Chapter 11 Atmosphere composition

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Chapter 11 Atmosphere composition

• There is little variability in the of the main
  components of the atmosphere, Nitrogen (78 ),
  and Oxygen (21 )
• The percentages of water vapor and CO2 are the
  most variable of all components of the
  atmosphere. Together, they play a major role in
  regulating the amount of energy (E) absorbed by
  the atmosphere.
• Water vapor (0 - 4)
• - source of clouds, rain, snow
• - Heat transfer from waters changes of state
  (liquid to solid, solid to liquid, liquid to gas)
  set atmosphere in motion creating weather and
  climate.
• CO2 (lt 1 ) can modify patterns of heat transfer
  within the atmosphere (see later)
• Air has mass It is composed of various gases,
  and various solids that are present as very small
  particles.

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The Earths primitive atmosphere

• Earths original atmosphere was probably composed
  mainly of methane and ammonia.
• In the 1st billion years after Earths formation,
  there was much more volcanic activity than today.
• Over geologic time, erupting volcanoes changed
  the atmosphere as they released gases (water
  vapor, chlorine, CO, CO2, hydrogen, nitrogen)
• The Argon (Ar) in todays atmosphere began to
  accumulate, as radioactive decay of potasium
  (K-40)
• Over millions of years, as the planet cooled
  water vapor condensed and absorbed most of the
  CO2. Then, storm clouds formed and torrential
  rains fell.
• It is thought that eventually this rain filled
  low basins in Earths surface, ultimately formed
  the early ocean.
• Oxygen was probably made by the dissociation of
  H2O molecules, and later by photosynthesis of
  primitive cyanobacteria, the oldest known dating
  3.5 billion years old. Like todays
  cyanobacteria, they used solar energy to convert
  CO2 and water into sugar, and released O2 as a
  waste product.
• - Cyanobacteria increased during Archaea are
  were most abundant during the Proterozoic.

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The importance of Oxygen

• Very important on Earth
• - Most animals require Oxygen for respiration.
• - Oxygen in the form of Ozone protects against
  UV radiation from the Sun (only a small fraction
  of the UV radiation that the Sun radiates
  reaches the Earth surface)
• - UV radiation is damaging to living tissues.
• Ozone (O3)
• - Gas formed when high-energy UV splits O2
  molecules. The single O atoms combine with other
  O2 molecules to form O3.
• - Occurs mainly as a layer 25 km above Earths
  surface.
• - Early life, mainly cyanobacteria, released
  large amounts of O2. Some of it became O3, which
  filtered UV radiation, and allowed other life
  forms to survive.
• It appears that nearly all the oxygen we breathe
  today, and the O2 that all animals have breathed
  in the geologic past, was released into the
  atmosphere by photosynthesis.

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Structure of the Atmosphere

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Lower Atmospheric layers

• Troposphere Layer closest to Earths surface
• - Contains most of atmosphere mass, including
  water vapor.
• - Most weather, and air pollution occurs here.
• - Within this layer, T decreases as altitude
  increases.
• - Tropopause upper limit of Troposphere
  (gradual T decrease stops).
• - at 16 km height in tropics
• - at 9 km height over poles.
• Stratosphere Mostly concentrated Ozone (O3)
  layer.
• - Absorbs more UV radiation than troposphere
• - Heated Within this layer, T increases as
  altitude increases.
• - Stratopause upper limit of Stratosphere
• - 50 km above Earths surface (between 25-50 km
  height)
• - gradual T increase stops

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Upper Atmospheric layers

• Mesosphere
• - No concentrated O3
• - Within this layer, temperature decreases with
  elevation.
• - Mesopause upper limit of mesosphere (gradual
  T decrease stops)
• Thermosphere
• - very little air mass. Air molecules very
  sparse.
• - Within this layer temperature increases with
  elevation, up to 1000 T.
• - ionosphere upper part of thermosphere, made
  of electrically charged particles and lighter
  gases.
• Exosphere
• - Light gases (He, Hydrogen are here)
• - Marks transition of atmosphere and
  interplanetary gas (outer space is beyond it).
• - Hydrogen and He molecules absorb solar
  radiation which increases their movement, and
  they escape the Earths gravitational field.
  Thus, there is a constant seepage of atmospheric
  gases into outer space.

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Air temperature vs Altitude

• Life on Earth flourishes in only a small layer of
  the atmosphere, closest to Earths surface ? The
  TROPOSPHERE.
• It is unlikely for life as we know it to exist in
  any other layer, partly due to
• - harsh temperatures (too hot in thermosphere
  and exosphere, too cold in mesosphere)
• - lower amounts of O2 (highest in troposphere)
• - absence of ozone (mesosphere, thermosphere,
  exosphere)

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Suns Energy transfer to Earth

• All energy in the atmosphere comes from the SUN
• Suns energy is transferred in 3 ways
• RADIATION - transfer of E through space by all
  forms of electromagnetic waves
• - all substances with T above absolute zero
  emit radiation.
• - The higher the T of the substance, the
  shorter the wavelength it emits.
• - Only 50 of all incoming solar radiation is
  absorbed directly or indirectly by Earths
  surface (15 is absorbed by the atmosphere)
• - 35 is reflected (6 by atmosphere, 25 from
  clouds, 4 by Earths surface).

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• (More on Radiation)
• - Different areas absorb E and heat up at
  different rates
• - water heats up and cools down slower than
  solids
• - darker objects absorb E faster, so they heat
  up faster.
• - Solar radiation does not heat up the air
  directly
• - Most solar radiation passing thru the
  atmosphere is of short wavelength, and therefore
  much of it passes thru the atmosphere and is
  absorbed by Earths surface.
• - The surface radiates energy in longer
  wavelengths, and thus does not pass again thru
  the atmosphere It is absorbed by the atmosphere
  and warms-up the air (by conduction and
  convection)
• CONDUCTION Transfer of E that occurs when
  molecules collide.
• - For conduction to occur among two substances,
  the 2 substances must be in contact.
• - conduction affects a very thin layer of the
  atmosphere, closest to Earths surface.
• CONVECTION Transfer of energy by the flow of a
  heated substance.
• - pockets of air near Earths surface are
  heated, become less dense than surrounding air,
  and rise.
• - as air rises, it expands and cools down. As it
  cools, it increases in density and sinks. Then
  they heat again and start to rise again
• - Convection currents are among the main
  mechanisms responsible for vertical movement of
  air masses, which cause different types of
  climate.

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Heat transfer thru the atmosphere -The car in
the parking lot analogy

• Why does the inside of a closed, parked car heat
  up, while the windows stay cool ?
• The windows are much like the atmosphere They
  allow incoming radiation to pass through without
  much absorption.
• The inside of the car, though, is like Earths
  surface It absorbs the incoming energy and
  converts it into heat. This heat, however, cannot
  pass back through the windows, and thus the
  inside of the car warms up.
• ? Never leave anything alive (plant, animal or
  person) inside a closed car. If is it hot
  outside, it will get hotter inside. If it is not
  hot.. it is still illegal and you should not do
  it anyway regardless of the temperature.

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3 ways of transfer of SUN energy through the
atmosphere
Lowest air density
Decreasing air density
Shorter wavelength
Increasing air density
CONVECTION
RADIATION
CONDUCTION
Longer wavelength
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Temperature vs Heat

• Heat- Transfer of energy that occurs because of a
  difference in temperature between 2 substances
  (Total kinetic energy of all the atoms and
  molecules in a substance)
• Temperature-
• - refers to intensity (the degree of hotness).
• Measurement of how rapidly or slowly molecules
  move around (measurement of the average kinetic
  energy of the individual atoms in a substance)
• - When heat is applied to a substance, its atoms
  move more and faster? its temperature rises
• - When heat is removed, its atoms move less and
  more slowly ? its temperature drops.
• Heat flows from higher areas of higher T to
  areas of lower T (Differences in temperature
  determine the direction of heat flow)
• 3 Temperature scales
• - Kelvin is the SI unit. At a temperature of
  absolute 0 (0 Kelvin, molecular activity
  theoretically stops).
• - Celsius is convenient because the difference
  between boiling and freezing points is exactly
  100
• - F is used because of tradition only.

Nothing can be colder than absolute (0 K)
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The Greenhouse effect

• 50 of solar energy that strikes the top of the
  atmosphere actually reaches Earths surface. Most
  of this energy is re-radiated upwards, at longer
  wavelengths than solar radiation.
• The atmosphere absorbs the longer wavelengths
  emitted by Earth (terrestrial radiation), but
  does not absorb much of the solar radiation
  (shorter wavelengths) instead, it lets much of
  the solar radiation pass thru to Earths surface.
• Water vapor and CO2 are the principal absorbing
  gases, but especially water vapor.
• The lower troposphere has the highest
  concentration of water vapor, and thus becomes
  warm (from terrestrial radiation). Therefore, the
  atmosphere heats up from the bottom up, not from
  the top down. This is the reason that within the
  troposphere, temperature decreases with altitude
  (the farther from the radiator the Earth, the
  lower the T)
• Gases in the atmosphere warm-up when they absorb
  terrestrial radiation. They then radiate that
  energy away, both upwards (towards other gas
  molecules) and downwards (back towards Earth).
  Since the water vapor is lower upwards, most of
  that energy is radiated downwards. As a result,
• Earths surface continuously receives heat from
  the atmosphere, as well as from the Sun (this is
  called the Greenhouse Effect). Without the
  atmosphere to absorb some of the heat, Earth
  would be inhospitable (too hot).
• Gases in the atmosphere act as the glass roof of
  a greenhouse (allow shorter wavelengths in, thus
  warming the objects inside, and then let longer
  wavelengths out)

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Dew Point and Condensation

• Dew Point Temperature to which air must be
  cooled at constant pressure to reach saturation.
• Saturation Point at which the air holds the
  maximum amount possible of water vapor.
• Condensation When matter changes state, from
  gas to liquid (water vapor becomes liquid).
• - condensation does not occur until saturation
  has been reached.
• - The dew point is often called the condensation
  temperature.

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Vertical temperature changes

• Masses of air moving upward thru the atmosphere
  experience changes in T.
• DALR (Dry Adiabatic Lapse Rate) The rate at
  which air will cool if no heat is added or
  removed. (10 C/1000 m)
• LCL (Lifted Condensation Level) The height to
  which air rises, and because its T has
  decreased, condensation occurs.
• - The base of clouds often corresponds to the
  LCL
• MALR (Moist Adiabatic Lapse Rate) the rate at
  which air cools above the LCL (slower than at
  LCL)
• - 4 C/1000 m (very warm air)
• - 9 C/1000 m (in very cold air)

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Pressure /T / Density relationship

• Particles of gas are constantly pulled towards
  the center of the Earth (gravitational forces).
• Thus, air exerts pressure on everything under it.
  
• - Near the bottom of the atmosphere (closest to
  Earth), pressure is highest
• - Pressure decreases with elevation, because
  there are less air particles exerting pressure.
• The density of air is proportional to the number
  of particles in a given space.
• - The lesser the of particles, the lesser the
  pressure.
• - The upper part of atmosphere exerts pressure
  on lower levels, increasing the density of the
  air at lower levels.
• - At the top of a mountain, the following
  variables are all less than at lower elevations
  temperature, pressure, and density
• T and pressure Directly proportional
• T and density Inversely proportional

• -Thus, air rises when its T increases, because
• it becomes less dense.
• In real situations, temperature is proportional
• to the ratio of pressure to density, which
• decreases with increasing altitude. (This is
• because all 3 variables are changing in
• in response to altitude)

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Temperature inversions

• Temperature inversion an increase in
  temperature with height in an atmospheric layer.
• - This is an exception to the general
  relationship between temperature and pressure in
  the troposphere. Thus the relationship is
  inverted.
• Several possible causes. The most common
• - When the lower layers of the atmosphere are
  not receiving heat from Earths surface (the
  surface is not heating up but cooling down). Thus
  the surface of the Earth is not re-radiating
  energy into the lower levels of the atmosphere.
  This can happen on cold, clear, winter nights
  when the wind is calm.
• In many large cities (Mexico City, Bogotá, Los
  Angeles, etc.), temperature inversions can trap
  pollution under the inversion layer (most
  commonly closest to Earth surface and areas of
  densest human habitation)
• - Air rises only when it is less dense than the
  air around it.
• - In a temperature inversion, a layer of cooler
  air exists under a layer of warmer air. The
  cooler air is denser so it cannot rise through
  the less dense air to be dispersed.
• - The cool air, with the pollutants produce at
  the surface (city pollutants) are trapped in the
  area below the less dense air.

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Wind

• The unequal heating and cooling of Earths
  surfaces, causes density imbalances (parcels of
  air having higher, or lower density).
• - Density imbalances create areas of high and
  low pressure.
• - Cool, denser air, sinks and forces warm, less
  dense air upward.
• - Air masses are therefore set in motion. The
  movement is generally from areas of high density
  to areas of low density
• Wind usually measured in miles/h, km/h, or knots
  (most frequently on ships but sometimes on land,
  particularly coastal areas).
• Friction Affects wind speed, depending on
  contact surfaces
• - close to Earths surface ? highest frictional
  loss
• - farther up into atmosphete ? lower friction
• - Solids (land) cause more frictional loss than
  liquids
• - Irregular surfaces cause more frictional loss
  than smooth surfaces.
• Some types of winds
• - Katabatic winds wind blowing downslope of a
  mountain
• - If the katabatic wind gets warmer during its
  downslope journey, it is called a foen. There
  are several types of foens, some capable of
  elevating the air temperature by as much as 30 C
  in just 15 minutes. Foens often cause
  grass/forest fires to spread rapidly.

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Relative Humidity

• Expressed as a percent ()
• It is the ratio of water vapor in a volume of
  air, relative to how much water vapor that same
  volume of air could actually hold.
• - If a volume of air is holding as much water as
  it can, then the relative humidity is 100
• - It the air holds ½ the maximum amount
  possible, then the relative amount is 50
• - Air is saturated when the ratio of water vapor
  in it is equal to the maximum volume of water it
  can hold (1/1 100 relative humidity). At
  saturation, condensation takes place (change of
  state of water, from vapor to liquid)
• The maximum amount of water a parcel of air can
  hold depends on the air temperature
• - Warm air can hold more water vapor (moisture)
  than cold air
• - Cold air can hold less water vapor, so it gets
  saturated first (at a lower total water volume)
• Importance of water vapor
• - The source of all clouds and precipitation
• - Absorbs heat given off by Earth (re-radiated)
• - Absorbs heat from Sun

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The Ozone layer and its Depletion

• For nearly 1 billion years, the ozone layer in
  the atmosphere has protected life from
  ultraviolet radiation.
• In the last 50-100 years, increased use of CFCs
  (chlorofluorocarbons) has reduced the ozone layer
  to dangerous levels, which allow more UV to pass
  thru. Chlorine in CFCs destroys ozone molecules.
• CFCs are convenient materials used in various
  industries
• - manufacturing of some plastics (foams like
  styrofoam)
• - coolants for air conditioning/refrigeration
• - propellants for aerosol sprays
• UV causes skin cancer, which has increased
  dramatically is some regions, particularly
  southern and northern hemispheres, and more in
  higher latitudes (north and south). UV also can
  damage DNA and induce mutations.
• In 1974, 3 scientists (Crutzen, Rowland Molina)
  alerted the world of this problem. At first they
  were not taken very seriously, but in 1995 were
  given the Nobel Prize in chemistry.
• Beginning in 1987, and increasing number of
  countries have agreed to efforts to reduce and
  eliminate CFC production and use, as part of the
  now called Montreal Protocol. Currently 172
  countries adhere to it.
• There are several alternatives now in use, in
  place of CFCs. They dont contain Chlorine, and
  therefore dont damage the ozone HFAs
  (hydrofluoroakanes), and HFCs (hydrofluorocarbons
  )

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Southern Hemisphere, Spring 2001 (the worst seen
yet)
Southern Hemisphere Spring 1998
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Clouds

• Air buoyancy- Tendency to rise or sink (warm,
  less dense air has higher buoyancy and therefore
  it rises).
• Condensation nuclei Small particles, actually
  many microscopic (dust, sea salt, etc.), around
  which water condenses when it reaches the air
  masses reach the dew point (and therefore
  saturation).

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