Atmospheric Stability Categories Stability CategoryConditionDescription Lapse Rate AExtremely UnstableSunny Summer Weather-1.9 BModerately StableSunny and Warm-1.9 to –1.7 CSlightly UnstableAverage Day-1.7 to –1.5 DNeutralOvercast Day or Night-1.5 to –0.5 ESlightly StableAverage Night-0.5 to 1.5 FModerately StableClear Night1.5 to 4.0 GHighly StableAdded by NRC> 4.0
Ground Level Sector Averaged Concentration – Ground Release Set H = 0
Time-Averaged Concentration Wind Directions in Each Sector are Distributed Randomly Over Period of Interest Calculate X/Q Using Joint Frequency Distribution: f( ,S,N) – Direction –SStability Class –NWind Speed Class
Ground Level Sector and Time Averaged Concentration – Ground Release
Estimation of Release from Area Source Assume Point Source at Center of Release –Very Conservative –Does not consider that source is initially distributed over large surface area.
Estimation of Release from Area Source Turner (Workbook of Atmospheric Dispersion Estimates, 1994) –Treat area source as having initial horizontal standard deviation - yo - related to area width. –Horizontal standard deviation for square source is approximated by L/4.3 (L= Length of a side of the area).
Estimation of Release from Area Source Turner (Workbook of Atmospheric Dispersion Estimates, 1994) –Select “Virtual Distance” - x y - based on yo. –Calculate X/Q using distance of x + x y.
Simple Case Calculate X/Q Assuming –Ground Level Release –Emission Source is One Mile Square –Receptor is Due West ½ Mile from Center of Source (i.e. at Boundary) –Assume Worst Case Met Conditions Extremely Stabile (Class G) Calm Conditions (0.04 m/s) Least Dispersion
Geometry for Point Source 1600 meters Receptor Point Source Distance – 800 m
Simple X/Q for Point Source u = 0.022 m/s x = 800 m zG = 7.5 m
Simple X/Q for Area Source Calculate yo based on 1 mile side –1600 meters/4.3 = 372 m From y table/plot look up distance corresponding to yo for stability class of interest. –20000 m Calculate X/Q for virtual distance: 20,000 + 800 = 20,800 m.
Geometry for Area Source 1600 meters Receptor Point Source Virtual Distance – 20,800 m
Simple X/Q for Area Source u = 0.022 m/s x = 20,800 m zG = 7.5 m
Applying JFD Data to X/Q XOQDOQ Provides Summary of JFD Data by Stability Class, Sector and Wind Speed JFD for Receptor in West Sector
Applying JFD Data to X/Q Use Average Wind Speed (Not Max Wind Speed) Determine yo for Each Stability Class Determine Virtual Distance (X v ) for Each Stability Class