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CE 511 - Session 11

CE 511 - Session 11. Metereology & Dispersion Modeling. Objectives. Discuss the sources of turbulence Discuss atmospheric stability and lapse rates Discuss the factors that effect dispersion Calculate ground-level pollutant concentrations using a point source gaussian dispersion model .

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CE 511 - Session 11

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  1. CE 511 - Session 11 Metereology & Dispersion Modeling

  2. Objectives • Discuss the sources of turbulence • Discuss atmospheric stability and lapse rates • Discuss the factors that effect dispersion • Calculate ground-level pollutant concentrations using a point source gaussian dispersion model

  3. Meteorology - Driving Processes • Heat from the Sun • Latitude/Orientation • Rotation • Heat Conductivity • In Sum - induce expansion, compression, heat exchange and interaction of the atmosphere with the earth’s surface.

  4. Pressure • Differences in Pressure induce motion

  5. Turbulence • Mechanical Turbulence • Random fluctuation in wind speed and direction • Cause: wind flowing past objects, 0 at surface, increases with elevation • Higher speed ---- greater turbulence • More turbulence -- more dispersion of pollutants • Thermal turbulence • Heating the ground --- heats air --- less dense air rises

  6. Stability - Dry Adiabatic Lapse Rate • Related to • wind speed • changes in air temperature with height • Dry Adiabatic Lapse Rate () • The rate of temperature decrease with elevation equals -1.00C/100m • Evaluate stability by comparing ambient lapse rate to 

  7. Neutral (Adiabatic) - Ambient =  • Neutral stability - thermal structure doesn’t affect mechanical turbulence

  8. Unstable (Superadiabatic)- ambient >  • Unstable air - mechanical turbulence enhanced by thermal structure - characterized by cyclones (low-pressure areas)

  9. Stable (Subadiabatic) ambient <  • Stable air - mechanical turbulence inhibited by thermal structure -- characterized by anticyclones (high-pressure)

  10. Special Subadiabatic Lapse Rates • Isothermal - no change in temperature with elevation • Inversion - temperature increases with elevation • Condition often associated with pollution episodes due to restricted air volumes

  11. Terrain Effects on Stability • Heat Islands (cities, large industrial sites) • atmosphere less stable than over farm land • Good news - ground level pollutants diluted • Bad news - smoke stack emissions mix more and some may come to ground in city • Land/sea breezes • Land heats/cools more rapidly than water • Night - offshore breeze • Day - onshore breeze • Valley breezes

  12. Factors That Affect Pollutant Dispersion • Emission point characteristics • Nature of pollutants • Meteorological conditions • Effects of terrain and structures

  13. Classes of Dispersion Models • Short-term • Estimate ambient concentrations where measurements are impractical • Estimate source reductions needed during pollution alerts or inversions • Estimate probably locations of high ground-level concentrations for siting • Climatological • Used to estimate average pollutant concentrations over a long time or during specific times

  14. Basic Point SourceGaussian Dispersion Model Assumptions • Layer into which contaminated gas is discharged is uniform in stability, wind speed and direction • Turbulence is random and can be defined by normal distribution • Contaminated gas is released at effective stack height • Degree of dilution is inversely proportional to wind speed • Pollutant is conservative • Does not decay • When hitting the ground is completely reflected

  15. Plume Dispersion Coordinate System

  16. Basic Point Source Model Equation c(x,y,0,H) = downwind concentration at ground level, g/m3 E = emission rate of pollutant, g/s sy = horizontal dispersion coefficient (std. dev.), m sz,= vertical dispersion coefficient (std. dev.), m u = wind speed m/s x,y,z,H = distances, m

  17. Effective Stack Height H = h + H H = effective stack height, m h = physical stack height, m H = plume rise vs = stack velocity, m/s d = stack diameter, m u = wind velocity, m/s P = pressure, kPa Ts = stack temp., K Ta = air temp., K

  18. Determining sz and sy • Dependent upon the turbulent structure of atmosphere (wind speed and sunlight intensity) - see Table 11-6 • Stability Classes A very unstable B unstable C slightly unstable to neutral D neutral to stable E stable F very stable

  19. Estimating sz and sy

  20. Calculation of sy and sz a,c,d,f from text (Table 11-7) depend on stability class (A - F) distance downwind, x (km)

  21. Inversion Aloft If an inversion occurs above the plume, the model needs to account for the fact that the plume cannot disperse vertically above the inversion

  22. Example • A 1000 MW coal-fired power plant emits SO2 at the legally allowed limit of 6.47 mg/sec. The stack has an effective height of 300 m. It is a cloudy summer day and the windspeed at 10-m is measured at 2.5 m/sec. What is the ground level concentration of SO2 4 km directly downwind.

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