FIELD TESTING AND EVALUATION OF DUST DEPOSITION AND ...

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downwind concentration profile is dependent on u * . In contrast, the ISC model prescribes the concentration profile based on downwind distance only. In practice, this means that the ISC does not allow for alteration of the concentration profile due to variations in u * . This point is illustrated in the next chapter. Figure 4-7 f. shows the fraction remaining in suspension according to the box model for various values of the parameter A in Equation 4-29. The deposition velocity for the box model was computed assuming a height of 1 meter and neutral stability. Note that the box model gives a result that is independent of distance downwind of the source. For all values of A, the box model captures the general behavior of particle deposition to a surface; the greater the dispersion rate (as characterized by A and u * ), the smaller the fraction of particles that deposit. However, comparison of Figure 4-7f. with the other five panels in the figure (ADE and ISC3 model results) underscores the basic difficulty of using the box model: Though in its derivation the box model does not depend on the height of the box or the downwind distance, the parameter A used in the model depends on both. A also depends on atmospheric stability. It is clear from Figure 4-5 that for a line source, the concentration profile through the surface layer is dynamic, becoming flatter in the vertical direction with downwind distance. This means that the concentration at the top of the surface layer (C SL in the previous section) is neither constant in time, nor is it negligibly small compared to the concentration in the box (C Box ). Thus, the formulation for dispersion in the Box Model is not valid for a plume that is expanding in the vertical direction with concurrent decay in the concentration gradients within the surface layer. In a dust storm (Gillette, 1974), the ground acts like an area source, which behaves quite differently in the context of the Box Model from a line source for three reasons. First, an area source is horizontally homogeneous. Therefore, the vertical concentration profile is, on average, invariant with downwind distance. Second, the ground is acting as a strong source and the net flux of material is always in the upward direction. This means that we do not have to account for particles re-entering the Box through the ceiling. Third, because in a dust storm, the ground is a very strong source, the concentration near the ground is much higher than it is aloft (i.e. C Box >> C SL ), or at least, the ratio C Box /C SL is constant over large distances in the horizontal. For these three reasons, dispersion in a dust storm can be estimated based only on the concentration near the ground (as in Equation 4-27), without considering the concentration profile aloft. For a dust plume that is expanding while traveling downwind, the assumptions behind Equation 4-27 do not hold as well. 4.3 Summary For unpaved road dust emissions, the Box Model provides an order of magnitude estimate of the dust particle removal due to deposition. For a more accurate assessment, a model that accounts for changes in the concentration profile with downwind distance is required. Among the models considered in this section, the algorithm of the ISC3 is most suited for simulating near-source dispersion. The ADE performs poorly in the near field, 4-20
though it is expected to improve at larger downwind distances (500 - 1,000 m). In the next two chapters, the ISC3 and ADE models are compared with field data. 4-21
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 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: gravitational settling. Stable cond
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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