NOVA Field Experiment
mid-July to mid-August 1996, scientists from NOAA's Air Resources
Laboratory participated in the Project NOVA (Natural emissions
of Oxidant precursors: Validation of techniques and Assessment)
field experiment in rural northeastern North Carolina. The
primary purpose of the project was to compare different methodologies
for estimating the release of nitric oxide (NO) from agricultural
soils. Secondary goals of the project included the quantification
of emission estimates of NO from agricultural crop land,
and characterization of tropospheric photochemistry at a
rural site in the southeastern U.S.
site of the NOVA field project was a private farm near
Plymouth, NC. The experiment was conducted over a field
of soybeans, with an upwind fetch of 1 kilometer. Fast-response
NO sensors were deployed at 3, 5, and 10 meters to provide
emission flux estimates via eddy correlation. Additionally,
eddy correlation flux measurements of NO2 and O3 were also
conducted, by scientists at ATDD and Argonne National Laboratory
(ANL). Slow-response concentration measurements of NO,
NOx, and O3 were conducted by ARL to provide selective,
specific measurements with which the photochemical perturbation
to the NO emission flux profiles could be inferred. Scientists
from ARL/ATDD deployed two eddy correlation systems (at
3 and 10 m), consisting of fast response measurements of
3-D winds and temperature (sonic anemometers); ozone (coumarin
chemiluminescence); NO2 (luminol chemiluminescence), and
water vapor and CO2 (IRGA). Additionally, a flux-gradient
tower was erected to provide gradient measurements of temperature
and water vapor concentrations, and wind speed and direction;
eddy diffusivities/transfer coefficients of sensible and
latent heat, and momentum were thus derived. Soil moisture
and heat fluxes were also measured, as were net radiation,
PAR, and UV radiation, to investigate and close the energy
balance at the site.
the impact of Hurricane Bertha before, and numerous heavy
and prolonged precipitation events during the project,
NOVA was an operational success, and most systems performed
well. Only 5-6 days of in-sector winds (SW flow) were observed,
making for a limited data set. However, initial observations
show excellent agreement among the three eddy correlation
measurements of NO, excellent agreement between static
and dynamic chamber (enclosure) methods, and reasonable
agreement between the micrometeorological and enclosure
techniques. The three fast response NO sensors were co-located
at 10 m for a side-by-side flux intercomparison at the
end of the project. Nitric oxide calibration standards
were cross-compared to ensure consistency of calibration.
Finally, while nitrogen-fixing crops such as soybean are
not generally fertilized, arrangements were made with the
farmer to apply ammonium nitrate fertilizer to a portion
of the field, in order to stimulate NO emission. Nitric
oxide emission fluxes of up to 1 ng N m-2 s-1 were observed
prior to fertilization; emission rates approximately doubled
with the application of NH4NO3
- To Compare NO Emission fluxes Derived from Micrometeorological
Techniques with Those from Chamber (Enclosure) Methods.
- To Quantify the NO Emission Flux from Agricultural
Lands Representative of the Southeastern U.S.
- To Measure Flux Profiles of NO and NO2 and to Investigate
the Role of Tropospheric Photochemistry in Modifying
the Surface Fluxes of NO and NO2 (Flux Divergence).
- To Characterize the Air Quality and Tropospheric Photochemistry
at a Rural Site in the Southeastern United States.
|Environmental Protection Agency,
In-Kind Contributions from:
- NIST-Traceable Calibration and Flow Measurement Standards
- Baseline Correction and Twice-Daily Calibrations of
ARL NO and NOx Sensors
- Cross-Calibrations of All NO Calibration Standards
(NOAA, NASA, UMD, NCSU)
- Common Sonic Anemometry and NO Flux Measurements at
10 m (NOAA, DOE. UMD)
Preliminary Results -- DO NOT CITE!
- Mid-Day NO emission fluxes from a soybean field in
July-August 1996 were typically less than 1 ng N/m2 sec,
and almost always less than 2 ng N/m2 sec. Emission fluxes
doubled immediately after fertilization with NH4NO3 (ca.
- Under convectively unstable conditions and light winds,
all 3 E/C NO flux estimates agreed well with one another
- Enclosure (static and dynamic) measurements of NO
emission flux agreed well with each other, but only sometimes
agreed with micromet methods.
- Cross calibration of NO standards revealed significant
(>40%) errors in some of the calibration standards. Correcting
for calibration errors significantly improved agreement.