Stability category

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).

Certainly the directional frequencies of these sites cannot be considered as representative of each other. However, this comparison would seem to indicate that the percentage of the stability categories may be more conservative over distance than the direction frequencies. 

Doty, S. R., Wallace, B. L., and Holzworth, G. C., "A Climatological Analysis of Pasquill Stability Categories Based on STAR Summaries." National Climatic Center, Environmental Data Service, National Oceanic and Atmospheric Administration, Asheville, NC, 1976. 

The range of directions used in the search is determined from a set of look-up tables based on the aspect ratio of the area source, the stability category, and the downwind distance. The SCREEN model also provides the user an option to specify a wind direction orientation relative to the long axis of the rectangular area. The second option may be used to estimate the concentration at a particular receptor location relative to the area. The output table for area sources includes the wind direction associated with the maximum concentration at each distance. 

STAR The STAR program summarizes National Weather Services (NWS) meteorological data, by generating joint frequencies of 6 wind speeds, 16 wind directions, and 6 stability categories for the station and time period desired. 

Figure 12.6.3. Variation of concentration profile downwind from point source A-F designate stability categories listed in Table 12.6.1...

A truck carr) ing two tanks containing a very unstable and hazardous gas is involved in an accident that results in tlie consecutive explosion of the tanks -one innncdiately, the second approximately a minute later. The total mass of tlie emission resulting from the explosion of each tank is 30,000 g. The wind velocity is 1 m/s from the north, and tlie effective height of emission is 30 meters at the time of tlie accident. Calculate the concentration of tliis gas at 500 meters south but 100 meters east from the site 10 minutes after tlie explosion of tlie first tank. Assume tliat stability category D applies. 

A 40-nietcr elevated vapor transfer line in a petroleum refinery suddenly ruptures. It is estimated that 375 g/s of a hazardous gas is being emitted. If the emission from the mpture is brought under control 2 minutes later, estimate the concentration 1000 meters downwind and 100 meters displaced from centerline eonditions 5 minutes later. The wind velocity is 4.0 m/s and the stability category is D. 

Legislation on air pollution of concern to the plant engineer 45/821 Table 45.2 Pasquill s stability categories... 

Atmospheric stability and mechanical turbulence (important near to the ground) are used to derive the vertical and horizontal dispersion coefficients. Table 45.2 shows Pasquill s stability categories used to derive the coefficients by reference to standard graphs. 

Relationship of Pasquill Stability Categories to Ranges of the Parameter a ... 

Weather conditions have a major effect on the mechanisms and extent a release spreads. Meteorological data are readily available from the National Oceanic and Atmosphere Administration (NOAA) and from airports in the vicinity of the facility. These data include wind direction, several wind speeds, and several atmospheric stability categories. A typical 16-point wind rose is... 

Precipitation during the 12-hour period before filter collection decreased the radon concentration, as shown in Table II for three stability categories. The effect is especially apparent at high stabilities radon concentrations were almost two-fold lower during precepitation at sta-... 

The calculation of population radiation effects is performed on the basis of different atmosphere stability categories. The estimated maximum annual effective dose for Severodvinsk population at the boimdary of the Sanitary Protection Area (500 m from radioactive release point) will be 0.1 pSv. It is considerably less than ImSv (the population dose limit under normal operation conditions in accordance with Radiation Safety Codes -NRB-99). 

There are limitations to estimating cry and atmospheric stability categories further errors may result from applying these models to settings that differ in local topography or climatic conditions. Typically, these models apply to high plumes and downwind distances less than 10 km. 

FIGURE 4-25a Standard deviations of mass distribution in a Gaussian plume, cry and az, given as a function of both distance downwind from a point source and Pasquill stability categories. Dispersion coefficient as used in this figure means the standard deviation of the plume width or height [L] rather than a Fickian coefficient [L2/T], (From Atmospheric Chemistry and Physics of Air Pollution, by J. H. Seinfeld. Copyright 1986, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

To calculate downwind concentrations, use Eq. [4-16]. From Table 4-6, a wind speed of 4.5 m/sec corresponds to Pasquill stability category C during the winter when insolation is low. By using Fig. 4-25, at a downwind distance of 8 km, cry is approximately 700 m and crz is approximately 400 m. Note thaty = 0 directly downwind and z = 0 at ground level. First, estimate Q/u ... 

From Table 4-6, the Pasquill stability category is D. Turbulent intensity in the vertical direction can be calculated from Eq. [4-21] ... 

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