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
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•
•
•
Introduction
Overview of mechanism
Direct measurement
Inferential measurement (using sand as a
surrogate)
• Wind tunnel measurement
• Soil condition considerations
• Roughness considerations
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 nonpolar 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.
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
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
Mechanisms of dust emission
http://www.weru.ksu.edu/
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
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
Sand flux
• Integrated samplers (e.g., BSNE, Cox sand
catcher)
– Reliable, inexpensive, does the job, provides physical
sample
– No time resolution, labor-intensive
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
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
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
Field wind tunnels
• To obey fluid and saltation scaling laws, wind
tunnel must be large
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)
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
Concept
• Use flat plate to generate shear
stress
• Use circular shape to get steady
conditions (axisymmetric flow)
Plate moving with
respect to ground
Velocity gradient
results in shear at
ground, suspending
loose particles
Ground
Various versions
Current version
Miniature PI-SWERL (MPS-2)
Mini PI-SWERL
1.1
1
Friction Speed:
0.9
y = -1E-12x3 + 8E-09x2 + 0.0001x + 0.0872
0.9
R2 = 0.9998
0.8
0.8
0.7
Shear
u*
Poly. (u*)
Poly. (Shear)
0.7
0.6
0.5
0.6
0.5
0.4
0.4
0.3
0.3
Shear:
3
0.2
2
y = -4E-12x + 5E-08x - 2E-05x + 0.0351
0.2
R2 = 0.9999
0.1
0.1
Mini_Swirler_RPM
6800
6400
6000
5600
5200
4800
4400
4000
3600
3200
2800
2400
2000
1600
800
1200
400
0
0
0
u* (m/s)
1
Shear stress (Pa)
• 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
How does soil respond to shear stress
OGS_Count_2
OGS_Count_3
OGS_Count_4
DT_PM10(mg/m3)
Flowrate (LPM)
2000 RPM or u* = 0.39 m/s
700
600
500
400
300
200
140
120
100
OGS2 and
OGS4 are
saturated
80
60
40
20
100
0
0
0
100
200
400
300
Seconds into test
500
600
700
DustTRak PM10 (mg/m3) or Flowrate (LPM)
OGS_Count_1
1000 RPM or u* = 0.24 m/s
RPM/10 or OGS counts/second
800
TRPM/10
4000 RPM or u* = 0.69 m/s
900
RPM/10
3000 RPM or u* = 0.55 m/s
1000
How else?
10
9
8
7
6
5
4
3
2
1
0
4500
PM10 or OGS peak area
PM10 concentration (mg/m3)
OGS Sensor 1 (peak area)
OGS Sensor 2 (peak area)
Target RPM
4000
3500
3000
2500
2000
1500
1000
500
0
100
200
300
400
Test Duration (s)
500
0
600
Useful for understanding differences
100
10
Burn
9MAB
12MAB
21 MAB
24 MAB
33 MAB
1
Burned-US
Burned-IS
0.1
Control
Emissions relative to control
1000
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
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.
Confounding Vegetation!
1. u* measured
here
2. ut* measured
here
3. Does soil
surface
experience u*?
Thanks! Questions?
Collocation/calibration
100
.
Non Gravel Surfaces
Gravel Surfaces
Saltation Above 100 mg/ms
1
All Data: R=0.76, b=1.0,
a=0.02
Non Gravel Surfaces: R =
0.75, b=1.13, a=0.17
Gravel Surfaces: R=0.85,
b=0.81, a=-0.35
0.1
Saltation above 100 mg/ms:
R=0.81, b=0.94, a=-0.15
0.01
100
10
0.0001
0.0001
Y =(0.57  0.08) X
(0.77  0.05)
2
0.001
Wind Tunnel PM10 Emissions (mg/m s)
-2 -1
Wind Tunnel PM 10 Emissions (mg.m s )
10
0.001
0.01
0.1
1
10
100
1
0.1
0.01
PI-SWERL PM10 Emissions (mg.m-2s-1)
0.001
0.001
0.01
0.1
1
10
2
PI-SWERL PM10 Emissions (mg/m s)
100
Shear stress
1.2
S-0
S-400
S-800
S-1200
S-1600
S-2000
S-2400
S-2800
S-3200
S-3600
S-4000
S-4400
S-4800
S-5200
S-5600
S-6000
S-6400
1
Shear Stress (Pa)
0.8
0.6
0.4
0.2
0
-7
-2
3
8
13
18