Air Pollution 207323

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Air Pollution 207323

• 5 NOV 2002
• LECTURE 3

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Circulation of Water in the Atmosphere
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Water in the Atmosphere
o vapor humidity o droplets clouds, fog, rain o
ice clouds, fog, hail, snow, pellets
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WATERS CHANGES OF STATE
Evaporation liquid -gt gas (vapor) requires
heat Condensation vapor -gt liquid releases
heat heat for evaporation Melting solid -gt
liquid requires heat
Freezing liquid -gt solid releases heat heat
for melting Sublimation solid -gt gas (e.g.,
shrinking ice cubes) Deposition gas -gt solid
(e.g., frost)
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Humidity
Humidity refers to the amount of water
vapor(H2OV)present in the atmosphere. There are
five (5) different methods to express humidity
1)Absolute humidity 2)Mixing ratio 3)Vapor
pressure 4)Relative humidity 5)Dew point
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Absolute Humidity
Absolute Humidity mass of water vapor/ volume
of air So, Absolute Humidity is like a water
vapor density, commonly express in grams m-3 Is
Absolute Humidity a good variable to use for
measuring moisture in air??ANSWER WHY???
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What happens to the volume of a rising or sinking
air parcel?

• Consider a parcel of air at 1000mb
• The parcel will exert 1000 mb of pressure to
  counteract the atmospheric pressure acting on the
  parcel
• Recall that Pressure Force/Area
• If no energy is added or taken away from the
  parcel, then the force of the molecules bumping
  into the side of the parcel will be constant,
  i.e.,
• Force Pressure x Area const

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What happens to the volume of a rising or sinking
air parcel

• The parcel rises to 500 mb
• since Force Pressure x Area const, then since
  the pressure decreases, as it rises, the area of
  the parcel must increase, or the parcel expands

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Absolute Humidity - is it a good variable to
measure humidity

• Consider a parcel of air that is rising and
  expanding
• recall that absolute humidity mass of H2O /
  volume
• Q does the amount of moisture in the parcel
  change as it rises?? ANSWER
• Q is the absolute humidity constant as the
  parcel rises?? ANSWER
• So, is absolute humidity a good moisture
  variable?? ANSWER

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Specific Humidity(q)

• specific humidity mass of water vapor/total
  mass of air
• example
• in a given parcel, the mass of water vapor is 1
  gm
• the total mass of the parcel (N2, O2, AR, H2O,
  other trace gasses) is 1 kg
• Q specific humidity?

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Mixing Ratio(r)

• mixing ratio mass of water vapor/mass of dry
  air
• example
• in a given parcel, the mass of water vapor is 1
  gm
• the mass of dry air in the parcel (N2, O2, AR,
  other trace gasses) is 1.0 kg
• then the mixing ratio is ANSWER

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Vapor Pressure and Partial Pressure

• Given a parcel of air comprised of only nitrogen,
  oxygen, and water vapor --gtgt
• The partial pressure due to water vapor is called
  the "vapor pressure"

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Relative Humidity

• Most common variable used to describe atmospheric
  moisture
• RH water vapor content / water vapor capacity
• RH vapor pressure/saturation vapor pressure
  e/es
• or RH mixing ratio/saturation mixing ratio

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RH can be changed by either
changing amount of moisture as water vapor is
added the RH increases until saturation
(RH100) changing the temperature as parcel
is cooled then the saturation mixing ratio
decreases, and hence RH increases (without change
in amount of water).
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Saturation
Liquid water and dry air in a closed volume
Some water molecules have enough energy to
escape liquid gtgtgtgtevaporation gtgtgtgt vapor
molecules As more vapor molecules in air o
vapor pressure (e) increases o some return to
liquid gtgtgtcondensationgtgtgtgt water When
evaporation rate equals condensation rate o
equilibrium (output input) o no more net water
to vapor gtgtgtgt no net evaporation o equilibrium
gtgtgtgt saturation