Ground level concentration

The NAAQS are expressed ia the form of ground level concentrations (GLC), which are the concentrations of pollutant ia the ambient air as measured at ground level, ia units of either micrograms per cubic meter or ppm. In order to convert a source s emission ia kilograms per hour to a GLC, dispersion modeling must be used. 

Maximum Ground-Level Concentrations The effective height of an emission having been determined, the next step is to study its path downward by using the appropriate atmospheric-dispersion formula. Some of the more popular atmospheric-dispersion calculational procedures have been summarized by Buonicore and Theodore (op. cit.) and include ... 

A pollutant is released from an effective height of 50 m and has a ground-level concentra of 300 ng m at a position directly downwind where the ground-level concentration ... 

Fig. 20-1. Concentration of an air pollutant at the point of maximum ground-level concentration as a function of wind speed and Pasquill stability category (A-F).

Air Pollution Dispersion Application of air dispersion modeling principles and EPA tools to assessing environmental impacts from stack and area releases of pollutants Dispersion theory Gaussian plume model Ground-level concentrations Worst case scenarios Air quality impact assessments Stationary source emissions... 

Maximum ground level concentration and its location can be calculated from the following expression ... 

Using the Gaussian plume model and the other relations presented, it is possible to compute ground level concentrations C, at any receptor point (Xq, in the region resulting from each of the isolated sources in the emission inventory. Since Equation (2) is linear for zero or linear decay terms, superposition of solutions applies. The concentration distribution is available by computing the values of C, at various receptors and summing over all sources. 

One major item remains before we can apply the dispersion methodology to elevated emission sources, namely plume height elevation or rise. Once the plume rise has been determined, diffusion analyses based on the classical Gaussian diffusion model may be used to determine the ground-level concentration of the pollutant. Comparison with the applicable standards may then be made to demonstrate compliance with a legal discharge standard. 

The values of u and A0/AZ are based on assumed conditions of stability class F and stack height wind speed of 2.5 m/s for the stable layer above the inversion. The value of hj incorporates the effect of buoyancy induced dispersion on a/, however, elevated terrain effects are igndred. The equation above is solved by iteration, starting from an initial guess of x , = 5,000 m. The maximum ground-level concentration due to inversion break-up fumigation, Xf, is calculated from ... 

As with the inversion break-up case, the distance to maximum ground-level concentration is determined by iteration. The equation used for the shoreline fumigation case is ... 

The maximum ground-level concentration due to shoreline fumigation, Xf. is calculated from ... 

The ground-level concentration in the recirculation region is calculated assuming the mass fraction of the plume, below HR at the downwind end of the region, is captured into the region. The calculation assumes a Gaussian... 

In this evaluation SCREEN was applied to calculating Ground Level Concentrations (GLCs). In this analysis, we assume an average emission rate over the length of the fire incident. Literature information supports that a fire of this magnitude and under uncontrolled conditions, would consume anywhere from 70 to 90 % of the products. The entire fire scenario is... 

Preferably, the HjS flare system should consist of a segregated header and separate line routed up the side of a conventional elevated flare stack, sharing the same structure, pilots and igniters. However, the HjS header may be tied into the regular flare seal drum if there are special mechanical design problems associated with the separate stack e.g., in the case of a flare which is to be dismantled for overhaul. Flare elevation must be sufficient to meet atmospheric pollution and ground level concentration requirements for the sulfur dioxide produced. 

Table 2. Maximum Ground Level Concentration for A 5-min. Blow (Worst Case)...

Three commonly used dispersion calculation methods for the prediction of ground level concentrations are based on the above expression. The variance in each method is the calculation of plume rise, Ah, and the horizontal and vertical plume dispersion parameters. These methods are ... 

Tables 6 provides a summary of the predicted GLCs under worst case meteorological conditions. Note that the ground level concentrations summarized do not consider wind direction nor variation. More specifically, two points within the plant vicinity are presented to summarize wind persistence as related to known landmarks ...

Maximum ground level concentrations estimated from the (episode) case condition assumed herewith within close proximity of the plant (up to 5-miles) range as follows ... 

Because of extreme venting conditions assumed, effective stack heights and resultant plumes from both 3- and 5-minute discharge conditions attain heights beyond the micro-meteorological conditions assumed in accepted computation models. It is therefore highly probable there will be considerably further atmospheric dispersion and diffusion of the VCM than predicted in the results shown. That is, the ground level concentration can be expected to be considerably lower than the values shown in Table 6. 

Table 6. Summary of Predicted/estimated Ground Level Concentration at Worst (Episode) Case Conditions and Weather.

Stability Class Surface Wind Worst Case Ground Level Concentration (ppm) Distance From... 

In the calculations that were made to predict ground level concentrations from a VCM reactor blow off, the Pasquill-Gifford-Holland dispersion model was used as a basis for these estimations. Calculations were made for six different stability classes and ground level concentrations, and at various distances from the point source of emission. 

No correlation was made initially as to wind direction, nor to probability of any wind direction/weather condition or percent time of occurrence, however, this is certainly an important factor in the probability of the pollutant being at a concentration predicted. The greatest significance is attached to predicting an ultimate ground level concentration from any potential episode. 

The range of ground level concentrations varies widely depending on wind speed, distance from the emission source, duration of the emission. 

Tables 7 and 8 give the short-term ground level concentrations for a 3-minute venting at 1/2, 1.0, 1-1/2, 2.0, and 5 miles from the discharge point respectively. The calculations assume meteorological conditions to be constant for approximately 10-minutes. Values in Tables 7 and 8 were computed for the six stability classes over a range of wind speeds.

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