Download The Earth`s Atmosphere-I

The Earth’s Atmosphere-I
GEOL 1350: Introduction To Meteorology
1
Overview
•  What is the composition of Atmosphere?
•  How did the atmosphere arrive at its current
state?
2
Earth’s Atmosphere
•  Earth’s atmosphere is a thin, gaseous envelope.
Earth radius ~ 6370 km; Atmosphere ~ 500 km
•  Why the atmosphere is so thin?
3
Earth’s Atmosphere
•  Gravity ---- Reason for “Orange skin”
atmosphere
Gravitational attraction has compressed the
atmosphere into a
shallow layer.
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What is the composition of the Earth’s
atmosphere?
Permanent Gases
Name of the gas
• 
• 
• 
• 
• 
• 
• 
Nitrogen
Oxygen
Argon
Neon
Helium
Hydrogen
Xenon
Molecular weight
(g/mol)
N2
O2
Ar
Ne
He
H2
Xe
28.01
32.00
39.95
20.18
4.00
2.02
131.30
Percentage
78.08%
20.95%
0.93%
0.0018%
0.0005%
0.00006%
0.000009%5
What is the composition of the Earth’s
atmosphere?
•  Mole: a unit of amount of substance
•  A mole has 6.022 x 1023 atoms or
molecules of pure substance being
measured
•  Molecular weight for N2 is 28.01 gram per
mole.
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Variable Gases
Name of the gas
• 
• 
• 
• 
• 
• 
• 
Molecular weight
Water vapor
H 2O
Carbon dioxide
CO2
Methane
CH4
Nitrous oxide
N 2O
Ozone
O3
Particles (dust, soot, etc.)
Chloroflourocarbons CFCs
Percentage
18.02
< 4.%
44.01
0.038%
16.04
0.00017%
44.01
0.00003%
48.00
0.000004%
0.000001%
0.00000002%
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Molecular Weight of Air
Air is a mixture of gases with different molecular weights
What is the molecular weight of the air ?
Molecular weight of air is the
average of the molecular weights
of its constituents
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Molecular Weight of Air
M air = ∑ (M i ⋅ Ci )
i
Mi = molecular weight of i-th constituent
Ci = fractional concentration of i-th
constituent
M air = 28.96 g mol
−1
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Molecular Weight of Air
M air = ∑ (M i ⋅ Ci )
i
Mair = MN2 CN2 + MO2 CO2 + MH2O CH2O +…
M air = 28.96 g mol
−1
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Atmospheric Composition
•  Major greenhouse molecules in order of
importance (H2O, CO2, O3)
•  Important for chemistry (O2, O3, CH4, N2O, CO,
CFCs).
•  N2 is important for background pressure.
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Composition & Structure
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Units for Concentration
Parts-per notation is used to describe relative
proportions in measured quantities, particularly in
low-value.
1 ppmv (parts per million by volume) = 10-6
1 ppbv (parts per billion by volume) = 10-9
1 pptv (parts per trillion by volume) = 10-12
Example: Current CO2 Concentration: 380 ppmv
It means that out of every million air molecules, 380 are CO2
molecules.
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Trace Constituents
Charles David Keeling
19N, 155W, 3.3 km
Increase in atmospheric CO2 from 1959 – 2004:
Δ(CO2) ≈ 377 ppmv - 315 ppmv = 62 ppmv
•  Longest continuous record
•  Regional CO2 trend
•  CO2 seasonal Cycle
High in winter (respiration)
Low in summer (photosynthesis)
Keeling Atmospheric CO2 Record
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Glacial ice cores, drilled from
great depths, provide a record
of atmospheric compositions.
Temperature are derived from
the geological information.
Sharp minima in CO2 coincide
with cold T in ice ages 20 and
140 Kyr ago.
CO2
Temp.
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Surface melt on Greenland
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Snow cover and Arctic sea ice are decreasing
Spring snow cover
shows 5% stepwise
drop during 1980s
Arctic sea ice
area decreased by
2.7% per decade
(Summer:
-7.4%/decade)
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Trenberth
Sea level is rising:
from ocean expansion and melting glaciers
Since 1993
Global sea level
has risen 41 mm
(1.6 inches)
•  60% from
expansion as
ocean
temperatures rise,
•  40% from melting
glaciers
Steve Nerem
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Satellite CO2 Data
Coordinated Observations
GLORY
1:34
1:26
CloudSat – 3-D cloud climatology
CALIPSO – 3-D aerosol climatology
aerosols,
polarization
TES – T, P, H2O, O3, CH4, CO
MLS – O3, H2O, CO
HIRDLS – T, O3, H2O, CO2, CH4
OMI – O3, aerosol climatology
AIRS – T, P, H2O,
CO2, CH4
MODIS – cloud,
aerosols, albedo
OCO - - CO2
O2 A-band
ps, clouds,
aerosols
CO2 data can be retrieved from AIRS on Aqua, TES on Aura, and GOSAT
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CO2 Retrieval from Atmospheric Infrared Sounder (AIRS)
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July 2003
Water Vapor
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Water Vapor
•  Tropospheric H2O is short lived due to the production
and destruction and rapid transport. Lifetime = a few
days.
•  H2O originates near the equator at the warm ocean
surfaces.
•  H2O is carried aloft by deep convective cells and
horizontally by large-scale eddies that disperse H2O
across globe.
22
Intertropical Convergence Zone (ITCZ)
•  The ITCZ is a belt of convection, which is parallel to
the equator, except over the tropical landmasses:
South America, Africa, and the maritime continent
over Indonesia, where the zone of convection
widens.
•  Inside ITCZ, the deep convection is supported by the
release of latent heat when moisture condenses.
•  ITCZ moves back and forth across equator following
the sun’s zenith point.
•  Variation in the ITCZ locations affect rainfall in the
tropics.
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•  O3 is concentrated in the stratosphere. It increases
sharply above tropopause.
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•  The zonal mean O3 mixing ratio is largest in the tropics,
where the flux of solar UV and photodissociation of O2
are large.
25
Column Ozone (Dobson Unit)
•  Dobson Unit (DU) is the most common unit for measuring O3
concentration.
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Column Ozone (Dobson Unit)
•  Dobson Unit (DU) is the most common unit for
measuring O3 concentration.
•  One DU is the number of molecules of O3 that would
be required to create a layer of pure O3 0.01 mm thick
at 0ºC and 1 atm pressure.
•  One DU would contain about 2.69x1016 O3 molecules
for every square centimeter of area at the base of the
column.
•  Over the earth surface. The ozone layers average
thickness is about 300 DU or a layer that is 3 mm
thick.
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Ozone Hole
Reduction of O3 by 30% in the southern hemispheric spring
over Antarctica and first reported in 1985.
Through the 1990s, column O3 in Sep and Oct have continued
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to be 40-50% lower than the preozone hole value.
Overall cause of O3 depletion is the
presence of chlorine-containing source
gases (primarily CFCs and related
Halocarbons).
Column O3 in Sep 21-30, 2006
NASA
The Cl-catalyzed O3 depletion
can take place in the gas phase,
but it is dramatically enhanced in
the presence of polar stratospheric
clouds (PSCs).
The PSCs form during extreme cold winter.
Low T at pole form cloud particles and are
composed by nitric acid (Type I PSC) or
Ice (Type II PSC). Both types provide surfaces
for chemical reactions that lead to O3
destruction.
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Methane (CH4)
•  CH4 is produced primarily by bacterial and
surface processes that occur naturally.
•  Anthropogenic sources such as mining and
industrial activities may constitute ~20% of CH4
production.
•  CH4 is long-lived and therefore well mixed in the
troposphere, where it is ~1.7 ppmv.
•  CH4 decreases with altitude as a result of
oxidation. This process ultimately leads to the
formation of stratospheric water vapor.
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Nitrogen Compounds
•  N2O is produced primarily by natural means
relating to bacterial processes in soils.
•  Anthropogenic sources of N2O include nitrogen
fertilizers and combustion of fossil fuels, which
accounts for ~25% of the total production.
•  N2O is long lived and well mixed in the
troposphere. N2O mixing ratio is ~ 300 ppbv.
•  N2O decreases with altitude in the stratosphere
due to dissociation of N2O into NO.
•  NO can destroy ozone catalytically. It is produced
as a by-product of inefficient combustion.
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How did the atmosphere
arrived at its currents state?
•  The earth’s first atmosphere (4.6 billion
years ago) consisted mostly of He, H2; Most
scientists feel that this early atmosphere
escaped into space from the earth’s hot
surface.
•  Outgassing (volcanic eruption) releases CO2,
N2, H2O, forming the earth’s second
atmosphere
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Eruption of Mt. St.
Helen in 1980,
adding to our
atmosphere’s
composition: water
vapor, carbon dioxide
and other gases.
Volcanic eruption:
water vapor
carbon dioxide
nitrogen
sulfur
85%
10%
1-5 %
1-5%
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Origin of the atmosphere
•  Water vapor condensed to form ocean
•  Carbon dioxide dissolved into the ocean
and locked up in carbonate sedimentary
rocks
•  Nitrogen, chemically stable, accumulated
Where did the oxygen in the atmosphere
come from ?
35
Where did the oxygen in the
atmosphere come from ?
•  Photodissociation:
2H2O → 2H2 + O2
•  O2 began an extremely slow increase in
concentration as energetic rays from the sun
split H2O into H2 and O2. H2, being lighter, rose
and escaped into space, while O2 remained in
the atmosphere.
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Photo-dissociation
uv
step 1 Oxygen formation
uv
O
O
H
H
H
2 H2
H
Escape to space
O2
2 H2O + ultra violet radiation
2 H2 + O2
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Where did the oxygen in the
atmosphere come from ?
•  This slow increase in oxygen may provide
enough of O2 for primitive plants to evolve,
perhaps 2 to 3 billion years ago.
•  After plants evolved, the atmospheric O2
increased more rapidly as a result of
photosynthesis (combine CO2 and H2O to
produce O2).
•  Photosynthesis: O2
2-3 billion years ago
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Where did the oxygen in the
atmosphere come from ?
•  O2 reach the present composition
about several hundred million years
ago.
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Summary
1.  Earth’s atmosphere is rich in nitrogen and oxygen as well
as smaller amounts of other gases, such as water vapor,
carbon dioxide, and other greenhouse gases.
2.  Nitrogen occupies about 78% and oxygen is about 21%.
Greenhouse gas, CO2, increases in its concentration by
more than 25% since early 1800s.
3.  The earth’s first atmosphere (4.6 billion years ago)
consisted mostly of He, H2. Outgassing (volcanic eruption)
releases CO2, N2, H2O, forming the earth’s second
atmosphere. After plants evolved, the atmospheric O2
increased more rapidly as a result of photosynthesis.
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