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Micromet Methods for Determining Fluxes of Nitrogen Species

Tilden P. Meyers NOAA/ARL Atmospheric Turbulence and Diffusion Division Oak Ridge, TN. Micromet Methods for Determining Fluxes of Nitrogen Species. Discuss various methodologies to measure NH 3 exchange Highlight specific sampling considerations and guidelines for each method

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Micromet Methods for Determining Fluxes of Nitrogen Species

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  1. Tilden P. Meyers NOAA/ARL Atmospheric Turbulence and Diffusion Division Oak Ridge, TN Micromet Methods for DeterminingFluxes of Nitrogen Species

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

  3. 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

  4. 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

  5. For characterizing NH3 emissions Integrated Mass Balance Average emission rate = what is advected past tower

  6. 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)

  7. 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 (> 1 Hz)  eddy covariance Slow but accurate gradient methods (AM,MBR) Accumulation methods  (conditional sampling (REA)

  8. 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.

  9. Advantages of Eddy Covariance • Good time resolution • Inherently a noise rejection method > many samples Considerations • Usually requires major power for pumps, etc. • Usually not “all” weather instrumentation • If using sampling tubes, tubes, and inlet losses

  10. 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

  11. 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%

  12. Advantages of MBR • Good for slow response trace gas sensors • Adequate time resolution (30 min -> hour) Considerations • Bias tests on surrogate scalars • Bias test on trace gas gradient systems • Sampling tube and inlet losses

  13. 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

  14. What constitutes an updraft, etc.? Separate “w” into three bins deadband +/- 0.10 m/s  dead accumulator updraft > 0.1 m/s  up accumulator downdraft < -0.1 m/s  down accumulator

  15. Downdraft Updraft Deadband (mid)

  16. 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

  17. These plants could use a drink.......mmh

  18. Average Loss 1.5 kg N/ha/day

  19. 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

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