Download Atmospheric Composition

Atmospheric Composition
The composition of Earth’s atmosphere both controls the
radiative environment and is significantly regulated by
biological activity. On geological timescales, the recycling
of carbon via plate tectonics is critical for Earth’s climate
(c.f. Mars & Venus). Table 1.1 lists, in order of
abundance, the constituents of Earth’s atmosphere.
3.9 x 10-4
3.8 x 10-4
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Atmospheric Mass:
The Hydrostatic Balance
Atmospheric pressure is the weight exerted by the overhead atmosphere on a
unit area of surface. Consider the mercury barometer:
"vacuum"
Patm
H
The weight of the mercury column, H, must equal the weight of the atmospheric
column, or:
Patm = ρHg g H = (at sea level) 13.6 g cm-3 × 9.8 m s-2 × 76 cm = 1.013 × 105 kg m-1 s-2
in SI units. The SI unit is Pascal (Pa); 1 Pa = 1 kg m-1 s-2. Other units for
atmospheric pressure in widespread use: the atmosphere, 1 atm = 1.013 x 105 Pa,
the bar (b) (1b = 1 x 105 Pa), the millibar (mb) (1mb = 100 Pa) and for Chemists, the
torr = 1 mm Hg = 134 Pa. To be good internationalists, Pa are it! Often you will now
see pressure in hectopascals, hPa, which are equivalent to mb.
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Consider the following forces acting on a slab of atmosphere:
Surface
area, A
Z+dz
Z
At equilibrium (or hydrostatic balance!) the weight (acting downward) must be
balanced by the pressure gradient force:
ρ g A dz = A × (P(z) - P(z+dz))
rearranging:
dP/dz = -ρ g
From the ideal gas law,
ρ = P Ma/RT
Substituting yields:
dP/P = - (Ma g / RT) dz
For isothermal atmospheres (and this is true on Earth to ~20%), we can integrate:
Pz = Po exp(- [(Ma g)/RT]z) = Po exp(- z / H)
where H, the scale height, is about 7 1/2 km.
Mass of Earth’s atmosphere
Mass = 4 π R2 P(surface)/ g ≈ 5 × 1018 kg. (1 kg / cm2). This is the same
weight as a column of water 10 meters deep. Air is 78% N2, 21 % O2, and 1%
Ar (+ some water and other things). The mean molecular weight, Ma, is: (0.78
× 28) + (0.21 × 32) + (0.01 × 40) = 29 g mole-1. For a column weight of 1 kg
cm-2 we have ~35 moles cm-2 or 2 × 1025 molecules cm-2 in the column.
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3.9 x 10-4
3.8 x
10-4
The Sun
The sun is a relatively small star whose projected lifetime on the main
sequence is ~ 11 billion years. Theory and observations of stars
similar to the sun suggest that the luminosity has increased 25-30%
over the last 4.5 billion years. Despite the increasing energy
delivered to Earth, many paleo climate records suggest that mean
surface temperatures have not changed much and in general,
perhaps, cooled. This has been called by some the faint-sun
paradox.
The current solar luminosity, Lo, is presently 3.9 x 1026 W. This energy
is emitted by the sun's photosphere whose radius is ~7 x 108 m. The
flux density at the photosphere is then:
Flux Density = flux/area = Lo/(4 π r2) = 6 x 107 W m-2
Compare San Onofre Nuclear Station 1 or 2 or 3 = 1 x 109 W
(actually not 1 as it is now decommissioned)
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The Solar "Constant"
Since space is effectively a vacuum, the amount of energy passing outward
through any sphere centered on the sun will be equal to the solar
luminosity, Lo. If the radiation is isotropic:
Flux = Lo = Sd 4 π d2
Sd, the Solar Constant (at distance d) = Lo/(4 π d2) – see figure to follow
At the mean distance of Earth from Sun (1AU = 1.5 x 1011 m):
So = 1368 W m-2.
For a rotating sphere at 1 A.U., average radiance:
So / 4 = 342 W m-2 [c.f. average new refrigerator (or average person) ~ 100
W.]
In addition to 25-30% change in Lo over 4.5 billion years, So changes on
various timescales:
1.  Annual due to orbital eccentricity (~3% change in d between perihelion
- presently Jan. 3rd - and aphelion)
2.  "11-year" (solar cycle) (0.25% change in Lo)
Numerous attempts to link decadal solar variation with climate have
been (largely) unsuccessful.
Figure 5. Ruddiman 2001 Emission Temperature of Planets
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Figure 6. Earth’s Orbit Climate Puzzles: The Solar "Constant"
Figure 7. Ruddiman 2001.
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Figure 7. Ruddiman 2001. The Solar "Constant"
The current climate epoch:
The Holocene
(though some now say we have entered a new epoch, the anthropocene)
The climate during the last 10,000 years has been notably quiescent
Ice core record from Greenland –
Emergence of civilization (see Diamond "Guns, Germs, and Steel")
Figure to follow
Some variability, however
Climate record of 20th century –
Figure to follow
Nevertheless, recent (and relatively small) local climate shifts have had
significant influence.
Dust Bowl – 1930s
Recent (last several decades): abnormal strength and consistency of
Indian monsoon
1816 - "year without a summer" (Tambora eruption)
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• 
Figure 2. Paleoclimate record for last 150,000 yrs. Ruddiman, 2001.
The current climate epoch - the Holocene
Schneider and Held, Discriminants of 20th Century Changes in Earth Surface Temperature,
J. Climate, 14, 249, 2001
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Some “Climate Puzzles”:
•  Faint Sun
•  Warm Ages - Cretaceous (100 Ma) Eocene (50
Ma) smaller pole-equator gradient(?)
•  Glaciation - No ice first 2.5 Ga; generally ice
free until 0.1 Ga.
•  Transition from 40 Kyr to ~100 Kyr glaciation
~0.7-1 million years ago.
•  Present - warm extratropics / ice covered poles
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