Air Pollution Dispersion

Transcription

1 Air Pollution Dispersion Dispersion Processes Convective Dispersion Air Parcel Dynamics Adiabatic Process Lapse Rate Equilibrium and Stability Atmospheric Stability Stability and Dispersion

2 Temperature Inversions Stability Formation/Types Mixing Height Application: Chimney Plumes Plume Type vs. Stability Enhancing Plume Dispersion

3 Diffusion Turbulence Convection Dispersion Processes Buoyancy Air Parcel Motion Stability Stable Unstable Neutral Lapse Rates Air Pollution Dispersion Temperature I nversion Mixing Height Radiation Advection Subsidence Adiabatic Processes Chimney Plumes

4 Types Temperature I nversions Radiation Advecti on Subsidence Sinking air Nighttime cooling Winds Regional Large-scale Mixing Height Air Pollution Dispersion Seasonal Ventilation Factor Chimney Plumes Types Dispersion Tall Enhancem ent Stacks Turbulence Exit Velocity Exit Temperature

5 Dispersion Processes Defn.: A substance mixes in and becomes diluted within a larger volume of another substance. Molecular Diffusion Turbulence Convection/Advection

6 Molecular Diffusion Molecules drift from regions of high concentration to regions of lower concentration Larger concentration gradient higher diffusion rate Length scale of motion = molecular Slow!!

7 Turbulence Bulk air motion in random directions y Wind x Plume Core Plume Edge }Plume Pollution Strong, gusty winds generate the most turbulence

8 Convection/Advection Mass transport of pollutants by winds Advection: horizontal motion Convection: vertical motion

9 Air Parcel Mechanics A specified volume of air (ex.: bubble, balloon) P out Constraints: P inside = P outside at all times No mixing of air P in

10 Parcel Buoyancy Buoyancy: up- or downward force from combination of atmospheric pressure and gravity Pressure Up- or downward motion of air parcel depends on buoyancy? Air Parcel Gravity

11 Air Density vs. Temperature ρ 1 T for P = constant Cold air is more dense than warm air

12 Buoyancy in Fluids Negative buoyancy ρ parcel > ρ Parcel colder than envir. envir Positive buoyancy ρ parcel < ρ envir Parcel warmer than envir.

13 Adiabatic Process No heat exchanged between a system and its surroundings Heat Adiabatic Compression: P, T Expansion: P, T

14 Lapse Rate Defn.: Rate of temperature decrease as altitude increases. z z 2 T 2 z 1 T 1 Lapse rate = γ or Γ ( gamma ) T 2 T1 = z 2 z1 = γ T z T as z : positive γ

15 Atmospheric Stability Describes behavior of air after it has been disturbed Indicates atmosphere s ability to mix vertically Related to air parcel buoyancy after perturbing parcel

16 Stability Behavior Behavior after disturbance of equilibrium characterizes stability Stable

17 Stability Behavior Behavior after disturbance of equilibrium characterizes stability Unstable

18 Stability Behavior Behavior after disturbance of equilibrium characterizes stability Neutral

19 Making Air Parcels Buoyant Need: parcel temp. envir. temp. Heat up air parcel at the ground Or, force parcel upward so it loses temp. via adiabatic expansion z T

20 z γ = 0 Parcel Environment Γ (= 10 C/km) γ > 0 γ < 0 T

21 z Neutral γ = Γ Γ Γ Γ Parcel Environment Stable γ < Γ Unstable γ > Γ T

22 Stability Criteria Stable: γ < Γ Unstable: γ > Γ Neutral: γ = Γ

23 Stability vs. Dispersion Turbulence gives parcel initial push Stability vs. mixing: Stable vertical motion suppressed vertical dispersion discouraged Unstable vertical motion encouraged vertical dispersion enhanced

24 Temperature I nversions Defn: Temp. increases with increasing altitude (γ < 0) z I nversions are extremely stable T

25 Temperature Inversion Aloft z Γ γ < 0 I nversion base T

26 Radiation Inversion Advection Inversion Regional Subsidence Inversion Large-scale Subsidence Inversion

27 Radiation Inversion Infrared radiation from ground to space Ground cools off at night; cools air next to it Shallow

28 Advection Inversion Cool m arine air Cool ocean Warm air inland Cool air flows underneath warm air Occurs as Marine Layer along SoCal coast

29 Regional Subsidence Inversion N.E. Deserts Weak Santa Ana Wind Air flows from high altitude to low altitude S.W. Coast Air compresses adiabatically while descending

30 Large-Scale Subsidence Inversion H Air aloft sinks (subsides) and warms from adiabatic compression L.A. has semi-permanent subsidence inversion

31 Mixing Heights z Inversion or stable layer Mixed Neutral Layer Mixing Height T Pollution concentration 1 Mixing Height

32 Mixing Height vs. Season MH ground temperature Winter: low temp. Summer: high temp. z Winter γ MH W (Morning, calm air) Summer γ MH S T

33 Ventilation Factor Volume Mixing height VF = MH WS Wind speed

34 Chimney Plume Dispersion Γ d γ Fanning (Top view) γ Γ d Looping

35 γ Γ d Coning Γ d γ Fumigation γ Γ d Lofting

36 Dispersion Enhancement Turbulence generated in flow around buildings Downwash Top Wake Eddy

37 Tall smokestacks Larger volume for mixing pollutant Taller chimney lower smoke concentration at ground

38 Inversion Layer Fanning plume Inversion base Fumigation Plume

39 Lofting Plume Inversion top Fanning Plume Fumigation Plume Inversion Layer

40 Increase Exit Velocity High V exit Higher exit velocity lower pollution concentration at ground Low V exit

41 How to Increase Exit Velocity Pump out faster with fan Fan uses energy $$ Reduce diameter of chimney tip Restricted flow

42 Increase Exit Temperature Hotter smoke is more buoyant Hot plume Cool plume

43 How to Increase Exit Temperature Heat up smoke Uses energy $$ for energy, maintenance Uses energy Hotter combustion Alters smoke composition