Local-Scale Outdoor Air Pollution

The smallest spatial scale at which outdoor air pollution is of concern corresponds to the air volume affected by pollutant chemical emissions from a single point source, such as a smokestack (Fig. 4-24). Chemicals are carried downwind by advection, while turbulent transport (typically modeled as Fick-ian transport) causes the chemical concentrations to become more diluted. Typically, smokestacks produce continuous pollutant emissions, instead of single pulses of pollutants thus, steady-state analysis is often appropriate. At some distance downwind, the plume of chemical pollutants disperses sufficiently to reach the ground the point at which this occurs, and the concentrations of the chemicals at this point and elsewhere, can be estimated from solutions to the advection-dispersion-reaction equation (Section 1.5), given a knowledge of the air (wind) velocity and the magnitude of Fickian transport. 

The smokestack plume problem is entirely analogous to the groundwater plume problem, although the mechanism for mixing is turbulent diffusion instead of mechanical dispersion. The traditional steady-state Gaussian plume 

FIGURE 4-24 Cross sections of pollutant concentrations at two locations downwind of a smokestack. Note that physical height of the stack is typically less than the effective height of the stack, which takes plume rise into account. The total flux of pollutant is identical at each downwind location, although concentration decreases as the plume widens. The shape of each concentration versus distance plot is a normal, or Gaussian, curve hence, this is often called a Gaussian plume (adapted from Boubel et al, 1994). 

One classic Gaussian plume model for smokestack emissions is the Pas-quill-Gifford model, which applies for steady emissions of a chemical over relatively level terrain. If no chemical sinks exist in the air (i.e., no reactions are degrading the chemical) and if there is an unlimited mixing height (i.e., no atmospheric inversion exists, and the plume can be mixed upward indefinitely), the Pasquill- Gifford model can be expressed in the form 

The final rise is the plume rise occurring downwind at the distance where turbulent updrafts and downdrafts result in vertical motions of similar magnitude to those caused by the buoyancy forces. Other formulae for estimating AH under different conditions, and for estimating final rise, are given by Boubel et al. (1994). 

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