Air Pollution Dispersion Dispersion Processes Convective Dispersion Air Parcel Dynamics Adiabatic Process Lapse Rate Equilibrium and Stability Atmospheric Stability Stability and Dispersion
Temperature Inversions Stability Formation/Types Mixing Height Application: Chimney Plumes Plume Type vs. Stability Enhancing Plume Dispersion
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
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
Dispersion Processes Defn.: A substance mixes in and becomes diluted within a larger volume of another substance. Molecular Diffusion Turbulence Convection/Advection
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!!
Turbulence Bulk air motion in random directions y Wind x Plume Core Plume Edge }Plume Pollution Strong, gusty winds generate the most turbulence
Convection/Advection Mass transport of pollutants by winds Advection: horizontal motion Convection: vertical motion
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
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
Air Density vs. Temperature ρ 1 T for P = constant Cold air is more dense than warm air
Buoyancy in Fluids Negative buoyancy ρ parcel > ρ Parcel colder than envir. envir Positive buoyancy ρ parcel < ρ envir Parcel warmer than envir.
Adiabatic Process No heat exchanged between a system and its surroundings Heat Adiabatic Compression: P, T Expansion: P, T
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
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
Stability Behavior Behavior after disturbance of equilibrium characterizes stability Stable
Stability Behavior Behavior after disturbance of equilibrium characterizes stability Unstable
Stability Behavior Behavior after disturbance of equilibrium characterizes stability Neutral
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
z γ = 0 Parcel Environment Γ (= 10 C/km) γ > 0 γ < 0 T
z Neutral γ = Γ Γ Γ Γ Parcel Environment Stable γ < Γ Unstable γ > Γ T
Stability Criteria Stable: γ < Γ Unstable: γ > Γ Neutral: γ = Γ
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
Temperature I nversions Defn: Temp. increases with increasing altitude (γ < 0) z I nversions are extremely stable T
Temperature Inversion Aloft z Γ γ < 0 I nversion base T
Radiation Inversion Advection Inversion Regional Subsidence Inversion Large-scale Subsidence Inversion
Radiation Inversion Infrared radiation from ground to space Ground cools off at night; cools air next to it Shallow
Advection Inversion Cool m arine air Cool ocean Warm air inland Cool air flows underneath warm air Occurs as Marine Layer along SoCal coast
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
Large-Scale Subsidence Inversion H Air aloft sinks (subsides) and warms from adiabatic compression L.A. has semi-permanent subsidence inversion
Mixing Heights z Inversion or stable layer Mixed Neutral Layer Mixing Height T Pollution concentration 1 Mixing Height
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
Ventilation Factor Volume Mixing height VF = MH WS Wind speed
Chimney Plume Dispersion Γ d γ Fanning (Top view) γ Γ d Looping
γ Γ d Coning Γ d γ Fumigation γ Γ d Lofting
Dispersion Enhancement Turbulence generated in flow around buildings Downwash Top Wake Eddy
Tall smokestacks Larger volume for mixing pollutant Taller chimney lower smoke concentration at ground
Inversion Layer Fanning plume Inversion base Fumigation Plume
Lofting Plume Inversion top Fanning Plume Fumigation Plume Inversion Layer
Increase Exit Velocity High V exit Higher exit velocity lower pollution concentration at ground Low V exit
How to Increase Exit Velocity Pump out faster with fan Fan uses energy $$ Reduce diameter of chimney tip Restricted flow
Increase Exit Temperature Hotter smoke is more buoyant Hot plume Cool plume
How to Increase Exit Temperature Heat up smoke Uses energy $$ for energy, maintenance Uses energy Hotter combustion Alters smoke composition