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Reading: Chap 3.1. Atmospheric Dispersion & Transport. Overview Wind Turbulence Ambient Stability & Inversion Plume Rise and Transport Plume Characteristics Long Range Transport Planetary Transport. Example?. The Atmosphere as a Sink .

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atmospheric dispersion transport

Reading: Chap 3.1

Atmospheric Dispersion & Transport
  • Overview
  • Wind
  • Turbulence
  • Ambient Stability & Inversion
  • Plume Rise and Transport
  • Plume Characteristics
  • Long Range Transport
  • Planetary Transport
the atmosphere as a sink


The Atmosphere as a Sink
  • Volcanoes and pollutants from other natural sources
  • Pollutants from human activities
    • From the discovery of the utility of fire to the present
  • Imperfect sink: limited ability to carry away (transport), dilute (dispersion) and remove (deposition) pollutants
    • Local or regional overloading
    • Topographical barriers
    • Scale of air motion
    • Atmospheric stability and inversion
    • Atmospheric chemical reactions
dispersion and transport

What’s the depth of PBL?

Dispersion and Transport
  • Transport - air motions carry pollutants from one region of the atmosphere to another
  • Dispersion - mixing of pollutants with air
  • Planetary Boundary Layer (PBL): Pollutants are initially released into PBL
    • PBL well mixed in the context of a few months
    • In shorter time scales pollutant mixing limited by atmospheric conditions
  • Air in Free Troposphere above the PBL is relative stable

Maximum height of wind profiles indicate where effects of surface roughness end and where gradient wind begins

Fig 3.1


What are the two components in describing wind?

  • Affected by:
    • Horizontal pressure gradients
    • Horizontal temperature gradients
    • Friction related to surface roughness

Meteorological Station

Is wind dilution mor effective in urban area or countryside?


Is wind speed measured at the ground level?

  • Effect of wind on emission: dilution of plume
    • Doubling of the wind speed decreases pollutant levels by 50 %
  • Wind Direction
    • Prevailing flows
    • (Anti-)Cyclonic flows
    • Effects of topography
      • In a valley
      • Along sea and lake coasts
wind in a valley
Wind in a Valley

What is the wind direction at night? During the day?

sea land breeze and circulation
Sea/Land Breeze and Circulation

What is the wind direction at night? During the day?

Will sea breeze continue inland for a long distance?

wind rose
Wind Rose

Wind speed and direction are typically quite variable

164/720 = 23%

16/720 = 2.2%, 24/720 = 3.3%, 70/720 = 9.7%




monthly wind rose of gainesville

What is the major wind direction in Gainesville?

What is the impact on local air quality if the wind is variable? What if it’s persistent?

Monthly Wind Rose of Gainesville

What is the effect of turbulence on pollution?

Is turbulence desired?

  • Circular eddies of air movements over short timescales than those that determine wind speed (unstable)
  • Mechanical Turbulence:
    • Caused by air moving over and around structures/vegetation
    • Increases with wind speed
    • Affected by surface roughness
  • Thermal Turbulence:
    • Caused by heating/cooling of the earth’s surface
    • Flows are typically vertical
    • Convection cells of upwards of 1000 - 1500 meters
atmospheric stability

Does dry or moist air have a larger temperature change for the same change in elevation? Why?

Does lapse rate have anything to do with air quality?

Atmospheric Stability
  • Concept that describes (non-)movement of air near the surface
  • Characterized by vertical temperature gradients (Lapse Rates)
    • Dry adiabatic lapse rate () = 0.976 oC/100 m ~ 1 oC/100 m
    • International standard lapse rate = 0.0066 oC/m

= 0 for adiabatic expansion

Lapse Rate

How much is dT/dZ if Cp = 1.0034103 m2/s2-K? What if Cp = 1.856103 m2/s2-K? (for dry air and moist air)

  • First Law of Thermodynamics
  • Barometric Equation
superadiabatic lapse rates unstable
Superadiabatic Lapse Rates (Unstable)
  • Temperature decreases are greater than -10o C/km
  • Occur on sunny days
  • Characterized by intense vertical mixing
  • Excellent dispersion conditions
neutral lapse rates
Neutral Lapse Rates
  • Temperature decreases are similar to the adiabatic lapse rate
  • Results from:
    • Cloudy conditions
    • Elevated wind speeds
    • Day/night transitions
  • Describes good dispersion conditions

Isothermal Lapse Rates (Weakly Stable)

  • Characterized by no temperature change with height
  • Atmosphere is somewhat stable
  • Dispersion conditions are moderate
inverted lapse rates strongly stable


Inverted Lapse Rates (Strongly Stable)
  • Characterized by increasing temperature with height

Does it occur during the day or at night?

Is it associated with high or low pressure systems?

Does it improve or deteriorate air quality?

  • Definition: temperature increases with altitude

  • Two major types of inversion:
    • Subsidence Inversion: descent of a layer of air within a high pressure air mass
    • Radiational Inversion: radiation at night from the earth’s surface into the local atmosphere
radiational inversions

Fig 3.3

What happens to inversion when sun rises?

Radiational Inversions
  • Result from radiational cooling of the ground
  • Occur on cloudless nights – nocturnal
  • Typically surface based
  • Are intensified in river valleys
  • Cause pollutants to be “trapped”
radiational inversions23
Radiational Inversions
  • Breakup after sunrise
  • Breakup results in elevated ground level concentrations
  • Breakup described as a fumigation

radiational inversions24
Radiational Inversions
  • Elevated inversions are formed over urban areas
    • Due to heat island effect
    • Due to dust dome

Fig 3.4

subsidence inversion
Subsidence Inversion
  • Associated with high-pressure systems
  • Inversion layer is formed aloft
  • Covers hundreds of thousands of square kms
  • Persists for days

Fig 3.5

subsidence inversion26

Where else on earth would have similar phenomenon?

Subsidence Inversion
  • Migrating high-pressure systems: contribute to the hazy summer conditions in Midwest, SE and NE
  • Semi-permanent marine high-pressure systems
  • Results in a large number of sunny calm days
  • Inversion layer closest to the ground on continental side
  • Responsible for air stagnation over Southern California

  • Frontal - warm air overrides cooler air
  • Advective - warm air flows over a cold surface or cold air

mixing height mh

Why is agricultural burning allowed only during the daytime?

Mixing Height (MH)
  • Height of air that is relatively vigorously mixed and where dispersion occurs

What is the MH in a radiational inversion?

When does the max MH occur during a day? Min MH?

Which season has the max MH? Min MH?

Why does Phoenix have a larger MH than New Orleans?

dispersion from point sources

Why does plume expand downwind?

What are the factors that influence the history of plume?

Impact on air quality depends on dispersion, which depends on the height of plume

Dispersion from Point Sources
  • Pollutants emitted in plume form

dispersion from point sources30

Under what conditions can we have a higher Effective Stack Height?

Dispersion from Point Sources
  • Plume rise affects transport
    • Effects maximum ground level concentrations (MGLCs)
    • Effects distance of MGLCs

stack plume looping
Stack Plume: Looping

Strong turbulence

Is it at stable or unstable condition? High or low wind speed?

Does it happen during the day or night?

Is it good for dispersing pollutants?


Stack Plume: Coning

Strong wind, no turbulence

What is the stability class? Good vertical mixing?

On sunny or cloudy days?

Good for dispersing pollutants?

What is the ground level concentration as a function of distance from the stack?


Stack Plume: Fanning

What is the stability class?

What is the top view of the plume?

What is the ground level concentration as a function of distance from the stack?


Stack Plume: Fumigation

Why can’t the pollutants be dispersed upward?

Does it happen during the day or night?

What is the ground level concentration as a function of distance from the stack?

What’s your opinion about requiring power plants to reduce their power output from 3 am to 3 hours after sunrise?


Stack Plume: Lofting

Why can’t the pollutants be dispersed downward?

When does it happen?

What is the ground level concentration as a function of distance from the stack?


Stack Plume: Trapping

What is the stability class?

What is the ground level concentration as a function of distance from the stack?

long range transport


Why only long-lived?

Long-Range Transport
  • Transport of pollutants hundreds/thousands of miles; resulting in air quality problems far away from the source

Planetary Transport

  • Stable air above PBL retards vertical mixing
  • Transport out of PBL to free troposphere takes few hours to few days
  • Transport to top of troposphere with uniform mixing takes about a week (for long-lived pollutants, e.g. CO2, CH4, CFC)
planetary transport


Planetary Transport
  • Significant concentration differences between two hemispheres; cross equatorial mixing takes approximately one year

Stratosphere -Troposphere Exchange

Is the exchange fast or slow? Why?

stratosphere troposphere exchange
Stratosphere - Troposphere Exchange
  • Mass exchange of troposphere with stratosphere takes 18 years
  • Mass exchange of stratosphere with troposphere takes 2 years

Fig 3.9

stratospheric circulation
Stratospheric Circulation
  • Characterized by horizontal airflows (due to thermal gradients between the equator and poles, and diabatic heating associated with O3 absorption of UV light)
    • North-South (meridianal)
    • East- West (zonal)
  • Driven by atmospheric pressure waves
    • Poleward in the winter Hemisphere
    • Significantly affects transport of O3
    • Affects the movement of CFCs