AOSC 620 Ozone and other Layers in the Atmosphere R' Dickerson

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AOSC 620The Ozone HoleR. Dickerson
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• Recap
• The general for of a catalytic ozone destruction
  cycle is
• X O3 ? XO O2
• XO O ? X O2
• O O3 ? 2O2 NET
• Where X is OH, NO, Cl, or Br.
• But nobody saw the Ozone Hole coming!

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October 24, 2009 From NASA http//ozonewatch.gsfc.
nasa.gov/index.html
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• Antarctic Ozone Hole
• In the Antarctic winter there is no sunlight and
  even in the spring there is too little UV to
  generate enough O atoms to destroy ozone. The
  annual loss of ozone over Antarctica is driven by
  heterogeneous chemistry and visible radiation. A
  good review is provided by Solomon Rev. Geophys.,
  1999, and Scientific Assessment of Ozone
  Depletion 2006 (WMO). The destruction of ozone
  is usually moderated by the production of
  chlorine nitrate, an important reservoir species.
• NO2 ClO M ? ClONO2 M
• In the Antarctic winter, heterogeneous reactions
  liberate chlorine from long-lived HCl and ClONO2
  and sedimentation of polar stratospheric clouds
  denitrifies the stratosphere (Solomon et al.,
  Nature, 1986 McElroy et al., Nature, 1986 Toon
  et al., GRL, 1986).

HCl ClONO2 ? Cl2 (gas) HNO3(aqueous)
Cl2 h? ? 2Cl HNO3(aqueous) sediments
(falls) out of stratosphere, removing a potential
source of NOx.
ice
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• Molina and Molina (1987)
• 2(Cl O3 ? O2 ClO)
• ClO ClO M ? (ClO)2 M?
• (ClO)2 hv ? Cl ClOO
• ClOO M ? Cl O2 M
• 2O3 ? 3O2 NET
• Two types of Polar Stratospheric Clouds (PSCs)
  exist.
• Type I HNO3 ? 3H2O Nitric acid trihydrate,
  formed at T 195K
• Type II H2O Water ice formed at T 190K
• They move NOy species from the vapor phase to the
  condensed phase as HNO3.
• They move chlorine from the reservoir species HCl
  and ClONO2 to ClOx.

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• Molina and Molina (1987)
• 2(Cl O3 ? O2 ClO)
• ClO ClO M ? (ClO)2 M?
• (ClO)2 hv ? Cl ClOO
• ClOO M ? Cl O2 M
• 2O3 ? 3O2 NET

McElroy, Salawitch, et al. (1986) Cl O3 ? ClO
O2 Br O3 ? BrO O2 ClO BrO ? Cl Br
O2 2O3 ? 3O2 NET
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Airborne Antarctic Ozone Expedition Punta
Arenas, Chile,1987
Anderson et al., Science, 1991
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THE ANTARCTIC OZONE HOLE
Southern Hemisphere ozone column seen from TOMS,
October
DU
1 Dobson Unit (DU) 0.01 mm O3 STP 2.69x1016
molecules cm-2
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Polar Stratospheric Clouds (PSCs)
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World Production of CFCs
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Multiphase Processes Gas-Surface Reactions
Key to ozone hole formation are reactions of
gaseous species with PSCs. Multiphase reactions
are important for tropospheric chemistry as well
the formation of HONO appears to take place
predominantly on organic surfaces. The
probability of a molecule A striking a unit area
is Z ¼ MA nA Where M is the molecular
number density and A is the mixing ratio of A,
and nA is the rms speed of molecules A (see
Kinetics Lecture). The number of collisions with
a single spherical particle of radius r is
proportional to the area of the particle, 4pr2.
For an ensemble of particles with total area Stot
(units area of particles per volume of air or cm2
cm-3) the total collision rate is R ¼ Stot MA
nA
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Aerosol particle size distribution
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Gas-Aerosol Reactions, cont.
Not all collisions result in a reaction, just as
with gas-gas collisions. The probability of a
collision resulting in a reaction is given by g,
called the accommodation coefficient, or uptake
coefficient. The rate of reaction is R ¼ g
Stot MA nA A pseudo first order rate ¼ g Stot
nA is a first order rate coefficient with units
of s-1. The value of g is a function of
temperature and the resistance due to diffusion
to the surface, the resistance to entering the
solution, and the fate of the molecule in the
condensed phase. The change in entropy on
solvation is negative thus g decreases with
temperature.
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What is an aerosol?

• An aerosol is a particle suspended in the
  atmosphere
• solid wind blown dust, volcanic ash
• liquid/partially liquid sulfates, sea spray
• Natural vs. Anthropogenic
• natural mineral dust, sea spray, volcanic ash,
• sulfates from biogenic sources or volcanic
• SO2
• anthropogenic industrial dust/soot, biomass
  burning,
• sulfates from pollutant sources

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What is an aerosol (2)?
106 ?m 1 m 1 mm 1000 ?m
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You can see aerosols from space
MODIS images of smoke (Quebec, July 2002, left)
and dust (Canary Islands, February 2004, right)
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Why do we care about aerosols?

• Climatic effects
• aerosols scatter and absorb solar radiation
  (direct effect)
• reduces radiation going to the ground (cooling)
• absorbing aerosols heat the air, changing
  dynamics
• aerosols modify cloud properties
• cloud droplets form on small aerosol particles
  called cloud condensation nuclei (CCN)
• more aerosols possibly more CCN
• for a given amount of water vapor, if there are
  more CCN the cloud droplets formed are smaller
  and the clouds look brighter from space (Twomey
  effect, 1st indirect effect)
• smaller cloud droplets dont coalesce efficiently
  to form large raindrops, so the clouds last
  longer (2nd indirect effect)

Do aerosol effects explain why global warming is
not as bad as greenhouse gas estimates alone
would lead us to expect?
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Why do we care about aerosols (2)?

• Chemistry
• aerosols provide surface area to drive some
  chemical reactions (heterogeneous chemistry)
• in the nighttime polar stratosphere, natural
  sulfate aerosols can form polar stratospheric
  clouds (PSCs)
• PSCs lock up Cl atoms during the polar night
• when sunlight returns, the clouds break-up,
  freeing the CL, which destroys ozone
• in the troposphere, aerosols also affect the
  amount of UV radiation available to drive
  chemical reactions

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Why do we care about aerosols (3)?

• Health/Environmental Concerns
• Aerosols affect visibility, with aesthetic but
  also safety implications (e.g., dust storms
  causing car accidents)
• Aerosols frequently pick up acids or even
  biological components hazardous to ecosystems
• Fine particulate matter (PM2.5) is regulated by
  the EPA
• particles smaller than 2.5 ??m diameter penetrate
  easily to lungs
• aggravates asthma, decreases lung capacity

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Sizes and sources of particles

• Aitken mode
• smallest particles (d lt 0.1 ?m)
• formed by gas to particle conversion (homogeneous
  nucleation) or condensation
• Accumulation mode
• 0.1 lt d lt 2.5 ?m
• Direct emissions (e.g., biomass burning),
  condensation on existing particles, or growth
  from Aitken mode by coagulation
• Coarse mode
• d gt 2.5 ?m
• mechanically generated (e.g., dust blown up by
  winds)

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Some concepts / terminology

• Aitken particles
• typically are larges concentrations / lowest
  mass concentrations
• Coarse mode particles
• Typically smallest concentrations / largest
  mass concentration

• Primary particles
• emitted fully formed into the atmosphere (e.g.,
  dust, soot)
• Secondary particles
• largely formed from gas to particle conversion
  (e.g., sulfates)

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Transformation Removal Mechanisms

• Particles grow by coagulating (sticking to each
  other) or by gases condensing onto their surfaces
• Particles shrink by evaporation of condensates or
  mechanically breaking
• Particles are removed by turbulent deposition to
  the surface (important for smallest particles),
  scavenging by precipitation, or gravitational
  settling (important for larges particles)

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Aerosol physical properties

• Chemical composition
• chemical reactions
• refractive index (i.e., optics)
• Shape
• light scattering
• efficiency for condensational growth
• Concentration
• efficiency of transformations (e.g., coagulation
  goes as the number concentration2)
• Size
• lifetime
• optical properties
• mobility

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Aerosol particle size distribution
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Difficult to evaluate particle size distribution
on linear scale because particles cover such a
wide range of size

• Use a logarithmic scale on the x-axis

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Distributions which look like Gaussian
distributions (normal distributions) when
plotted with a logarithmic x-axis are called
lognormal
This size distribution has 2 lognormal modes
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Lognormal Distributions
N number of particles per unit volume
cm-3 dN/dr number of particles per unit
volume per unit radius with radius between r and
r dr cm-3 cm-1
Where rm is the median radius and ? is the width
of the distribution
Problem What is the constant A for the number
distribution? What is the total volume of a
lognormal distribution?
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Some simple problems
An observed size distribution has three size
modes with these parameters

• How many particles are present per unit volume?
• What is the surface area of particles per unit
  volume?
• What is the volume of particles per unit volume?
• Plot the number, surface area, and volume size
  distributions as dN/d(ln r), dS/d(ln r), and
  dV/d(ln r) versus r. Use a logarithmic axis for
  radius. Be careful with units! Hint dN/d(ln r)
  rdN/dr.
• You can do this in Excel!

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