UV Remote Sensing of O3 and SO2
The Ozone Layer
•  The stratospheric ozone layer is a consequence
of molecular photodissociation
•  UV-C radiation dissociates molecular oxygen:
O2 + hv (λ < 0.2423 µm)  O + O
•  The large amount of oxygen in the atmospheric
column absorbs most solar radiation at λ < 0.24 µm
by this mechanism
•  The free oxygen atoms from the above reaction
then combine with other O2 molecules to produce
ozone:
O + O2  O3
•  Ozone is then dissociated by UV radiation:
O3 + hv (λ < 0.32 µm)  O + O2
•  Ozone is also destroyed by this reaction:
O3 + O  O2 + O2
The Chapman
Reactions
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The Ozone Layer
•  Fortunately for life on Earth, ozone absorbs strongly between 0.2 and
0.31 µm via electronic transitions – removing most UV-B and UV-C not
absorbed by O2
•  UV-A radiation (λ > 0.32 µm) is transmitted to the lower atmosphere
•  Plus a small fraction of UV-B (0.31-0.32 µm) – responsible for sunburn
•  Widening of this UV-B window (due to ozone depletion) would have
serious impacts on life
•  Absorption of solar radiation by ozone also locally warms the
atmosphere to a much higher temperature than would be possible if
ozone was absent – hence the increase in T in the stratosphere
•  Hence in an atmosphere without free oxygen, and hence without ozone,
the temperature would decrease with height until the thermosphere. There
would be no stratosphere, and weather would be vastly different...
Ozone hole
Antarctic ozone hole on Sept 11, 2005
Observed by Ozone Monitoring
Instrument (OMI)
•  Ozone destruction peaks in the Spring, as
UV radiation returns to the polar regions
•  Catalyzed by the presence of CFC
compounds (which supply chlorine), and by
polar stratospheric clouds (PSCs) at very
cold temperatures
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Ozone is not just in the stratosphere..
•  The UV-A radiation that reaches the troposphere is a key player in
tropospheric chemistry
•  Photochemical reactions involving unburned fuel vapors (organic
molecules) and nitrogen oxides (produced at high temperatures in car
engines) produce ozone in surface air (tropospheric ozone)
•  Ozone is good in the stratosphere, but a hazard in the troposphere (it is
a strong oxidant that attacks organic substances, such as our lungs)
•  Ozone is a major ingredient of photochemical smog
λ < 0.4 µm
Los Angeles: sunshine (UV) + cars + trapped air = smog
Atmospheric Constituents
The Ozone
Layer
The Air We
Breathe
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Satellite trace gas retrievals in the UV
Antarctic Ozone from TOMS and OMI
October
1, 1999
550
440
October
1,
2003
October
1, 2006
330
220
Dobson Units
October
1,
1994
110
http://macuv.gsfc.nasa.gov/
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Pinatubo SO2 cloud
Maximum SO2 column: ~800 DU
Total SO2 Mass: ~20 Mt
Pintatubo (Philippines) erupted in June 1991 and produced the largest
SO2 cloud measured to date (i.e. since 1978)
Satellite viewing geometry
Solar zenith
angle
d
Sensor zenith
angle
Absorbing gas
N
Solar azimuth
angle
Sensor azimuth angle
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Beer’s Law in this case
I λ = I 0,λ exp(−σ λ N d m)
m = sec θs + sec θsat = airmass factor (AMF)
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UV SO2 and O3 absorption spectra
Flyspec, UV camera
OMI has ~720
UV channels,
compared to 6
on TOMS
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Ideal Gas Law
•  The equation of state of an ideal gas – most gases are assumed to be ideal
PV = nRT
k=
PV = NkT
R
NA
•  P = pressure (Pa), V = volume taken up by gas (m3), n = number of moles, R =
gas constant (8.314 J mol-1 K-1), T = temperature (K)
•  k = Boltzmann constant (1.38×10-23 J K-1), N = number of molecules, NA =
Avogadro constant (6.022×1023 molecules mol-1)
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•  Neglects molecular size and intermolecular
attractions
•  States that volume changes are inversely
related to pressure changes, and linearly related
to temperature changes
•  Decrease pressure at constant volume =
temperature must decrease (adiabatic cooling)
Ideal gases
•  Standard temperature and pressure (STP): varies with organization
•  Usually P = 101.325 kPa (1 atm) and T = 273.15 K (0ºC)
•  Sometimes P = 101.325 kPa and T = 293.15 K (20ºC)
•  At STP (101.325 kPa, 273.15 K) each cm3 of an ideal gas (e.g., air)
contains 2.69×1019 molecules (or 2.69×1025 m-3)
•  This number is the Loschmidt constant and can be derived by
rearranging the ideal gas law equation:
N=
PV
kT
•  At higher altitudes, pressure is lower and the number density of
molecules is lower
•  Mean molar mass of air = 0.02897 kg mol-1 (air is mostly N2)
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Column density
•  Another way of expressing the abundance of a gas is as column density (Sn),
which is the integral of the number density along a path in the atmosphere
Sn =
∫c
n
(s) ds
path
•  The unit of column density is molecules cm-2
•  The integral of the mass concentration is the mass column density Sm (typical
units are µg cm-2)
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∫c
Sm =
m
(s) ds
path
•  Usually the path is the entire atmosphere from the surface to infinity, called the
total column, giving the total (vertical) atmospheric column density, V:
∞
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V=
∫c
n
(z) dz
0
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Dobson Units
•  A Dobson Unit [DU] is a unit of column density used in ozone research, and in
measurements of SO2
•  Named after G.M.B. Dobson, one of the first scientists to investigate atmospheric
ozone (~1920 – 1960)
•  The illustration shows a column of
air over Labrador, Canada. The total
amount of ozone in this column can
be conveniently expressed in
Dobson Units (as opposed to typical
column density units).
•  If all the ozone in this column were
to be compressed to STP (0ºC, 1
atm) and spread out evenly over the
area, it would form a slab ~3 mm
thick
•  1 Dobson Unit (DU) is defined to be 0.01 mm thickness of gas at STP; the ozone
layer represented above is then ~300 DU (NB. 1 DU also = 1 milli atm cm)
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Dobson Units
•  So 1 DU is defined as a 0.01 mm thickness of gas at STP
•  We know that at STP (101.325 kPa, 273.15 K) each cm3 of an ideal gas
(e.g., air, ozone, SO2) contains 2.69×1019 molecules (or 2.69×1025 m-3)
•  So a 0.01 mm thickness of an ideal gas contains:
2.69×1019 molecules cm-3 × 0.001 cm = 2.69×1016 molecules cm-2 =1 DU
•  Using this fact, we can convert column density in Dobson Units to mass of
gas, using the cross-sectional area of the measured column at the surface
•  For satellite measurements, the latter is represented by the ‘footprint’ of the
satellite sensor on the Earth’s surface
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