Particle bounce

Size cut enhances resolution, optically important aerosol analysis, low artifact potential, particle bounce amenable to automated compositional analysis automated versions available large networks under development... 

Particle bounce, wall losses labor intensive... 

Geiger and Marsden s observations astonished everyone. Although almost all the a particles did pass through and were deflected only very slightly, about 1 in 20 000 was deflected through more than 90°, and a few a particles bounced straight back in the direction from which they had come. It was almost as incredible, said Rutherford, as if you had fired a 15-inch shell at a piece of tissue paper and it had come back and hit you. ... 

This experiment eliminated the plum pudding model as a possible structure of the atom. But what did an atom look like Rutherford figured that the only way to make alpha particles bounce backward... 

The transmission coefficient T in equation (2.58) is the relative probability that a particle impinging on the potential barrier tunnels through the barrier. The reflection coefficient R in equation (2.59) is the relative probability that the particle bounces off the barrier and moves in the negative v-direction. Since the particle must do one or the other of these two possibilities, the sum of T and R should equal unity... 

The HVI plates were leached with 0.1 M HC1. Be-7 was measured using intrinsic germanium coaxial and well detectors. Lead-210 was determined 30 days after collection stopped by separating and measuring Bi-210 (Poet et al., 1972). When Pb-210 was measured, the upper two HVI stages were coated with a thin layer of petroleum jelly to minimize soil particle bounce. 

Combined Electrostatic and Steric Stabilization. The combination of the two mechanisms is illustrated in Figure 4, taken from Shaw s textbook, (13) where the repulsion of the steric barrier during a collision falls off so rapidly as the colliding particles bounce apart that the dispersion force attractions hold the particles together in the "secondary minimum". This is exactly what happens in the system investigated in this paper. 

Saltation is also used to transport settling slurries through pipelines [Condolios and Chapus (1963a)]. In this case the solid particles bounce and roll along the bottom of a horizontal pipe. 

In solids, the particles vibrate in liquids, they slide over one another and in gases, the particles bounce around at high speed. Temperature is a measure of the average speed at which the particles are moving. At higher temperatures. 

Most of the published composition/size distribution data have been obtained by analyzing cascade impactor samples. Some of these data suffer from poor size classification as a result of particle bounce or reentrainment, seriously limiting size resolution. Even when this problem is overcome, the data obtained with conventional cascade impactors are not capable of resolving many details of the distribution of submicron particles. These instruments typically classify only those particles larger than 0.3-0.5 tam aerodynamic diameter. All smaller particles are collected on a filter downstream of the impactor. Some measurements of the variation of composition with size below this limit have been attempted by aerodynamically classifying resuspended ash ( ). These data suffer from incomplete disapregation as well as poor classification of the smaller particles. 

In addition to these chemical artifacts, physical artifacts can also occur. For example, the problems of particle bounce (e.g., see Wedding et al., 1986) and reentrainment in impactors were discussed earlier. In addition, air turbulence is known to have a significant effect on the overall sampling efficiency of particle inlets (e.g., Wiener et al., 1988 Francois et al., 1995). 

The limits to the areal density of deposit for filters are set by clogging of the filter that sets in at typically 100 xg/cm2. The limit of areal density for impactors is set by the problem of particle bounce. This is a serious problem for coarse, dry aerosols but less so for fine, wet, secondary aerosols. Nevertheless, sticky substrates are universally used (19), and deposits are generally limited to a few monolayers of particles for a 2.5- xm particle. This limit amounts to no more than 7 xm of deposit, or, for 1.5- xg/m3 aerosols (per stage), about 1000 xg/cm2 of deposit. 

There is only a severely limited amount of mass available for analysis, and efforts to increase size information through more stages simply makes the available mass even less. Attempts to collect more mass by longer runs are limited by particle-bounce effects. 

For the deposition of particles, bounce-off and blow-off, which must be carefully distinguished, may be considered analogous to the accommodation coefficient and to re-evaporation respectively. 

Experiments on the impaction of monodisperse water droplets on cylinders (May Clifford, 1967) have shown C,- correlating with Sto as predicted theoretically. Droplets are almost always captured on impact, but this is not true of solid particles, for which the efficiency of capture Cp may be less than C(. The same factors which tend to increase Q, namely large dp, large ux and small L, also tend to increase the possibility of the particle bouncing off the surface, and this may result in a decline in Cp with increase in Sto. Bounce-off of particles from fibres is a well known factor limiting the efficiency of filters. 

The principal problems in determining size distribution parameters with cascade impactors are wall losses, inefficient collection due to particle bounce, deposition of gas-phase species on impaction substrates, and deposition of fine particles from boundary layers. 

Particle bounce. When particles bounce off the collection surface, they may be carried to subsequent stages, where they may stick or again bounce off. The result is that subsequent stages collect more mass than is appropriate, and the inferred particle-size distribution is biased towards the smaller particles. Apparently, because of increasing velocity, particles that bounce off one stage continue to bounce off the subsequent stages and are finally collected on the afterfilter. As discussed below, such collection can severely limit the utility of afterfilter data. Typically, sticky substances are applied to impaction surfaces to reduce particle bounce. Compounds that can be "wicked" by the collected particles tend to be the most effective. 

Afterfilter data. As indicated in Table I, the minimum D50 in this study was about 0.5 pm, and particles smaller than this were collected on an afterfilter. Aerosols from combustion of pulverized coal typically are distributed bimodally, with a fine-particle mode at about 0.1 pm and a large-particle mode at supermicrometer sizes the modal diameter of the latter depends strongly on the efficiency characteristics of the control device. The elemental concentrations in the fine-particle mode are of interest in health-impact and source-apportionment studies because of the typically high enrichment of the concentrations of many potentially toxic elements and useful tracer elements in particles in this size range. Large-particle con-taimination of the afterfilter due to particle bounce can, however, limit the value of these data. 

Table III. Ratios of Observed Elemental Masses on a Typical After-Filter to Those Adjusted for Particle Bounce.

We have used both the horizontally and vertically configured impactors to sample coal-combustion aerosols in elevated plumes and in plumes at ground level near Deep Creek Lake in western Maryland. Few problems were encountered in this study however, particle bounce could not be evaluated for ambient samples because the individual submicrometer particles could not be discerned. Marple and Rubow (8) have performed extensive calibrations with monodisperse aerosols these show that impaction plates covered with aluminium foil have slightly larger D50S than plates covered with Teflon-fiber filters. 

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