FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

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4.2.2 Comparison of results for particle removal between predictions from ADE, ISC3, and the Gillette Box Model The results of numerically solving the Atmospheric Diffusion Equation are segregated by atmospheric stability and shown in Figure 4-4. The fraction of particles remaining in suspension increases as the atmospheric conditions vary from very stable to very unstable. For all atmospheric stability conditions, the fraction remaining in suspension decreases with increasing friction velocity. This is a very important point. Increasing the friction velocity not only enhances turbulent dispersion, but it also increases the impaction of particles and therefore the deposition velocity. Table 4-1 gives approximate values for the wind speed at 10 meters corresponding to the friction velocities shown in Figure 4-4. For a roughness height of 0.01 m and friction velocity of 0.5 m/s, the equivalent 10 m wind speed is 13.8 m/s (30 mph), a value that corresponds to a fairly windy day at most locales. For low to moderate winds in arid to semi-arid areas (i.e. u * < 0.5 m/s), the fraction of particles remaining in suspension after 500 seconds is greater than 70% except for the very stable case. Note however that in the case of a stable atmosphere, the wind speed is unlikely to exceed a few meters per second, since at higher wind speeds, mechanical mixing at the surface would disallow the formation of a stable layer to any appreciable height (i.e. more than a few tenths of a meter). The trend of reduced deposition with increasing instability is consistent with expectation. During stable conditions, dust particles stay close to the ground for longer periods of time. Therefore, they are removed more quickly. In contrast, during unstable conditions, particles are lofted to higher elevations quickly by buoyant eddies. This means that those particles are not immediately available for deposition to ground surfaces. The concentration profiles are compared among different conditions of atmospheric stability in Figure 4-5. It is clear from the figure that unstable conditions result in the rapid vertical dispersion of particles. The removal of dust in the area near a source is often assumed to be significant because concentrations of particles downwind of a source rapidly attenuate to the background. This is an erroneous deduction. Watson and Chow (2000) and Countess (2001) both cite an earlier study (Watson et al., 1996) where the concentration of PM 10 dust was found to decrease by 90% only 50 m downwind from the source. The concentration of particles may decrease rapidly with downwind distance, but it is incorrect to assume that the decrease is due solely to deposition. This concept is illustrated in Figure 4-6. The figure clearly shows that while concentrations decrease rapidly downwind of the source, the actual fraction of particles remaining in suspension may be quite high. For example, according to the ISC3 model, under neutral conditions, the concentration at a height of 1 meter 200 meters downwind of the road is 6% of its value near the road (within a few meters). However, 90% of the particles are still in suspension at the same downwind distance. Clearly, the decrease in concentration can be due primarily to the vertical mixing of particles and is not necessarily due to deposition. 4-14
1 0.9 0.8 1 0.9 0.8 Fraction remaining 0.7 0.6 0.5 0.4 0.3 ADE u*=0.1 ADE u*=0.3 0.2 ADE u*=0.5 0.1 ADE u*=0.8 ADE u*=1.2 0 0 50 100 150 200 250 300 350 400 450 500 Time since release (s) Fraction remaining 0.7 0.6 0.5 0.4 0.3 ADE u*=0.1 ADE u*=0.3 0.2 ADE u*=0.5 0.1 ADE u*=0.8 ADE u*=1.2 0 0 50 100 150 200 250 300 350 400 450 500 Time since release (s) a. very stable (L=10 m) b. stable (L=1,000 m) 1 0.9 0.8 1 0.9 0.8 Fraction remaining 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 ADE u*=0.1 ADE u*=0.3 ADE u*=0.5 ADE u*=0.8 ADE u*=1.2 0 50 100 150 200 250 300 350 400 450 500 Time since release (s) Fraction remaining 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 ADE u*=0.1 ADE u*=0.3 ADE u*=0.5 ADE u*=0.8 ADE u*=1.2 0 50 100 150 200 250 300 350 400 450 500 Time since release (s) c. neutral |L|>100,000 d. unstable (L=-1,000 m) Fraction remaining 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 ADE u*=0.1 ADE u*=0.3 ADE u*=0.5 ADE u*=0.8 ADE u*=1.2 0 50 100 150 200 250 300 350 400 450 500 Time since release (s) e. very unstable (L=-10 m) Figure 4-4. Fraction of particles remaining in suspension vs. time since emission in the near field (0
Page 1:
FIELD TESTING AND EVALUATION OF DUS
Page 5 and 6:
EXECUTIVE SUMMARY The Western State
Page 7 and 8:
models. Countess (2001) summarized
Page 9 and 10:
TABLE OF CONTENTS 1. INTRODUCTION 1
Page 11 and 12:
TABLE OF FIGURES Figure 2-1. Develo
Page 13 and 14:
Figure 4-6. The fraction of particl
Page 15 and 16: Figure 5-16. Comparison of fraction
Page 17 and 18: 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 =
Page 25 and 26: C D u* = ur The overall drag c
Page 27 and 28: St a vg u g = cA L * Equation 2
Page 29 and 30: Table 2-1. Parameters used in the S
Page 31 and 32: 1E+01 1E+00 z0 = 4.4 cm z0 = 6.4 cm
Page 33 and 34: 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: of such a height is somewhat arbitr
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 96 and 97: to the ground) giving an approximat
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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