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6.4 Stability

Stability typing is employed in air quality dispersion modeling to facilitate  estimates oflateral and vertical dispersion parameters [e.g., the standard deviation of plume concentration in the lateral (y ) and vertical (z )] used in Gaussian plume models. The preferred stability typing scheme, recommended for use in regulatory air quality modeling applications is the scheme proposed in an article by Pasquill in 1961 [33]; the dispersion parameters associated with this scheme [often referred to as the Pasquill-Gifford (P-G) sigma curves] are used by default in most of the EPA recommended Gaussian dispersion models.

Table 6-3 provides a key to the Pasquill stability categories as originally defined;  thoughimpractical for routine application, the original scheme provided a basis for much of the developmental work in dispersion modeling. For routine applications using the P-G sigmas, the Pasquill stability category (hereafter referred to as the P-G stability category) should be calculated using the method developed by Turner [34]; Turner's method is described in Section 6.4.1. Subsequent sections describe alternative methods for estimating the P-G stability categorywhen representative cloud cover and ceiling data are not available. These include a radiation-based method which uses measurements of solar radiation during the day and delta-T at night (Section 6.4.2) and turbulence-based methods which use wind fluctuation statistics (Sections 6.4.3 and 6.4.4). Procedures for the latter are based on the technical note published by Irwin in 1980 [35]; userís are referred to the technical note for background on the estimation of P-G stability categories.

Table 6-3
Key to the Pasquill Stability Categories
Surface Wind
Speed (m/s
)
Daytime InsolationNighttime cloud cover
StrongModerateSlightThinly overcast or 4/8 low cloud3/8
< 2A - BB--
2 - 3A - BBCEF
3 - 5BB - CCDE
5 - 6CC - DDDD
> 6CDDDD
Strong insolation corresponds to sunny, middlay, midsummer conditions in England; slight insolation corresponds to similar conditions in midwater. Night refers to the period from one hour after sunrise. The neutral category, D, should be used regardless of wind speed, for overcast conditions during day or night

6. METEOROLOGICAL DATA PROCESSING
  6.1 Averaging and Sampling Strategies 
  6.2 Wind Direction and Wind Speed 

      6.2.1 Scalar Computations 
      6.2.2 Vector Computations 
      6.2.3 Treatment of Calms  
      6.2.4 Turbulence 
      6.2.5 Wind Speed Profiles  
  6.3 Temperature 
     
6.3.1 Use in Plume-Rise Estimates  
      6.3.2 Vertical Temperature Gradient 
  6.4 Stability 
      6.4.1 Turner's method  
      6.4.2 Solar radiation/delta-T (SRDT) method 
      6.4.3  E method 
      6.4.4 Amethod 
      6.4.5 Accuracy of stability category estimate
  6.5 Mixing Height 
      6.5.1 The Holzworth Method  
  6.6 Boundary Layer Parameters  
      6.6.1 The Profile Method 
      6.6.2 The Energy Budget Method  
      6.6.3 Surface Roughness Length 
      6.6.4 Guidance for Measurements in the Surface Layer 
  6.7 Use of Airport Data 
 
6.8 Treatment of Missing Data  
      6.8.1 Substitution Procedures 
  6.9 Recommendations


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