Puff model, described

The puff model describes near-instantaneous releases of material. The solution depends on the total quantity of material released, the atmospheric conditions, the height of the release above ground, and the distance from the release. The equation for the average concentration for this case is (Growl and Louvar, 1990, p, 143) ... 

Two types of neutrally buoyant vapor cloud dispersion models are commonly used the plume and the puff models. The plume model describes the steady-state concentration of material released from a continuous source. The puff model describes the temporal concentration of material from a single release of a fixed amount of material. The distinction between the two... 

Dispersion models describe the airborne transport of toxic materials away from the accident site and into the plant and community. After a release the airborne toxic material is carried away by the wind in a characteristic plume, as shown in Figure 5-1, or a puff, as shown in Figure 5-2. The maximum concentration of toxic material occurs at the release point (which may not be at ground level). Concentrations downwind are less, because of turbulent mixing and dispersion of the toxic substance with air. 

The puff model can be used to describe a plume a plume is simply the release of continuous puffs. However, if steady-state plume information is all that is required, the plume model is recommended because it is easier to use. For studies involving dynamic plumes (for instance, the effect on a plume of a change in wind direction), the puff model must be used. 

Several types of models are commonly used to describe the dispersion of atmospheric contaminants. Among these are the box, plume, and puff models. None are suitable, however, for describing the coupled transport and reaction phenomena that characterize atmospheres in which chemical reaction processes are important. Simulation models that have been proposed for the prediction of concentrations of photochemically formed pollutants in an urban airshed are reviewed here. The development of a generalized kinetic mechanism for photochemical smog suitable for inclusion in an urban airshed model, the treatment of emissions from automobiles, aircraft, power plants, and distributed sources, and the treatment of temporal and spatial variations of primary meteorological parameters are also discussed. 

The Gaussian plume and puff models, which describe the concentration distribution of an inert species downwind of a point, line, or area source, characterize the next level of complexity of airshed models. In the usual applications of these models ... 

Portions of the material described here are derived from a comprehensive airshed modeling program in which the authors are participating (17). This chapter focuses on urban airshed models however novel models have been proposed for urban air pollution problems of a more restricted scale— particularly, the prediction of concentrations in the vicinity of major local sources, notably freeways, airports, power plants, and refineries. In discussing plume and puff models earlier we pointed out one such class of models. Other work is the model proposed by Eschenroeder (18) to predict concentrations of inert species in the vicinity of roadways and the modeling of chemically reacting plumes, based on the Lagrangian similarity hypothesis, as presented by Friedlander and Seinfeld (19). 

Puff models such as that in Reference 5 use Gaussian spread parameters, but by subdividing the effiuent into discrete contributions, they avoid the restrictions of steady-state assumptions that limit the plume models just described. A recently documented application of a puff model for urban diffusion was described by Roberts et al, (19). It is capable of accounting for transient conditions in wind, stability, and mixing height. Continuous emissions are approximated by a series of instantaneous releases to form the puffs. The model, which is able to describe multiple area sources, has been checked out for Chicago by comparison with over 10,000 hourly averages of sulfur dioxide concentration. 

HPAC (Hazard Prediction Assessment Capability) (DTRA, 2001, 2004 [153, 154]) is a multipurpose code that could be described as a Gaussian puff model. It is widely used for non-urban areas in U.S. Department of Defense (DOD) applications and has been previously evaluated for several field experiments (Chang and Hanna, 2004 [113]). The latest version (DTRA, 2004 [154]) includes also the urban algorithms. HPAC incorporates UDM (Hall et al., 2002 [248]) for distances less than 2 km, after which it switches to its standard puff dispersion algorithm. 

To model accidental releases of the kind described above requires both a source emission model and a transport and dispersion model. When an accidental source emission occurs, the initial emission and acceleration phase gives way to a regime in which the internal buoyancy of the puff or plume dominates the dispersion. This regime is followed by transition to a regime in which the internal turbulence dominates the dispersion. There is then a transition from dominance of internal buoyancy to dominance of ambient turbulence. Models describing source emissions are beyond the scope of this article. This article will describe only the transport and dispersion models where ambient turbulence dominates. 

Two types of dispersion models are used to describe these releases when the puff or plumes are neutrally or positively buoyant. When there is an instantaneous release or a burst of material, we make use of a puff model. In this model the puff disperses in the downwind, cross wind, and vertical directions simultaneously. Computer codes written for puff models usually have the capability of tracking multiple puff releases over a period of time. When the release rate is constant with time, the puff model can be mathematically integrated into a continuous model. In this case, dispersion takes place in the cross wind and vertical directions only. The mathematical expressions are those discussed in Section III. The dispersion coefflcients used, however, may differ from those described in Section IV. Plume rise equations from Section V for positively buoyant plumes may be used in conjunction with these dispersion models. The equations of current models indicate that they are well formulated, but the application of the models suffers from poor meteorological irrformation and from poorly defined source conditions that accompany accidental releases. Thus, performance of these models is not adequate to justify their use as the sole basis for emergency response planning, for example. 

The well-known Gaussian models describe the behavior of neutrally buoyant gas released in the wind direction at the wind speed. Dense gas releases will mix and be diluted with fresh air as the gas travels downwind and eventually behave as a neutrally buoyant cloud. Thus, neutrally buoyant models approximate the behavior of any vapor cloud at some distance downwind from its release. Neutrally or positively buoyant plumes and puffs have been studied for many years using Gaussian models. These studies have included especially the dispersion modeling of power station emissions and other air contaminants used for air pollution studies. Gaussian plumes are discussed in more detail in Section 2.3.1. 

The dispersion model is typically used to determine the downwind concentrations of released materials and the total area affected. Two models are available the plume and the puff. The plume describes continuous releases the puff describes instantaneous releases. 

Twenty-four subjects completed an 8-period balanced crossover study. For each subject, pharmacodynamic responses are taken at the test baseline, test 1 puff, test 4 puffs, reference baseline, reference 1 puff, and reference 4 puffs. A population model fit (Section 16.6.1.2) was conducted to describe the data. Responses at baseline were similar for both formulations, although they appeared to differ somewhat at 1 puff and 4 puffs. Overall, the profiles of the two formulations were judged as similar. 

Box models are used to describe instantaneous heavy gas releases (puffs). Physical aspects of interest for a box model are ... 

The flash fire can occur as a result of delayed ignition of a cloud of gas. In this case a time-limited steady release of gas is simulated as series of puffs, each of which is considered as a separate release described by Pasquill-Gifford model. Both atmosphere stability and direction and speed of wind are taken into account, as the gas dispersion depends on these factors. The computation of individual risk for flash fire reflects the fact that the heat flux, the probability of ignition of dispersed gas and exposure time vary in time and space. 

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