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Windblown dust measurement (field)

Windblown dust measurement (field). Vic Etyemezian DRI Las Vegas, NV. Prepared for The Southwest Border Symposium on Air Quality and Climate, April 22-23, 2013, Las Cruces, NM. Outline. Introduction Overview of mechanism Direct measurement

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Windblown dust measurement (field)

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  1. Windblown dust measurement (field) Vic Etyemezian DRI Las Vegas, NV Prepared for The Southwest Border Symposium on Air Quality and Climate, April 22-23, 2013, Las Cruces, NM

  2. Outline • Introduction • Overview of mechanism • Direct measurement • Inferential measurement (using sand as a surrogate) • Wind tunnel measurement • Soil condition considerations • Roughness considerations

  3. Windblown dust (WBD) • Among largest sources of atmospheric aerosol (about 1,000 terragrams or 1 billion tons per year) • Mostly natural (75%, Ginoux et al., 2012) and significant fraction (30%, Ginoux et al., 2012) related to hydrologic processes. • Largely from African continent, but all non-polar continents have significant dust sources Ginoux, P., J. M. Prospero, T. E. Gill, N. C. Hsu, and M. Zhao (2012), Global-scale attribution of anthropogenic and natural dust sources and their emission rates based on MODIS Deep Blue aerosol products, Rev. Geophys., 50, RG3005, doi:10.1029/2012RG000388.

  4. Scales • Global(104 km) /Continental (103 km)/Regional (102 km)/ Local (10 km) • Remote sensing • Models (e.g. NAAPS, http://www.nrlmry.navy.mil/aerosol/ ) • Local (10 km)/ Field (1 km) • Remote sensing – source identification tricky • Models – inhomogeneous terrain and parameter specification tricky • On-site measurements can provide useful information

  5. Mechanisms of Windblown Dust Suspension • Creep • 0.5 - 2 mm particles roll due to pressure differential • Saltation • 0.1 - 0.5 mm particles suspended, travel parallel to ground 1-5 m, re-impact • Cause release of additional particles • Emission • 0.001 - 0.1 mm particles suspended and transported between 10 – 10,000 m

  6. Mechanisms of dust emission http://www.weru.ksu.edu/

  7. Direct measurement of WBD • Dale A. Gillette (DAG) largely championed approach • Measure profile of wind speed and dust concentration during WBD event • Use theoretical considerations to calculate upward flux of dust • In practice, DAG used sand flux as surrogate for dust concentration for measurement ease • Advantage: Defensible measure of emissions under real-world conditions • Disadvantage: Requires substantial field infrastructure and presence during dust event(s); assumes homogeneous source strength

  8. Inference from sand flux • Saltation (sand ballistic impacts on soil) is primary means of emitting dust • The amount of sand that is carried across a line should relate to the amount of dust that has been emitted from the surface • In some instances, dust emission is assumed proportional to sand flux (e.g., Gillette et al., 2004) Gillette, D.A., Ono, D. and Richmond, K. 2004. A combined modeling and measurement technique for estimating windblown dust emissions at Owens (dry) Lake, California. Journal of Geophysical Research. Earth Surface 109, F01003, doi:10.1029/2003JF000025

  9. Sand flux • Integrated samplers (e.g., BSNE, Cox sand catcher) • Reliable, inexpensive, does the job, provides physical sample • No time resolution, labor-intensive

  10. Sand flux (cont.) • Electronic sensors (e.g., saltiphone, SensitTM, Safire, Wenglor, other optical gate sensors) • Vibration/sound • Extinction/optical gate • Advantages: Time resolution for linking with wind conditions • Disadvantages: No physical sample, sensitivity issues, requires electronic infrastructure

  11. Sand Flux (cont.) • NSF-funded project to develop new real-time sensor using optical gate technology • Includes WS/WD, T, Sand sensor + trap, power source, data logger w/memory card • Promisingfor sand sizing as well as mass

  12. In-situ measurement of WBD potential • Conduct measurements in field using wind tunnel or similar technology • Identify soil/landform characteristics that can be grouped • Conduct erodibility measurements to obtain response of soil to wind stress • Use range of results to estimate response of soil/landform • Apply other corrections for weather, soil conditions, and surface cover

  13. Field wind tunnels • To obey fluid and saltation scaling laws, wind tunnel must be large

  14. Field wind tunnels (cont.) • Advantages: • No need to wait for dust • Can examine subsets of landscape, soil conditions, wind strengths, treatment efficacy (if applicable) • Disadvantages • So much land, so little wind tunnel • Labor intensive (4-8 hours per measurement) • Finite length can deplete erodible portion • Despite scaling compliance, ability to quantify saltation effect questionable • Addition of “external” sand source results in undue sandblasting • Can only examine subsets of landscape, soil conditions, wind strengths, treatment efficacy (if applicable)

  15. PI-SWERL • Portable In-Situ Wind ERosion Lab (PI-SWERL) • Like a wind tunnel • But not • Does not comply with scaling laws • Motivated by need for portable measure of wind erodibility where wind tunnels are not practical

  16. Concept • Use flat plate to generate shear stress • Use circular shape to get steady conditions (axisymmetric flow)

  17. Various versions

  18. Current version Miniature PI-SWERL (MPS-2) • 30 cm diameter • Rotates up to 6000 RPM or approximately u*=1 m/s or WS  60 mph • Use DustTrak (8520 or 8530) to estimate PM10 concentrations • Uses optical gate sensors to detect and quantify sand motion (not quite sand flux) • “clean” air is pumped into chamber • “dirty” air is exhausted • Emissions potential can be estimated at varying shear stresses

  19. How does soil respond to shear stress

  20. How else?

  21. Useful for understanding differences

  22. Confounding factors 1 • Wind tunnel measurements are point-in-time measurements. Do not account for: • Soil moisture • Temperature • Soil crusting • Disturbance level • Need to account for these • Robust measurement plan • Corrections after the fact

  23. Confounding Factors 2 • Surface cover • WT is placed over bare or light vegetation • PI-SWERL only on very light vegetation • Both directly include accounting for gravel • Vegetation + large scale roughness – trickier • Techniques available (e.g., Raupach ,1992) • Imperfect Raupach, M.R. 1992. Drag and drag partitioning on rough surfaces. Boundary-Layer Meteorology 60(4):375-395.

  24. Confounding Vegetation!

  25. Thanks! Questions?

  26. Collocation/calibration

  27. Shear stress

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