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OCR for page 141
instrumentation Development
Needs for Use of Mass-Balance Technique
BY D. LENSCHOW
In Part I of this report, several of the investigations of
the proposed research program require the measure-
ment of fluxes of chemicals. Such measurements are
required in the mass-balance or box-budget experi-
ments proposed under the general heading "Noncyclic
Transformation and Removal," and also for field stud-
ies of dry deposition and for advanced experiments un-
der the heading "Sources of Atmospheric Chemicals,
Biological and Abiological." In~this brief section, we
outline the principles of such measurements and the
general requirements they place on instrumentation.
The budget for the mean concentration of a species s
can be written as
i dxi in s s (8.1)
where the overbar denotes an average over a time or
horizontal distance long enough to obtain a stable esti-
mate, and the prime denotes a departure from the
mean. For aircraft flux measurements, the time or dis-
tance over which a mean is calculated is typically at least
5 min or 25 km. This equation states that the time rate of
change, plus the mean advection of s, plus the vertical
turbulent flux divergence of s is equal to the mean inter-
nal sources Ps and sinks Ls of s. Integrating vertically and
applying (8.1) to a convective boundary layer that is
vertically well mixed to a depth h and capped with a well-
defined inversion layer,
141
8
h at +
OCR for page 142
142
to direct eddy correlation measurements (Hicks et al.,
1980~. These techniques include (1) box methods,
where an enclosure is placed over a surface of interest
and the time rate of change of the species concentration
is measured; and (2) profile or gradient methods, where
differences in concentration at several levels close to the
surface can be related to flux at the surface. These meth-
ods require only mean concentration measurements.
However, the box method assumes that the enclosure
does not affect the surface flux, or at least that such
effects can be corrected, and the profile method requires
accurate measurements of concentration differences (on
the order of 1 percent of the mean concentration or
better), as well as supporting micrometeorological mea-
surements to determine the surface stress and stability.
Further details on use of different platforms and sensors
for measurements of fluxes and other micrometeorolog-
ical variables are presented by Dobson et al. (1980) and
in a special issue of Atmosph~r~c Technology. ~
Absolute accuracy for turbulence measurements is
not important, as long as the sensor output does not drift
significantly during the period of a flux-measuring run
(typically about 10 min), and the sensor gain is known
accurately. This is because a flux measurement is calcu-
lated from departures from a mean value; the mean of
each quantity is not used in the calculation of a vertical
turbulent flux. However, the fluctuations must be re-
solved. In a convective boundary layer where the fluctu-
ations are generated by a flux at the surface, the mea-
sured resolution of a trace species should be at least
about 10 percent of its surface flux divided by the con
. .
vect~ve ve oc~ty,
w* = [(glT) (W'8~)oh]~/37
where (w'b')O is the surface virtual temperature flux and
g/T is the buoyancy parameter (gravitational accelera-
tion divided by the mean temperature). Typically, w* is
of the order of 1 m/s.
In order to resolve the eddies important for vertical
transport through most of the convective boundary
layer, a sensor must resolve wavelengths at least as small
as 30 m and preferably 5 m. Close to the surface, the
requirements are even more stringent since there the
eddies scale with height above the ground. At 10-m
height, for example, wavelengths as small as 3 m must
be resolved accurately to measure all the significant con-
tributions to the turbulent flux (Kaimal et al., 19723.
For a stably stratified boundary layer (e. g., the clear-air
"Instruments and Techniques for Probing the Atmospheric
Boundary Layer." Atmospheric Technology No. 7, 1975, edited by
D. H. Lenschow. Available from National Center for Atmospheric
Research, PO. Box 3000, Boulder, CO 80307-3000.
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
nocturnal boundary layer over land), somewhat smaller
spatial resolution is required to resolve the turbulent
fluctuations. A sample rate of 50 per second would be
desirable if aircraft measurements are proposed for this
situation.
For measurements above the boundary layer, in the
free troposphere and lower stratosphere, a slower sam-
pling rate seems adequate. Possibly two per second
would be satisfactory. If at all possible, however, a higher
rate (say, 5 to 10 per second) should be used.
Whatever rate is used, it should be remembered that
the Nyquist frequency (i.e., the maximum frequency
that can be resolved by a spectral decomposition of the
data) is half the sampling rate. Thus a 20-per-second
sampling rate will resolve 10-Hz spectral variables. At
an aircraft speed of 100 m/s, this is equivalent to a 1 O-m
wavelength.
Another factor to remember is that a first-order in-
strument time constant must be multiplied by ~2,r
before taking the reciprocal in order to estimate the
response of the instrument in the frequency domain.
Thus, if one has a first-order time constant of 0.1 s, the
amplitude of its variance is reduced to 72 percent of the
variance of the input signal at a frequency of 1 Hz. For
flux calculations, the phase angle between the input and
the output is also important. At 1 Hz, the output signal
lags the input by 32° in the above example. This lag
reduces the contribution to the eddy flux at 1 Hz to 85
percent of the input signal. This phase-angle require-
ment means that sampling of variables must be simulta-
neous, or at well-defined time intervals so that correc-
lions can be made, if necessary, to match the phase
angles of variables before calculating fluxes.
For flux measurements, measurement noise that is
not correlated with the vertical velocity does not contrib-
ute to the vertical flux. The noise may, however, necessi-
tate a longer averaging time. This is important to keep
in mind for measurements of trace gases that involve
counting a limited number of photons. In this case, the
noise is likely to have a flat "white noise" spectrum with
a Poisson distribution, while the signal spectrum typi-
cally decreases with frequency with a-5/3 slope. Thus
the noise may surpass the signal above some frequency
that depends on the magnitude of both the signal and the
noise.
By evaluating the left side of (8.1), it is possible to
estimate the net internal production or loss of s (Ps and
Ls) This has been done to estimate the net photochemi-
cal production of O3 in the boundary layer (Lenschow et
al., 1981). In addition, the species flux at low levels in
the boundary layer is a direct measurement of surface
deposition (or emission) if chemical reactions are not
significant on a time scale of a few hundred seconds or
less. Even if the lowest measurement level is several tens
OCR for page 143
INSTRUMENTATION DEVELOPMENT NEEDS
of meters above the surface, the surface flux may be
estimated by extrapolating to the surface a flux profile
obtained from measurements at several levels through
the boundary layer, since the flux profile is linear for a
conserved species in a horizontally homogeneous well-
mixed boundary layer.
Because of the usefulness of resolving turbulent fluc-
tuations of trace species in the boundary layer, it is im-
portant to develop this capability for a larger number of
species. In this way, measurements of chemical and pho-
tochemical production and loss, surface sources and
sinks, and transport through and across the top of the
boundary layer can be obtained for direct comparison
with model predictions, or as fundamental data in them-
selves.
In evaluating the mean concentration budget, the
horizontal mean advection term is obtained by measur-
ing the horizontal gradient of a species s. The required
accuracy of this measurement can be estimated by as-
suming that the removal at the surface is equal to the
horizontal advection term. Accuracy of surface removal
can be specified in terms of a deposition velocity, which is
defined as va' = - (w's')O/s. Equating the surface removal
to the advective term in (8.2), one obtains
s ~ ( uh )
(8.3)
where L is the horizontal distance across which the dif-
ference bs-s2 -so is measured. As an example to
illustrate the magnitude of the horizontal changes ex-
pected in an aircraft experiment, let L = 105 m, u = 5 m/
s, and h = 103 m. Thus
as =20tsm-~]v~.
(8.4)
For many species, a reasonable accuracy goal for mea-
suring vet is 5 x 10-4 m/s. Thus bs/s = 1 percent. In
many cases, L can be increased by as much as a factor of
10. Therefore, bs/s = 1 to 10 percent.
A potential alternative to eddy correlation flux mea-
surements is implementation of the eddy accumulation
technique (Desjardins, 1977~. In this technique, mean
concentrations of trace species in two gas samples are
measured. The rate of flow ofthe sampler is controlled to
be proportional to the magnitude of the vertical air ve-
locity. One sample is obtained from upward moving air,
and the other is obtained from downward moving air.
The difference in concentrations between the two air-
streams is proportional to the vertical flux. The main
advantage of this technique is that fast-response concen-
tration measurements are not required for flux mea-
surements; instead, fast-response, accurate, and sensi-
tive air flow control is necessary. However, very accurate
mean concentration differences are required. These lat
143
ter two requirements may preclude application of this
technique for many species, particularly if their removal
rate is small.
Surface-tower techniques provide an important com-
plement to the aircraft methods described above.
Whereas aircraft provide direct measurements of spatial
averages of dry deposition fluxes, tower instrumenta-
tion provides a more detailed investigation of the factors
that control these fluxes on a time-evolving basis. A
comprehensive study would necessarily involve both
techniques. Instrumentation developed to meet the re-
quirements for aircraft eddy flux applications will also
satisfy the requirements for tower operation. In some
cases, however, the requirement for rapid response can
be relaxed slightly, and sometimes it can be replaced by a
demand for extremely accurate difference measure-
ments. This is the case if the desire is for instruments
suitable for measurement of concentration gradient in-
stead of covariance.
One of the important applications of direct flux mea-
surements is to provide detailed knowledge of deposition
velocities for various species, and the variables that de-
termine them. These deposition velocities can then be
used in numerical models to parameterize surface
fluxes. Field verification of these modeling studies re-
quires concentration data from a network of surface
observation sites. Simple, but reliable sampling meth-
ods need to be developed for this purpose. Methods
analogous to high-volume filtration for airborne parti-
cles appear to offer special promise. Such methods are
already in operation in some networks (e.g., in Canada
and Scandinavia), and the methods need to be im-
proved to permit routine and inexpensive operation on a
global basis.
BIBLIOGRAPHY
Desjardins, R. E., 1977. Energy buclget by an eddy correlation
method.J. Appl. Meteorol. 16:248-250.
Dobson, F., L. Hasse, and R. Davis, 1980. Air-Sea Interaction Instru-
mentsar~dMethods. Plenum, New York, 801 pp.
Hicks, B. B., M. L. Wesely, and J. L. Durham, 1980. Critique of
methods to measure dry deposition: Workshop summary. EPA-
600/9-80-050. Environmental Protection Agency, Washington,
D.C., 83 pp.
Kaimal,~J. C., J. C. Wyngaard, Y. Izumi, and O. R. Cote, 1972.
Spectral characteristics of surface-layer turbulence. Quart. J. Roy.
Meteorol. Soc. 98:563-589.
Lenschow, D. H., R. Pearson, Jr., and B. B. Stankov, 1981. Esti-
mating the ozone budget in the boundary layer by use of aircraft
measurements of ozone eddy flux and mean concentration. J.
Geophys. Res. 86:7291-7297.
Lenschow, D. H., R. Pearson, Jr., and B. B. Stankov, 1982. Mea-
surements of ozone vertical flux to ocean and forest. I. Geophys.
Res. 87:8833-8837.
Lilly, D. K., 1968. Models of cloud-topped mixed layers under a
stronginversion. Quart. J. Roy. Meteorol. Soc. 94:292-309.
Representative terms from entire chapter:
flux measurements
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