PROPERTIES OF THE ATMOSPHERE

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PROPERTIES OF THE ATMOSPHERE

• Troposphere 0 - 11 km above ground
• Stratosphere above 11 km
• 99 of atmospheric mass within 30 km
• Equivalent to a large pancake of 25,000 km
  diameter
• Horizontal movements more pronounced than
  vertical movements

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Figure 1.1 Seinfeld Pandis

• Temperature vs height in different layers of the
  atmosphere

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HORIZONTAL ATMOSPHERIC MOTION - GLOBAL

• Solar heating maximum at the equator
• (2.4 X heating at the poles, annual average)
• Atmosphere carries heat from equator to poles
• Long horizontal distance vs short height,
  break-up into tropical, temperate, and polar
  cells (Figure 5.2 de Nevers)
• Rotation of Earth gives rise to different
  surface wind patterns in these three zones
• Tropical southeasterly and northeasterly (trade
  winds)
• Temperate Westerlies
• Polar Easterlies

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Figure 5.2 de Nevers

• General circulation of the atmosphere

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HORIZONTAL ATMOSPHERIC MOTION - LOCAL

• Land surface heats and cools faster than
  ocean/lake surface.
• Daily and seasonal differences result in wind
  patterns between land and water bodies.
• (Figure 5.13 de Nevers)
• Random wind patterns between high (anticyclone)
  and low pressure (cyclones) zones
• superimposed on global and land-water winds

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Figure 5.13 de Nevers

• Onshore, offshore breezes

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ANTICYCLONES - HIGH PRESSURE

• 1020 - 1030 mb
• Sinking air near the ground
• Evaporating moisture, clearing sky
• Weak winds, outward from center, clockwise in
  the nothern hemisphere

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CYCLONES - LOW PRESSURE

• 980 - 990 mb
• Rising air near the ground
• Condensing moisture, clouds and precipitation
• Strong winds, inward toward center,
  counter-clockwise in the nothern hemisphere

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WINDS

• GROUND LEVEL
• Maximum, tornadoe 200 mph (90 m/s)
• Typical 10 mph (4.5 m/s)
• Velocity gradient in planetary boundary layer
• Frictionless velocity above 500 m
• (Figure 3.13 Wark Warner)
• Wind rose used for reporting annual wind speed
  and direction variation (Figure 5.14 de Nevers)

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Figure 5.14 de Nevers

• Wind rose

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Figure 3-13 Wark Warner

• Wind speed profile

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TEMPERATURE LAPSE RATETHE STANDARD ATMOSPHERE

• Compared with soil and water, the atmosphere is
  relatively transparent to infrared radiation
• Soil and water surface absorb solar radiation,
  heat up and heat the adjacent air by convection
• Atmospheric temperature decreases from
  temperatures of 20 C at the surface, to around
  - 50 C at the troposphere-stratosphere
  boundary
• Standard atmospheric lapse rate 6.5 C/km
• (average over day and night, summer and winter)
• A positive value is quoted for lapse rate
  although temperature decreases with increasing
  height (Figure 3-8 Wark Warner)

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ADIABATIC LAPSE RATE
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SUPERADIABATIC LAPSE RATE

• Lapse rate more than the adiabatic 10 C/km,
• e.g. 12 C/km
• Small adiabatic displacements in the vertical
  direction are enhanced by existing temperature
  profile
• UNSTABLE conditions, leading to effective mixing
  and dispersion

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SUBADIABATIC LAPSE RATE

• Lapse rate less than the adiabatic 10 C/km,
• e.g. 8 C/km
• Small adiabatic displacements in the vertical
  direction are inhibited by existing temperature
  profile
• STABLE conditions, leading to poor mixing and
  dispersion

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Figure 3-7 Wark, Warner Davis

• Standard atmosphere and adiabatic temperature
  profiles

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Figure 3-8 Wark, Warner Davis

• Lapse rate as related to atmospheric stability

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INVERSIONS

• Temperature increases with height above ground
• (I.e. positive dT/dz, negative lapse rate)
• Extremely stable conditions
• Radiation inversion daily occurrence due to
  cooling of ground surface at night
• Subsidence inversion (elevated inversion,
  inversion aloft) large regions cold air sinking
  from above due to weather patterns, heating at
  adiabatic lapse rate
• Drainage inversion due to horizontal motions,
  cold air sliding in under warm air, or warm air
  riding up on cold air

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Subsidence inversion

• Adiabatic compression and warming of a layer of
  air as it sinks to lower altitudes in the region
  of a high pressure center.
• For an ideal gas
• Cp constant, ? higher at the bottom
• Top warming faster, positive temperature gradient
  could be established

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Figure 3-11 Wark, Warner Davis

• Radiation inversion, Oak Ridge

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Figure 3-10 Wark Warner

• Subsidence, radiation and combination inversions

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FUMIGATION

• The daily radiation inversion starts breaking up
  near the ground as the ground heats.
• This can lead to a sandwich phenomenon with an
  inversion layer bounded by a stable layer above
  and an unstable layer below.
• As the unstable layer from below reaches the
  height of a pollutant plume in the inversion
  layer the plume mixes downward, producing
  temporary but high ground level concentrations.
• (Figure 5.15 de Nevers)

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Figure 5.15 de Nevers

• Fumigation

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Potential temperature, ?

• The temperature that a volume of air would have
  if brought by an adiabatic process from its
  existing pressure P to a standard pressure P0,,
  of 1000 mbar
• k Cp/Cv,
• T absolute

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Figure 3-9 Wark, Warner Davis

• Potential temperature

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Potential temperature gradient

• stable
• - unstable
• 0 neutral

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Atmospheric stability

• Two governing factors
• Temperature gradient (lapse rate)
• Turbulence due to wind
• Dry adiabatic lapse rate 10 C / km
• Saturated adiabatic lapse rate 6 C /km
• Standard profile 6.6 C / km

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Atmospheric Stability Classes (Pasquill 1961,
Turner 1970)

• Determinations based on inexpensive observations
  of wind speed, solar radiation, cloudiness
• A Strongly unstable
• B moderately unstable
• C slightly unstable
• D neutral
• E slightly stable
• F moderately stable
• G strongly stable

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Stability Classes

• Table 3-1 Wark, Warner Davis
• Table 6-1 de Nevers

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Atmospheric Stability Classes

• Direct measurement of temperature gradient and
  variation of wind direction.
• ?y , std deviation of horizontal wind direction
• ?z, std deviation of vertical wind direction

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Table 3-2 Wark, Warner Davis

• Comparison of different stability techniques

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Wind velocity profile and Stability Classes

• p varies with atmospheric stability class
• Table 3-3 Wark, Warner Davis

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MIXING HEIGHT

• Common to find superadiabatic lapse rate near
  ground level in the early afternoon under a
  strong sun.
• This gives rise to an UNSTABLE well mixed layer,
  above which there can be an adiabatic (NEUTRAL)
  or subadiabatic (STABLE) atmosphere. (Figure 5.9
  de Nevers)
• Pollutants released at ground level will be
  dispersed in this well mixed layer, the lower the
  mixing height the higher the resultant pollutant
  concentration

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Figure 5.7 de Nevers

• Vertical temperature distribution at various
  times during day

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Figure 5.9 de Nevers

• Mixing height

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MIXING HEIGHT

• Lower at night than during the day
• Lower in the winter than in the summer
• Can be strongly influenced by weather patterns
• Typical values, 0 - 2000 m
• (Figure 3.15 Wark, Warner Davis
• Winter mean mixing heights for U.S.)

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MIXING HEIGHT MEASUREMENT

• Environmental temperature profile determined by
  sending up a balloon that transmits temperature
  vs height data for several km
• A dry adiabatic temperature line from the maximum
  monthly surface temperature intersects the
  previous line at the maximum mixing height
  (Figure 3-14 Wark, Warner Davis)

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Figure 3-14 Wark, Warner Davis

• Establishment of MMD under various atmospheric
  conditions.

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Figure 3-15 Wark, Warner Davis

• MMMD for winter mornings and afternoons in U.S.

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Plume behaviour

• Figure 3-18 Wark, Warner Davis