Micrometeorological studies of NO{sub x} and O{sub 3} fluxes above agricultural crops Page: 3 of 5
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8.1
MICROMETEOROLOGICAL STUDIES OF NOX AND 03 FLUXES ABOVE AGRICULTURAL CROPS
M. L. Wesely,* W. Gao, D. R. Cook, T. J. Martin, and J. Qiu
Environmental Research Division
Argonne National Laboratory, Argonne, IL1. INTRODUCTION
The emission of NO from microbial processes in soils
can contribute significantly to levels of NOx (NO + NO2)
in the planetary boundary layer and can lead to
modifications of 03 concentrations there. Accordingly,
the rates of NO emission should be simulated in
atmospheric chemistry models used to investigate
methods of reducing tropospheric concentrations of 03.
Developing good simulations of the rates is complicated
by the large seasonal and spatial variability of NO
emissions from soils (e.g., Williams et al., 1992). The
availability of efficient methods of surveying NO
emissions would result in more complete information for
evaluating the model outputs. In addition, the
atmospheric chemistry models need to take into account
the rapid conversion of NO to NO2 in the lower
atmosphere (e.g., Gao et al., 1991; Kramm, 1989)
because this rapid conversion permits some of the N02
to be removed by dry deposition before it mixes into the
atmospheric boundary layer above the surface layer.
Such small-scale processes usually must be
parameterized rather than explicitly simulated in
regional- or large-scale atmospheric numerical models
used to construct tropospheric budgets of NOx. Here,
preliminary results are described from experimental
efforts (1) to evaluate methods of estimating NO
emissions from soils and (2) to gather information that
could lead to parameterization of the rapid conversion of
NO to NO2 in the lowest few meters of the atmosphere.
2. MEASUREMENTS
Measurements of the vertical mass flux densities of
NO, N02, and 03 were made in rural eastern North
Carolina (350 44' 47" N and 760 41' 14" W) in a field 6f
maize in 1995 and in the same field planted with
soybeans in 1996, as part of a collaborative,
multiagency-supported study known as the Natural
Emission of Oxidant Precursors-Validation of
Techniques and Assessment (NOVA) project. The
primary objectives and methods of NOVA were
described by Aneja (1994). Several observational
approaches were taken, including the use of
environmental enclosures placed on the soil surface.
Here, the focus is on N02 and 03 fluxes measured by
Argonne National Laboratory by application of the
micrometeorological approach of eddy correlation.
*Corresponding author address: Marvin L. Wesely,
Bldg. 203, ER, Argonne National Laboratory, Argonne,
IL 60439; e-mail <mlwesely@anl.gov>.The eddy fluxes of NO2 and 03 were measured
simultaneously at heights of 5 m and 10 m above the
soil surface during a two-week period in May 1995 and
during a four-week period in July and August of 1996.
In 1995, the canopy of maize was not closed because
the plants were immature; the canopy height changed
from approximately 40 cm to 70 cm during the course of
the two weeks. The field of maize extended at least
1 km to the southwest, the favored wind direction for
data reported here. The corn rows were oriented
southeast-northwest and were spaced about 0.91 m
apart. Nitrogen fertilizer (urea, NH4+, and N031 had
been applied at the rate of approximately 73 kg N ha-1
(30% solution) at planting, and another
102 kg N ha-1was applied on May 20. During the 1996
effort, the soybean canopy was closed and ranged in
height from 40 cm to 100 cm over the four-week
experimental period. No fertilizer had been applied
previously to the soybean field, but an area to the
southwest of the measurement site was fertilized with
nitrate on July 31 in an attempt to increase the amount
of NO emitted from the soils.
The eddy correlation measurements were made with
three-dimensional sonic anemometers in conjunction
with fast-response 03 analyzers that detected the
vapor-phase chemiluminescent reaction of 03 with
C2H4 and with fast-response N02 analyzers that
detected chemiluminescence produced when NO2
contacted surfaces wetted with a specially formulated
solution. The response times of these analyzers were
approximately 0.2 s for 03 and 0.7 s for NO2. In
addition, sensible heat fluxes were measured by using
temperature signals from the sonic anemometers, and
water vapor fluxes were measured by using krypton
hygrometers. The turbulence data were sampled at a
rate of 20 s 1, and a running mean filter of 200 s was
applied in the computation of the covariances of the
vertical velocity with the scalar quantities. Subsequent
analysis made adjustments to compensate for the
limited time response of the chemical sensors and for
the effects of density variations on the flux calculations.
3. RESULTS FROM 1995
The downward (negative) 03 fluxes were
consistently greater in magnitude at 10 m than at 5 m,
while the N02 fluxes were directed upward (positive)
and were larger in magnitude at the 10-m level
(Figure 1). These changes with fluxes with height were
apparently caused by reactions involving the large
amounts of NO emitted from the soil in the fertilized
maize field. Data from environmental enclosures placed
on the soil surface indicated that NO was emitted at a
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Wesely, M. L.; Gao, W.; Cook, D. R.; Martin, T. J. & Qiu, J. Micrometeorological studies of NO{sub x} and O{sub 3} fluxes above agricultural crops, article, March 1, 1997; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc676870/m1/3/: accessed May 7, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.