 30 to 60 minutes, the net vertical flux of
the trace gas species of interest is proportional to
the difference in gas concentration between the
sampling reservoirs. The method is attractive as it
can be used for trace gases or pollutants for which
no suitable fast-response sensor is available and
hence the alternative technique of eddy covariance
is unsuitable (e.g., Majewski et al. 1993). The requirement for a fast sensor to measure vertical
windspeed remains, but there is considerable relaxation in the speed requirement for the chemical analyzer. A further advantage is that by accumulating
gas into reservoirs, the difference between the samples is enhanced and it is then possible to use highprecision gas analyses in the laboratory to determine the differences (Businger and Delaney 1990)
The method was first suggested by Desjardins
(1977) for sensible heat flux and given the name
eddy accumulation. Although attractive in principle, early studies using this method sampled air at
a rate proportional to the magnitude of the vertical
John B. Moncrieff, Paul G. Jarvis, and Ricardo Valentini
windspeed and this was technically difficult to
achieve with the required accuracy. Hicks and
McMillen (1984) suggested, almost as an aside,
that a simplified method might work which would
eliminate some of the practical difficulties. The
simple method would be to sample air at a constant
rate into either bag rather than at a rate proportional
to vertical windspeed. One bag will then contain air
collected in updraughts and the concentration of
CO 2 , say, would be c +; the downdraught bag will
have a CO 2 concentration of c - . The idea was taken
up by Businger and Oncley (1990) who wrote the
flux (Fc) for a gas with concentration c, as
Fc = {Jaw(c+ - c-)
(11.32)
where {J is an empirical coefficient usually determined by experiment, O"w is the standard deviation
of the vertical windspeed, and (c + - c -) is the gas
concentration difference between the two sampling
bags at the end of the sampling period. As Businger
and Oncley point out, the measurement of (c +
- c -) is a direct difference of concentration not
weighted by the magnitude of vertical wind speed
in the original method. Thus in this method they
relax the conditions for the original eddy accumulation and the method is based simply on sampling
air into different reservoirs on the condition of either upward- or downward-moving air. The technique is known equally as conditional sampling or
relaxed eddy accumulation (CSIREA). Figure 11.6
shows a schematic of a typical conditional sampling system in which fluxes of methane (CH4)
were routinely monitored on-line within a CSIREA
system. In general, the CSIREA method has been
well validated over the past few years although for
fluxes of carbon and water, eddy covariance remains the more appropriate choice (pattey et al.
1992; Oncley et al. 1993). Conditional sampling
has a role in measuring fluxes of biogenic compounds and other trace gases for which no suitable
eddy covariance sensors exist, for example, nitrous
oxide (N 2 0), non-methane hydrocarbons, and CH 4
(Moncrieff et al. 1998).
Errors in Long-Term Measurements
of Fluxes of Carbon and Water
There are a number of sources of uncertainty associated with flux measurements made above the
canopy by the micrometeorological methods de-