Gaussian Dispersion Model II

Gaussian Dispersion Model II - 9/13/11 Aerosol...

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Unformatted text preview: 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 11 Effect of Sampling Time Period Peak to 1 hour 1 hour 1.0 30 minutes 1.3 10 minutes 2.3 3 minutes 4 1 minute 4-7 30 seconds 4-10 Peak to mean concentrations ratio (observed at ground level ) Variation of calculated conc. with sampling time ( observed at height of release ) Sam pling tim e Ratio of calculated conc. to 3 m in conc. 3 m inutes 1.00 15 m inutes 0.82 1 hour 0.61 3 hours 0.51 24 hours 0.36 Q: Why does sampling time matter? 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 22 p t t C C = 2 1 1 2 p = 0.17-0.2 good for < 10 min sampling time p = 0.5 good for > 10 min sampling time C2 : desired concentration C1 : concentration estimated by the short sampling time t1 : 10 minutes (typical standard for Gaussian model) t2 : sampling period Q: Whats the sampling time used in the dispersion coefficients based on Pasquill-Gifford stability? 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 33 Dispersion Using Fluctuation Statistics y a y f x ) tan( = z e z f x ) tan( = a : standard deviation of the azimuth angle (wind direction) of the wind over a period of time e : standard deviation of the elevation angle of the wind over a period of time t : travel time between source to receptor [ ] o t t f / 9 . 1 / 1 + = For fy , t0 = 1000 For fz , t0 = 500 (unstable, daytime neutral) = 50 (stable, nighttime neutral) For non-rural/open country locations where Pasquills values are not applicable, use measured meteorological data 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 44 ASTM method 71 . 71 . 15 . 15 . x x e z a y = = 86 . 86 . 045 . 045 . x x e z a y = = (stable, T/ z > 0) (unstable, T/ z < 0) Q: Where should we measure the wind velocity? 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 55 Buoyancy-Induced Dispersion Entrainment of ambient air into the plume during vertical rising http://meted.ucar.edu Causes the rapid growth of the plume; hence, not an infinitely small source Momentum plume: results from the shearing motion between the vertical rising plume and the outside air Buoyantly rising plume: like boiling with circular eddies 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 66 5 . 3 / h z y = = 2 2 2 2 z y ze y y ye + = + = The effects near the point of release are equal for horizontal and vertical plume rises Q: How does it affect dispersion? Q: Is buoyancy-induced dispersion important for a large x? Effect small near the source; max effect at the point of max rise As the plume is affected by the wind and becomes horizontal, the plume reaches final size and is symmetrical in the horizontal and the vertical Initial plume sizes Inclusion in dispersion parameters 9/13/11 Aerosol & Particulate 9/13/11 Aerosol & Particulate 77 Q: A small source with an effective height of 12 m (physical height of 8 m) is releasing 1.5 g/s ofm (physical height of 8 m) is releasing 1....
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Gaussian Dispersion Model II - 9/13/11 Aerosol...

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