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

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

2 Presentation Objectives Discuss various methodologies to measure NH 3 exchange Highlight specific sampling considerations and guidelines for each method Present results from various field experiments

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 or NH 3, we often are interested in characterizing eposition as well as emission ission sources of NH 3 generally are aracterized by small areas that have atial scales less than a typical flux otprint (swine lagoons, poultry farms,.) position targets are vegetation asslands, forests, crops, soil) that can e considerable spatial extent, but are en located in areas that have small NH 3 centrations

5 For characterizing NH 3 emissions Integrated Mass Balance Average emission rate = what is advected past tower

6 Flux = z 0 u z c z dz ( ) ( ) r 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 he application of micromet methods is not limited the micromet state of the art but by the current ethods used to measure the trace gas of interest H 3 ). st time response (> 1 Hz) eddy covariance ow but accurate gradient methods (AM,MBR) ccumulation methods (conditional sampling (REA)

10 Eddy covariance 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. Emission flux+ w+,c+, w-,c-, w ' c' + Deposition fluxw-,c+, w+,c-, w ' c' -

11 Good time resolution Advantages of Eddy Covariance 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

12 radient (Modified Bowen Ratio Method) Measure the flux (eddy covariance) and vertical gradient of constituent (heat, water vapour, CO 2 ) over specified height. Compute effective transfer coefficient (flux/gradient) Measure vertical gradient of NH 3 over same height interval and apply computed transfer coefficient to obtain a measure of the flux?c F NH3 =H??

13 Water vapor and CO 2 gradients To remove bias error, use same analyzer for both heights, switching at 30 sec to 5 min intervals and allowing for representative sampling. T c =cycletime Relative error e= 6 ( T / t) 0.8 c For T c = 60 s, τ = 30 s,? = 10% t=turbulencetimescale

16 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

17 Conditional Sampling Relaxed Eddy Accumulation For sampling gases and aerosols in accumulation devices like annular denuders, filterpacks, etc. Flux = βσ w (C up - C dn ) = empirical coefficient, 0.6 = standard deviation vertical velocity = average concentration of updrafts, downdrafts w up, C dn

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

19 Updraft Downdraft Deadband (mid)

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

22 These plants could use a drink...mmh

23 Average Loss 1.5 kg N/ha/day

24 Advantages of REA When used with denuders and filterpacks, can sample several species at once (NH 3, NH 4, SO 2, SO 4, HNO 3, NO 3 ) 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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