Pasquill stability classes

In addition to short-term emission estimates, normally for hourly periods, the meteorological data include hourly wind direction, wind speed, and Pasquill stability class. Although of secondary importance, the hourly data also include temperature (only important if buoyant plume rise needs to be calculated from any sources) and mixing height. 

Other estimations of o-y and cr by Briggs for two different situations, urban and rural, for each Pasquill stability class, as a function of distance between source and receptor, are given in Tables 19-6 and 19-7 (12). 

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 ... 

Numerous analyses of data routinely collected in the United States have been performed by the U.S. National Climatic Center, results of these analyses are available at reasonable cost. The joint frequency of Pasquill stability class, wind direction class (primarily to 16 compass points), and wind speed class (in six classes) has been determined for various periods of record for over 200 observation stations in the United States from either hourly or 3-hourly data. A computer program called STAR (STability ARray) estimates the Pasquill class from the elevation of the sun (approximated from the hour and time of year), wind speed, cloud cover, and ceiling height. STAR output for seasons and the entire period of record can be obtained from the Center. Table 21-2 is similar in format to the standard output. This table gives the frequencies for D stability, based on a total of 100 for all stabilities. 

Data for one full year (1964) for Nashville, Tennessee, and Knoxville, Tennessee, 265 km (165 mi) apart, were compared to determine the extent to which the frequencies of various parameters were similar. Knoxville is located in an area with mountainous ridges oriented southwest-northeast Nashville is situated in a comparahvely flat area. The data available are the number of hours during which each of 36 wind directions (every 10° azimuth) occurred, the average wind speed for each direction, the number of hours of each Pasquill stability class for each direchon, and the mean annual wind speed. 

Fig. 21-9. Stability rose (direction-Pasquill stability class) for O Hare Airport, Chicago, 1965-1969,...

Figure 21-9 is a stability wind rose that indicates Pasquill stability class frequencies for each direction. For this location, the various stabilities seem to be nearly a set proportion of the frequency for that direction the larger the total frequency for that direction, the greater the frequency for each stability. Since the frequencies of A and B stabilities are quite small (0.72% for A and 4.92% for all three unstable classes (A, B, and C) are added together and indicated by the single line. 

The Monin-Obukhov length L is not a parameter that is routinely measured. Colder (1972), however, established a relation between the stability classes of Pasquill, the roughness height zo (see Section VII,B), and L. The results of his investigation are shown in Fig. 4. Alternatively, the local wind speed and cloud cover measurements are used to estimate the Pasquill stability class (Table IV). In addition, Colder developed a nonogram for relating the gradient Richardson number R-, to the more easily determined bulk Richardson number / (, ... 

Fig. 4. Relationship between Monin-Obukhov length L and roughness height Zo for various Pasquill stability classes. From Myrup and Ranzieri (1976).

Stability condition Pasquill stability class Coefficients ... 

Table 1 gives the equivalence between the Monin-Obukhov lengthscale and the Pasquill stability classes. 

Table 1. The equivalence between the Monin-Obukhov lengthscale, L (expressed in terms of the dimensionless ratio z/L, where z (m) is the height above the surface) and the Pasquill stability classes (Jacobson, 1999)...

Use of the equations derived in this section requires estimation of the Monin-Obukhov length L. A number of approaches are available, including the profile and gradient methods using available measurements (Arya 1999). The simplest approach based on the Pasquill stability classes will be discussed in the next section. 

The most widely used ay and ct- correlations based on the Pasquill stability classes have been those developed by Gifford (1961). The correlations, commonly referred to as the Pasquill-Gifford curves, appear in Figures 18.4 and 18.5. 

FIGURE 18.4 Correlations for crv based on the Pasquill stability classes A to F (Gifford 1961). These are the so-called Pasquill-Gifford curves. 

Colder (1972) established a relation between the Pasquill stability classes, the roughness length zo and L (Figure 16.8). To simplify calculation of /L, Colder s plot can be approximated by the correlation... 

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