Atmospheric stability, pollution

In another review, Hoffert discussed the social motivations for modeling air quality for predictive purposes and elucidated the components of a model. Meteorologic factors were summarized in terms of windfields and atmospheric stability as they are traditionally represented mathematically. The species-balance equation was discussed, and several solutions of the equation for constant-diffusion coefficient and concentrated sources were suggested. Gaussian plume and puff results were related to the problems of developing multiple-source urban-dispersion models. Numerical solutions and box models were then considered. The review concluded with a brief outline of the atmospheric chemical effects that influence the concentration of pollutants by transformation. 

Because of their electrical, optical, and redox properties as well as the thermal and chemical stability, the Pcs also have been tried in the detection of volatile organic compounds and poisonous gases, which is very important for environment and human health. In the past decades, the possible applications of Pc thin film as sensor for atmospheric gaseous pollutants have been extensively studied [73, 74], Langmuir-Blodgett films of some multinuclear and multidouble-decker lutetium Pcs have also been used for those measurements [75,76], More details about conductivity and sensing properties of Pcs can be found elsewhere [77,78]. 

The mean residence time of pollutants depends upon various factors such as the presence of specific components of the atmosphere, atmospheric stability and precipitation frequency. 

One commonly used suite of models that is based on Gaussian plume modeling is the Industrial Source Complex (ISC) Dispersion Models (US EPA, 1995). This suite includes both a short-term model (ISCST), which calculates the hourly air pollutant concentrations in an area surrounding a source, as well as a long-term model (ISCLT), which calculates the average air pollutant concentrations over a year or longer. ISCLT uses meteorological data summarized by frequency for 16 radial sectors (22.5° each) this data format is referred to as a stability array (STAR). Within each sector of STAR, joint frequencies of wind direction, wind speed, and atmospheric stability class are provided. 

This solution describes a plume with a Gaussian distribution of pollutant concentrations, such as that in Figure 5, where cy (.x) and cy (.x) are the standard deviations of the mean concentration in the y and directions. The standard deviations are the directional diffusion parameters, and are assumed to be related simply to the turbulent diffusivities, and iW In practice, G (x) and GjAj are functions of. x U, and atmospheric stability (2,31—33). 

The lapse rate in the lower portion of the atmosphere has a great influence on the vertical motion of air. Buoyancy can resist or enhance vertical air motion of airmasses, thus affecting the mixing of pollutants. Comparing the environmental lapse rate A with the adiabatic lapse rate T, we can define three regimes of atmospheric stability (Figure 16.2) ... 

In this section we consider the variation of wind with height in the surface and Ekman layers, which constitute the so-called planetary boundary layer. Most of our attention will be devoted to the surface layer, the region in which pollutants are usually first released. The exact vertical distribution of wind velocity depends on a number of parameters, including the surface roughness and the atmospheric stability. 

In these computations we have assumed a constant dry deposition velocity, although it is actually dependent, among other factors, on wind speed and atmospheric stability. In particular, the dry deposition velocity is typically smaller in stable conditions. However, the loss of pollutant from the plume depends also on the vertical concentration distribution. For stable conditions, the plume is substantially more shallow dry deposition therefore causes an increased decrease of concentration at larger distances, compared with the corresponding case in neutral stability. The apparent convergence of the concentration curves for the two stability classes is fortuitous. 

How the atmosphere behaves when air is displaced vertically is a function of atmospheric stability. A stable atmosphere resists vertical motion air that is displaced vertically in a stable atmosphere tends to return to its original position. This atmospheric characteristic determines the ability of the atmosphere to disperse pollutants emitted into it. To understand atmospheric stability and the role it plays in pollution dispersion, it is important to imderstand the mechanics of the atmosphere as they relate to vertical atmospheric motion. 

An inversion occurs when air temperature increases with altitude. Temperature inversions (extreme cases of atmospheric stability) create a virtual lid on the upward movement of atmospheric pollution. This situation occurs frequently but is generally confined to a relatively shallow layer. Plumes emitted into air layers that are experiencing an inversion (inverted layer) do not disperse very much as they are transported with the wind. Plumes that are emitted above or below an inverted layer do not penetrate that layer rather, these plumes are trapped either above or below that inverted layer. High concentrations of air pollufants are often associated with inversions, as they inhibit plume dispersions. Two types of inversions are important from an air quality standpoint radiation and subsidence inversions. 

Perrino, C., A. Pietrodangelo, and A. Febo. An atmospheric stability index based on radon progeny measurements for the evaluation of primary urban pollution Atm. Environ. 35 5235-5244. 

Inversions are of considerable interest in relation to air pollution because of their stabilizing influence on the atmosphere, which suppresses the vertical motion that causes the vertical spreading of pollutants. 

As an example of the use of the Gaussian plume equations using the Pasquill-Gifford dispersion parameters, assume that a source releases 0.37 g s of a pollutant at an effective height of 40 m into the atmosphere with the wind blowing at 2 m s . What is the approximate distance of the maximum concentration, and what is the concentration at this point if the atmosphere is appropriately represented by Pasquill stability class B ... 

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