Micromet Methods for Determining Fluxes of Nitrogen Species

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Micromet Methods for DeterminingFluxes of
Nitrogen Species

• Tilden P. Meyers
• NOAA/ARL
• Atmospheric Turbulence and Diffusion Division
• Oak Ridge, TN

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Presentation Objectives

• Discuss various methodologies to measure NH3
  exchange
• Highlight specific sampling considerations and
  guidelines for each method
• Present results from various field experiments

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All micrometeorological methods are constrained
by 1. Fluxes don't vary in X,Y plane in the
flux footprint 2. The exchange at the surface
is same as that at sensor height
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For NH3, we often are interested in
characterizing deposition as well as emission
Emission sources of NH3 generally are
characterized by small areas that have spatial
scales less than a typical flux footprint (swine
lagoons, poultry farms, etc.)
Deposition targets are vegetation (grasslands,
forests, crops, soil) that can have considerable
spatial extent, but are often located in areas
that have small NH3 concentrations
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For characterizing NH3 emissions
Integrated Mass Balance
Average emission rate what is advected past
tower
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Flux
Similarly, inverse Lagrangian methods have been
used to estimate the mean source area emission
strength from measurements of NH3 concentrations
downwind of the source area. (Flesch et al,
1995, J. Applied Meteorology) (Kljun, et al.,
2002, Boundary-Layer Meteorology)
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The application of micromet methods is not
limited by the micromet state of the art but by
the current methods used to measure the trace gas
of interest (NH3). Fast time response (gt 1
Hz) ? eddy covariance Slow but accurate ?
gradient methods (AM,MBR) Accumulation methods ?
(conditional sampling (REA)
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Eddy covariance
Emission ? flux w,c, w-,c-,
?

Deposition ? flux- w-,c, w,c-,
?
-
Eddy covariance is inherently a noise rejection
method as the high number of samples in an
averaging period (30 min 60 min) will average
out if errors are random with a mean of 0.
Useful results can be obtained even with low
signal/noise ratios.
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Advantages of Eddy Covariance

• Good time resolution
• Inherently a noise rejection method gt many
  samples

Considerations

• Usually requires major power for pumps, etc.
• Usually not all weather instrumentation
• If using sampling tubes, tubes, and inlet losses

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Gradient (Modified Bowen Ratio Method)

• Measure the flux (eddy covariance) and vertical
  gradient of constituent (heat, water vapour,
  CO2) over specified height.
• Compute effective transfer coefficient
  (flux/gradient)
• Measure vertical gradient of NH3 over same
  height interval and apply computed transfer
  coefficient to obtain a measure of the flux

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Water vapor and CO2 gradients
To remove bias error, use same analyzer for both
heights, switching at 30 sec to 5 min intervals
and allowing for representative sampling.
Relative error
For Tc 60 s, ? 30 s, ? 10
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Advantages of MBR

• Good for slow response trace gas sensors
• Adequate time resolution (30 min -gt hour)

Considerations

• Bias tests on surrogate scalars
• Bias test on trace gas gradient systems
• Sampling tube and inlet losses

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Conditional SamplingRelaxed Eddy Accumulation
For sampling gases and aerosols in accumulation
devices like annular denuders, filterpacks, etc.
Flux ??w(Cup - Cdn)
? empirical coefficient, 0.6 ?w standard
deviation vertical velocity Cup, Cdn average
concentration of updrafts, downdrafts
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What constitutes an updraft, etc.?
Separate w into three bins deadband /- 0.10
m/s ? dead accumulator updraft gt 0.1 m/s
? up accumulator downdraft lt -0.1 m/s
? down accumulator
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Downdraft
Updraft
Deadband (mid)
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Ammonia Fluxes (REA)

• USDA/ARS-BARC J. Meisinger
• NOAA/ARL W. Luke
• 20 l/min flow
• cyclone/impactor 2.5 ?m cut-point
• citric acid (phosphoric) coated denuders
• 3-4 hour sample intervals

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These plants could use a drink.......mmh
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Average Loss 1.5 kg N/ha/day
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Advantages of REA

• When used with denuders and filterpacks, can
  sample several species at once (NH3, NH4, SO2,
  SO4, HNO3, NO3)

Considerations

• Very manual intensive with denuders (cleaning,
  coating, exposing, extracting, IC analysis
• Sample flows and extraction volumes need to be
  measured very accurately