FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

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to the ground) giving an approximate wake height of 1.7 meters according to this regression (see Figure 5-15). Turbulence is a surrogate for mixing. The wake height measured in this portion of the study is important because it gives an indication of the height to which the dust plume is mixed behind a vehicle traversing an unpaved road. As a first approximation, we may assume that the dust emitted behind a vehicle is well-mixed up to the height of the turbulent wake (approximately 1.7 times the height of the vehicle itself). This “injection” height may play an important role when determining the fraction of particulate matter that deposits close to the road vs. the fraction that is transportable; as a rule, the further a particle is from the ground initially, the less likely it is to deposit in a given period of time. As an aside, the injection height also poses a lower limit on the height that can be used in Gillette’s model of a hypothetical box. The approximation for the injection height suggested here is quite crude and simplistic. The height of the wake is likely to depend on a number of parameters, most notably, the atmospheric stability, the shape of the vehicle, and the angle of the ambient wind with respect to the direction of vehicle travel. The analysis is based on data obtained under stable conditions. Under unstable conditions, turbulent mixing is not inhibited by buoyancy forces. Therefore, it is likely that the effective injection height will be larger than under stable conditions. For the present purpose, it is sufficient to note that the height to which a dust plume is thoroughly mixed in the wake of a vehicle is dependent on the size of the vehicle, increasing in a roughly linear relationship with height. Detailed discussion of vehicle wake characteristics is outside the scope of this project. Hosker (1982) provides a thorough literature review of flows around bluff bodies. More recently, Rao et al. (2002) have measured wake properties with a trailer that was drawn by a cargo van and equipped with multiple sonic anemometers. Hider et al. (1997) have examined a modeling approach for particles dispersing in a vehicle wake. Though preliminary and based only on a single vehicle type, their data suggest that the turbulent kinetic energy in the wake of the vehicle is greatest at approximately the vehicle height and approaches background levels at twice the vehicle height. This is qualitatively consistent with our assumption of a wake plume that is completely mixed from the ground up to 1.7 times the vehicle height. 5.2.3.2 The effect of injection height on removal of particles downwind of an unpaved road The injection height is least important under unstable atmospheric conditions, and most important under stable atmospheric conditions. This is because under unstable conditions, the dust plume is lofted up high quickly anyway, more or less regardless of the starting height. Under stable conditions, the extent of vertical mixing is retarded by buoyancy, and a lower injection height allows the particles to be closer to the ground for a longer period of time, thereby enhancing deposition. According to the ADE model, overall, the effect of injection height on particle removal is quite small. The reason for this was discussed earlier in section 5.2.1, namely that the dispersion near the source (nominally less than 1,000 m downwind) is greatly 5-22
overestimated by the ADE since some of the basic model assumptions are violated in that region. The ISC model more accurately reflects the degree of dispersion that occurs in the first 1,000 or so meters downwind of the unpaved road source. Recall that the injection height of the dust plume can be accounted for in the model through the initial dispersion parameter σ z , assumed to equal one-half of the injection height, and a “virtual” upwind distance (see section 4.1.2). Comparison of downwind removal rates (Figure 5-16) indicates that for unstable conditions, the injection height has a non-trivial, but small effect. For very stable conditions, the injection height has a large effect, approaching a factor of two difference in estimated removal rates at 1,000 m downwind of the source. The difference between the removal of particles for two plumes with different injection heights is confined to the near-source region. That is, the difference does not continue to grow past about 1,000 meters downwind (Figure 5-16). With this in mind, and noting as before that most unpaved road dust emissions are likely to occur in the daytime, when conditions are neutral to unstable, the effect of the injection height is secondary to the uncertainties associated with deposition velocities and dispersion parameters. 1.2 1.2 Fraction of particles remaining in suspension 1 0.8 0.6 0.4 0.2 IH=1 IH=2 IH=4 IH=6 Fraction of particles remaining in suspension 1 0.8 0.6 0.4 0.2 IH=1 m IH=2 m IH=4 m IH=6 m 0 0 0 200 400 600 800 1000 0 200 400 600 800 1000 Distance Downwind (m) Distance Downwind (m) a. Moderately unstable (Class B) b. Neutral (Class D) 1.2 1.2 Fraction of particles remaining in suspension 1 0.8 0.6 0.4 0.2 IH=1 IH=2 IH=4 IH=6 Fraction of particles remaining in suspension 1 0.8 0.6 0.4 0.2 IH=1 IH=2 IH=4 IH=6 0 0 200 400 600 800 1000 Distance Downwind (m) c. Very stable (Class F) 0 0 2000 4000 6000 8000 10000 Distance Downwind (m) c. Same as c. except x-axis extends to 10 km Figure 5-16. Comparison of fraction of particles removed according to the ISC3 model for various values of the injection height under a. moderately unstable conditions; b. neutral conditions; c. very stable conditions; and d. same as c. but x-axis extends to 10 km. The injection height is manifested in the model by setting the initial value of σ z to 0.5×IH. Another aspect of the injection height that is not examined here is its effect on the removal efficiency for particles in the “impact” region. This region (labeled Region A in Figure 2-1) may play an important role in the removal of particles, especially if the injection height is lower than or comparable to the vegetative canopy height. None of the 5-23
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FIELD TESTING AND EVALUATION OF DUS
Page 5 and 6:
EXECUTIVE SUMMARY The Western State
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models. Countess (2001) summarized
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TABLE OF CONTENTS 1. INTRODUCTION 1
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TABLE OF FIGURES Figure 2-1. Develo
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Figure 4-6. The fraction of particl
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Figure 5-16. Comparison of fraction
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LIST OF TABLES Table 2-1. Parameter
Page 19 and 20:
1. INTRODUCTION 1.1 Background The
Page 21 and 22:
2.BACKGROUND Fugitive dust is emitt
Page 23 and 24:
Planetary Boundary Layer (PBL) F =
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C D u* = ur The overall drag c
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St a vg u g = cA L * Equation 2
Page 29 and 30:
Table 2-1. Parameters used in the S
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1E+01 1E+00 z0 = 4.4 cm z0 = 6.4 cm
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pathway for deposition for particle
Page 35 and 36:
An alternative method for modeling
Page 37 and 38:
3. MEASUREMENT OF DEPOSTION VELOCIT
Page 39 and 40:
Department at the University of Uta
Page 41 and 42:
In the laboratory, the deposited ma
Page 43 and 44:
Table 3-2. Log of all flat substrat
Page 45 and 46: 16 Normalized Deposition Velocity 1
Page 47 and 48: 1.E+05 Deposition Velocity - cm/s 1
Page 49 and 50: are deposition from a horizontally
Page 51 and 52: single element that is intended to
Page 53 and 54: 4. MODELS USED TO STUDY PARTICLE RE
Page 55 and 56: κu ( ) * z 8 fz K zz = exp −
Page 57 and 58: C = −6 ( 1× 10 ) QV 2π u s σ
Page 59 and 60: oad. M is the mass of particles in
Page 61 and 62: By making the approximation that dm
Page 63 and 64: diameters less than 10 microns. Dis
Page 65 and 66: of such a height is somewhat arbitr
Page 67 and 68: 1 0.9 0.8 1 0.9 0.8 Fraction remain
Page 69 and 70: Fraction of Particles Remaining in
Page 71 and 72: gravitational settling. Stable cond
Page 73 and 74: though it is expected to improve at
Page 75 and 76: 5. DETERMINATION OF PARTICLE REMOVA
Page 77 and 78: PM 10 measured by the DustTrak must
Page 79 and 80: 100% 90% 80% Relative Frequency (pr
Page 81 and 82: average wind speed measured at 12 m
Page 83 and 84: where the subscript i indicates tha
Page 85 and 86: k C int = peakend k C peeakstart d
Page 87 and 88: Height z (m) 14 12 10 8 6 T1_data T
Page 89 and 90: Horizontal fluxes of PM 10 measured
Page 92 and 93: Table 5-2 enumerates the speed-depe
Page 94 and 95: 6 Ratio of AP-42 Emission Factor to
Page 98 and 99: models analyzed in this study is su
Page 100 and 101: Table 6-1. Screen analysis of test
Page 102 and 103: value which was used up to the midp
Page 104 and 105: measured at 3,1. Downwind the dust
Page 106 and 107: trips, the average readings for the
Page 108 and 109: Fraction of particles of stated siz
Page 110 and 111: the ground, where particles have a
Page 112 and 113: particles through the top of the bo
Page 114 and 115: deposition. Much of the decrease in
Page 116 and 117: 7-8
Page 118 and 119: Gifford, F.A. (1961). Use of Routin
Page 120: Venkatram, A. (1993). The Parametri
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