Atmospheric dispersion factors affecting

It is apparent from equations 3.2.4-3.2.7 that the determination of the concentration field is dependent on the values of the Gaussian dispersion parameters a, (or Oy in the fully coupled puff model). Drawing on the fundamental result provided by Taylor (1923), it would be expected that these parameters would relate directly to the statistics of the components of the fluctuating element of the flow velocity. In a neutral atmosphere, the factors affecting these components can be explored by considering the fundamental equations of fluid motion in an incompressible fluid (for airflows less than 70% of the speed of sound, airflows can reasonably be modeled as incompressible) when the temperature of the atmosphere varies with elevation, the fluid must be modeled as compressible (in other words, the density is treated as a variable). The set of equations governing the flow of an incompressible Newtonian fluid at any point at any instant is as follows ... 

The third factor affecting dispersion is turbulence. Mechanical turbulence is caused by the roughness of the Earth s surface. Away from the surface, convective turbulence (heated air rising and cooler air falling) becomes increasingly important. The amount of turbulence and the height to which it operates depends on the surface roughness, wind speed and atmospheric stability. 

In most real-world applications for which the controlhng meteorological parameters are measured from a tower, conditions are reasonably steady and horizontally homogeneous (less than 50%) variation from the spatial and temporal average dirring the experiment) and no exceptional circimistances exist that could affect the atmospheric dispersive capacity in ways not accoimted for by the model, accmacy within a factor of 2 can be ejqrected. 

The objective must be to reduce atmospheric emissions to a minimum or at least below legislative requirements. However, there is inevitably some residual emission and this must be safely dispersed in the environment. The factors that affect the dispersion of gases to atmosphere are3 ... 

Enokida et al. (1991) measured hole mdbilities of PMPS before and after ultraviolet exposures. The exposures were of the order of 1 erg/s-cm2. Prior to the exposures, the mobilities were approximately 10-4 cm2/Vs and weakly field dependent. Following the exposures, a decrease in the mobility was observed. Under vacuum exposure conditions, a decrease of approximately 40% was observed for a 1 h exposure. Under atmospheric conditions, however, the decrease was approximately a factor of 4. Enokida et al. attributed the decrease in mobility to the formation of Si-O-Si bonds in the Si backbone chain. A similar study of PMPS was described by Naito et al. (1991). While the field and temperature dependencies of the mobility were not affected by the ultraviolet exposures, the dispersion in transit times increased significantly. The change in dispersion could be removed by subsequent annealing. The authors attributed the increase in transit time dispersion to a reduction in the hole lifetime, induced by Si dangling bonds created by the ultraviolet radiation. 

The residence time of chemicals in soil and water is, among other factors, determined by the volatility of the substances - the tendency to evaporate into the air compartment. Partitioning and transport of substances between environmental media are thus affected by vapour pressure ip ). Highly volatile chemicals have the potential for rapid, long-distance dispersion in the atmosphere, and can be taken up by terrestrial animals by inhalation or skin absorption, by terrestrial plants through stomatal pores or the cuticle, as well as into water bodies. 

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