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(10 4 km) /Continental (10 3 km)/Regional (10 2 km)/ Local (10 km) – Remote sensing – Models (e.g. NAAPS, http://www.nrlmry.navy.mil/aerosol/ ) 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/

9. 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

11. 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

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

13. Sand flux (cont.) Electronic sensors (e.g., saltiphone, Sensit TM, 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

14. 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 Promising for sand sizing as well as mass

15. 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

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

18. 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)

19. 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

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

21. Various versions

22. 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 PM 10 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

23. How does soil respond to shear stress

24. How else?

25. Useful for understanding differences

26. 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

27. 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.

28. Confounding Vegetation!

29. Thanks! Questions?

30. Collocation/calibration

31. Shear stress