Aerosol sedimentation velocity

The physical meaning of the above criteria is as follows. The external force acting on an aerosol is generally gravitation. This means that the lifetime of a particle in the system is determined by its sedimentation velocity. If the particle radius is greater than the mean free path of gas molecules, the vs falling velocity is given by the well-known Stokes equation ... 

Dry deposition velocity (A) and sedimentation velocity (B) of aerosol particles. Curve A refers to flow over grass (Hidy, 1973). (By courtesy of Plenum Press)... 

Aerodynamic diameter The diameter of an imaginary spherical aerosol particle, having unit density, that has the same sedimentation velocity (in air) as the particle(s) under consideration... 

Sedimentation velocities of aerosol particles depend on particle shape. The models of aerosol behaviour that are now available are derived for perfect spheres that have no porosity. The deviations of real particles from this ideal are handled by correction factors called shape factors. In the case of gravitational settling, the dynamic shape factor is used to account for deviations from sphericity and for porosity. These shape factors are not known well and frequently are estimated by back calculation from experimental data for simulant aerosols. This, of course, is not a reliable procedure. There have been some attempts to predict shape factors based on the fractal nature of particles that have grown by coagulation [A-7b]. 

Sedimentation velocity is proportional to particle density, but Brownian motion is independent of density. Table 5.6 shows that sedimentation of unit density particles is more effective in causing deposition than Brownian diffusion when dp exceeds 1 pm, whereas the reverse is true if dp is less than 0.5 pm. For this reason, it is appropriate to use the aerodynamic diameter Dp equal to p] dp when this exceeds 1 pm, but the actual diameter for submicrometre aerosol particles. 

A different approach which also starts from the characteristics of the emissions is able to deal with some of these difficulties. Aerosol properties can be described by means of distribution functions with respect to particle size and chemical composition. The distribution functions change with time and space as a result of various atmospheric processes, and the dynamics of the aerosol can be described mathematically by certain equations which take into account particle growth, coagulation and sedimentation (1, Chap. 10). These equations can be solved if the wind field, particle deposition velocity and rates of gas-to-particle conversion are known, to predict the properties of the aerosol downwind from emission sources. This approach is known as dispersion modeling. 

Although with an aerosol each particle will settle at its own terminal settling velocity, settling rarely takes place in absolutely still air since there is always some circulation and mixing. This mixing has the effect of producing a uniform aerosol concentration which decreases with time because of sedimentation. 

As discussed previously, terminal settling velocities of aerosol particles are generally quite small. Under normal circumstances it is unreasonable to expect that simple sedimentation as such will be an effective removal mechanism. 

Pulmonary deposition of an aerosol preparation is determined primarily by its size. Aerosols with a mass median aerodynamic diameter of 1-5 xm produce the best therapeutic results and are the target particle size for inhalation therapy. These small particles penetrate deep within the respiratory tract to ensure drug deposition in peripheral airways. The cross-sectional area (cm ) of the lung increases dramatically at the level of the respiratory zone therefore, the velocity of gas flow during inspiration rapidly decreases at this level. Moderate-sized particles (5-10 (xm) frequently settle out by sedimentation in larger more central airways because the velocity of gas falls rapidly in the region of the terminal bronchioles. 

In particular, the propellant-driven metered-dose inhalers release the aerosol cloud at the very high velocity caused by the pressure of the propellant. The open-mouth technique of inhalation [79] helps to slow down the droplets (and to evaporate the volatile excipients). An even more effective solution is to use spacer devices [4,79-87], in which the aerosol cloud can slowed down, the volatile constituents can evaporate, and any large particles will sediment out. Moreover, the patient can then inhale the remaining aerosol under optimal conditions for pulmonary delivery [4,8,56,79], that is, with a slow inspiratory flow rate. 

Let us consider diffusion to the surface of a circular cylinder of radius a in a flow with velocity Ui directed along the normal to the cylinder axis. This is a model problem used in chemical engineering for calculating mass transfer to prolate particles it is used even more widely in mechanics of aerosols for analyzing diffusion sedimentation of aerosols on fibrous filters [139,461]. 

Most of the methods used for aerosol degradation are based on intensifying the processes of coagulation, coalescence, adhesion of aerosol particles on different surfaces (on solid walls of filters, or water drops, as in artificial irrigation), and sedimentation (by changing the velocity and direction of aerosol streams during the inertial settling e.g. in so called cyclones). 

Friedlander (11) has examined the effects of flocculation by Brownian diffusion and removal by sedimentation on the shape of the particle size distribution function as expressed by Equation 9. The examination is conceptual the predictions are consistent with some observations of atmospheric aerosols. For small particles, where flocculation by Brownian diffusion is predominant, p is predicted to be 2.5. For larger particles, where removal by settling occurs, p is predicted to be 4.75. Hunt (JO) has extended this analysis to include flocculation by fluid shear (velocity gradients) and by differential settling. For these processes, p is predicted to be 4 for flocculation by fluid shear and 4.5 when flocculation by differential settling predominates. These theoretical predictions are consistent with the range of values for p observed in aquatic systems. 

The major causes of aerosol loss from a pMDI delivered into a spacer are illustrated in Fig. 3. The larger particles impact on the spacer wall due to the inertia in the jet of particles from the pMDI. Particles sediment due to the progressively reduced velocity of the aerosol. In addition, particles can be adsorbed to the spacer wall if this carries electrostatic charges. Aerosol loss is partly instanta-... 

Sedimentation of drug particles in the aerosol is another factor that reduces the available aerosol and shortens the time available for inhalation after actuation. The velocity of sedimentation is proportional to the aerodynamic diameter of the particles. In a narrow spacer, the sedimentation distance is short and therefore the loss of aerosol is faster in small-volume tube spacers than in large-volume spacers. This principle is taken to its extreme in a vertical spacer, compared with the traditional horizontal spacer. 

For the two main processes determining aerosol stability, sedimentation was discussed already in Section 2.5.1, and aggregation is discussed further in Section 5.5. For quite large particles, and Kn<, Stokes law describes the sedimentation process. Thermophoretic velocity is not strongly influenced by interactions between the aerosol particles and molecules of the gas. 

A washing liquid is co-injected and the two streams co-mingle as they enter a throat section, in which the rapidly moving gas stream causes the washing liquid to break up into droplets that in turn trap the desired aerosol particles and/or droplets. The mixture then exits the throat and enters an inverted funnel-shaped section whose diameter increases rapidly, causing the flow velocity to decrease rapidly. As the velocity drops, the droplets (with their trapped materials) are sedimented out directly, are collected on impingement plates or are concentrated with a cyclone. 

The deposition of activity to the ground is the result of two main processes, termed dry deposition and wet deposition. Dry deposition covers the combined effect of sedimentation of larger particles under gravity, of impaction of particulates and aerosols on leaves, etc., and of absorption of reactive gases by the soil and vegetation. The dry deposition depends on the product of the concentration of the material close to the surface and a so-called deposition velocity For gases like iodine-131 and sulphur dioxide, is about 1 cm/s in normal turbulent surface layers. Small aerosols and particulates (like the caesium-137 from Chernobyl) have a much smaller... 

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