LECTURE 13 Atmospheric Aerosols

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LECTURE 13Atmospheric Aerosols

• METO/ENCE 434
• AIR POLLUTION
• RUSSELL R. DICKERSON

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Atmospheric Aerosols Bibliography Seinfeld
Pandis, Atmospheric Chemistry and Physics, Chapt.
7-13 Finlayson-Pitts Pitts, Chemistry of the
Upper and Lower Atmosphere, Chapt. 9. Classic
papers Prospero et al. Rev. Geophys. Space
Phys., 1607, 1983 Charlson et al. Nature 1987
Charlson et al., Science, 1992. Recent Papers
Ramanathan et al., Science, 2001 Andreae and
Crutzen, Science, 1997 Dickerson et al., Science
1997 Jickells et al., Global Iron Connections
Between Desert Dust, Ocean Biogeochemistry and
Climate, Science, 308 67-71, 2005.
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EPA REGIONAL HAZE RULE FEDERAL CLASS I AREAS TO
RETURN TO NATURAL VISIBILITY LEVELS BY 2064
will require essentially total elimination of
anthropogenic aerosols!

moderately polluted day
clean day
Acadia National Park
http//www.hazecam.net/
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INTRODUCTION Particles are one of the most
important and certainly the most visible aspects
of air pollution. The effects span the areas of
health (1 increase in mortality per 10 µg m-3)
acid rain, visibility degradation, radiation and
photochemistry and cloud microphysics changes
(and thus climate changes), and the Antarctic
ozone hole. For a view into the "bad old days"
see Killer Smog by William Wise.
NOMENCLATURE Particle refers to a solid or
liquid, larger than a molecule, diameter 0.01
µm, but small enough to remain in the atmosphere
for a reasonable time, diameter µm. Particulate is an adjective, in spite of
what EPA tries to say. Aerosol is a suspension
of particles in a gas.
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Particles, like gases, are characterized by
chemical content, usually expressed in mg m-3,
but unlike gases, particles also have a
characteristic size. We may want to start
discussion the characteristics of atmospheric
aerosols by addressing the question "What is the
mean diameter of the particles?" The answer to
this question changes with your point of view.
A. Size Number Distribution If your concern is
the mass of some pollutant that is being
transported through the air for biogeochemical
cycles, then you want to know the mean diameter
of the particles with the mass or volume. In
other words, "What size particles carry the most
mass? If your concern loss of visibility then
you want to know the diameter of the particles
that have the largest cross section or surface
area. In other words, "What size particles cover
the largest surface area?" If your concern is
cloud formation or microphysics then you want to
know the range of diameters with the largest
number of particles. In other words, "What is the
size of the most abundant particles?" If your
concern is human health then you need to know
about both the mass and number of the particles,
because only a certain size particle can enter
the lungs.
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Here we define the number distribution function,
fn (Dp), and the number of particles with
diameter between Dp and Dp dDp in a cm3 of air
as follows fn(Dp) dDp (particles
cm-3/mm) The total number of particles, N, is
given by the following integral (everywhere we
integrate from 0 to infinite diameters) N ?
fn(Dp) dDp (particles cm-3 )
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We can define a surface area distribution
function, fs(Dp), for spherical particles as
follows fs(Dp)dDp pDp2 fn(Dp ) (mm2
mm-1 cm-3 ) This gives the surface area of
particles in a size range per cm3 of air. The
total surface area of the particles, S, is given
by the integral over all diameters S ? fs(Dp)
dDp p? Dp2 fn (Dp) dDp (mm2 cm-3)
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Likewise the volume distribution function and the
total volume fv (Dp) dDp p/6 Dp3 fn (Dp)
(mm3 mm-1 cm-3 ) V ? fv(Dp) dDp ? p/6
Dp3 fn(Dp) dDp (mm3 cm-3) The
distributions based on log Dp can be defined in a
similar manner, where n(log Dp)dlogDp is the
number of particles in one cm3 with diameter from
Dp to Dp log Dp. The total number is N ?
n(log Dp) d(logDp) (particles cm-3 ) The
normalized distribution functions based on log Dp
for surface area and volume are similar. For the
differential number of particles between Dp and
Dp dDp we use the notation dN, and likewise dS
and dV, we can represent the size distribution
functions as - n (log Dp) dN / N dlogDp
ns (log Dp) dS / S dlogDp nv (log Dp)
dV / V dlogDp This is the common notation
for expressing the variation in particle number,
surface area or volume with the log of the
diameter.
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B. Chemical Composition The bimodal nature of
the size-number distribution of atmospheric
particles suggests at least two distinct
mechanisms of formation, and the chemical
composition of the particles reflects their
origins. Fine particles have a diameter
smaller than about 2.5 mm, and are produced by
the condensation of vapors, accumulation, and
coagulation. They have a chemical composition
that reflects the condensable trace gases in the
atmosphere SO2, NH3, HNO3, VOCs, and H2O. The
chemical composition is water with SO4-2, NO3-,
NH 4, Pb, Cl-, Br-, C(soot), and organic
matter where biomass burning is prevalent,
K. Coarse Particles have a diameter greater
than about 2.5 mm, are produced by mechanical
weathering of surface materials. Their
lifetimes, controlled by fallout and washout, are
generally short. The composition of particles in
this size range reflects that of the earth's
surface - silicate (SiO2), iron and aluminum
oxides, CaCO3 and MgCO3 over the oceans , NaCl.
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ORIGIN OF THE ATMOSPHERIC AEROSOL
Aerosol dispersed condensed matter suspended in
a gas Size range 0.001 mm (molecular cluster) to
100 mm (small raindrop)
Soil dust Sea salt
Environmental importance health (respiration),
visibility, radiative balance, cloud formation,
heterogeneous reactions, delivery of nutrients
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AEROSOL NUCLEATION
1 2 3
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molecules
DG
Surface tension effect
cluster size
Critical cluster size
Thermo driving force
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Atmospheric Aerosols
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Aerosol Distributions

• Number
• cloud formation
• Surface
• visibility
• Volume
• mass
• Mass Number
• human health

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TYPICAL U.S. AEROSOL SIZE DISTRIBUTIONS
Fresh urban
Aged urban
rural
remote
Warneck 1999
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SAMPLE AEROSOL SIZE DISTRIBUTION (MARINE AIR)
Sea salt
Sulfate (natural)
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COMPOSITION OF PM2.5 (NARSTO PM ASSESSMENT)
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C. Optical Properties and Visibility The optics
of aerosol science follow the most rigorous
physics. Traditionally defined visibility is the
distance at which a large dark object, such as a
hill or a barn can just be seen. A more
quantitative definition can be obtained by
considering the change in intensity of light
reflecting off an object as a function of the
scattering of light by the atmosphere. DI/I
e(-bDX) Where I is the intensity of light, b
(or bext) is the extinction coefficient with
units of m-1, and X is the distance in m. The
limit to visibility for the human eye is a 2
change in intensity relative to the background
or DI/I 0.02
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Radiation and fine particles
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Atmospheric Visibility (absorption scattering)

• 1.Residual
• 2.Scattered away
• 3.Scattered into
• 4.Airlight

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The extinction coefficient represents the sum of
the extinctions from gases and particles, each of
which can in turn be divided into extinction due
to absorption or scattering. bext bgas
bparticles bext babs bscatt babs (gases)
Beer's Law absorption bscatt (gases) Rayleigh
Scattering babs (particles) Usually extinction bscatt (particles) Mie Scattering
(bsp) The ultimate limit to visibility in a
very clean atmosphere is Rayleigh scattering, but
Mie scattering usually dominates. The range of
bsp is 10-5 m -1 to 10-3 m-1. Single scattering
albedo, w, is a measure of the fraction of
aerosol extinction caused by scattering w
bsp/(bsp bap)
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Optical Properties of Small Particles

• m n ik
• m complex index of refraction
• n scattering (real part)
• k absorption (imaginary part)
• The real part of the index of refraction is only
  a weak function of wavelength, while the
  imaginary part, ik, depends strongly on
  wavelength.

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Refractive indicies of aerosol particles at ?
589 nm
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The scattering cross section is the product of
the mass loading, and the surface area per unit
mass note the ln of 0.02 is about -3.9,
thus Visibility 3.9(bsp)-1 bsp S?m
Where bsp is the scattering coefficient in units
of m-1 m is the mass loading in units of g m-3 S
is the surface area per unit mass in units of
m2g-1 For sulfate particles, S is about 3.2 m2
g-1 where the humidity is less than about 70
for other materials it can be greater. Visibility
3.9/(3.2 m) 1.2 /(m)
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Example Visibility improvement during the 2003
North American Blackout
Normal conditions over Eastern US during an air
pollution episode bsp 120 Mm-1 1.2 x 10-4
m-1 at 550 nm bap 0.8 x 10-5 m-1 bext
1.28 x 10-4 m-1 Visual Range 3.9/bext 30
km During blackout bsp 40 Mm-1 0.4 x 10-4
m-1 bap 1.2 x 10-5 m-1 bext 0.52 x 10-4
m-1 Visual Range 3.9/bext 75 km
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Example Visibility improvement during the 2003
North American Blackout
Single scattering albedo, w, normal 1.20/1.28
0.94 Blackout 0.4/0.52
0.77 With the sulfate from power plants
missing, and the soot from diesel engines
remaining the visual range is up, but the single
scattering albedo is down. Ozone production
inhibited. See Marufu et al., Geophys Res.
Lett., 2004.
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Extinction Coefficient as a PM2.5 Surrogate
PM2.5 7.6 ?g/m3
PM2.5 21.7 ?g/m3
PM2.5 65.3 ?g/m3
Glacier National Park images are adapted from
Malm, An Introduction to Visibility (1999)
http//webcam.srs.fs.fed.us/intropdf.htm
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ANNUAL MEAN PARTICULATE MATTER (PM)
CONCENTRATIONS AT U.S. SITES, 1995-2000NARSTO PM
Assessment, 2003
PM10 (particles 10 mm)
PM2.5 (particles 2.5 mm)
Red circles indicate violations of national air
quality standard 50 mg m-3 for PM10
15 mg m-3 for PM2.5
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AEROSOL OPTICAL DEPTH (GLOBAL MODEL)
Annual mean
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AEROSOL OBSERVATIONS FROM SPACE
Biomass fire haze in central America (4/30/03)
Fire locations in red
Modis.gsfc.nasa.gov
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BLACK CARBON EMISSIONS
DIESEL
DOMESTIC COAL BURNING
BIOMASS BURNING
Chin et al. 2000
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RADIATIVE FORCING OF CLIMATE, 1750-PRESENT
IPCC 2001
Kyoto also failed to address two major
pollutants that have an impact on warming  black
soot and tropospheric ozone.  Both are proven
health hazards.  Reducing both would not only
address climate change, but also dramatically
improve people's health. (George W. Bush, June
11 2001 Rose Garden speech)
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Climate radiative forcing revised 2002.
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ASIAN DUST INFLUENCE IN UNITED STATESDust
observations from U.S. IMPROVE network
April 16, 2001 Asian dust in western U.S.
April 22, 2001 Asian dust in southeastern U.S.
0 2 4
6 8 mg m-3
Glen Canyon, AZ
April 16, 2001 Asian dust!
Clear day
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TRANSPACIFIC TRANSPORT OF ASIAN DUST PLUMES
GEOS-CHEM Longitude cross-section at 40N, 16
April, 2001
Subsidence over western U.S.
Source region (inner Asia)
Asian plumes over Pacific
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0
ASIA
UNITED STATES
T.D. Fairlie, Harvard
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Aerosols in the Atmosphere

• Abundance and size
• Aerosol concentration is highly variable in space
  and time. Concentrations are usually highest near
  the ground and near sources.
• A concentration of 105 cm-3 is typical of
  polluted air near the ground, but values may
  range from 2 orders of magnitude higher in very
  polluted regions to several lower in very clean
  air.
• Radii range from 10-7 cm for the for small ions
  to more than 10 µm (10-3 cm) for the largest salt
  and dust particles.
• Small ions play almost no role in atmospheric
  condensation because of the very high
  supersaturations required for condensation.
• The largest particles, however, are only able to
  remain airborne for a limited time

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Aerosol Size Naming Convention

• Usually divided into three size groups ( D -
  diameter)
• 1. Aitken Nuclei 2 x 10-3 µm
• 2. Large Nuclei 0.2 µm
• (also called the accumulation mode)
• 3. Giant Nuclei D 2.5 µm

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Other Naming Convention

• Nucleation mode D 0.1 µm
• Accumulation or coagulation mode 0.1 µm µm
• Thought to be most important in natural cloud
  formation
• Coarse Particle Mode D 1 µm

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Origins of Atmospheric Aerosols

• Condensation and sublimation of of vapors and the
  formation of smokes in natural and man-made
  combustion.
• Reactions between trace gases in the atmosphere
  through the action of heat, radiation, or
  humidity.
• The mechanical disruption and dispersal of matter
  at the earths surface, either as sea spray over
  the oceans, or as mineral dusts over the
  continents.
• 4. Coagulation of nuclei which tends to produce
  larger particles of mixed constitution

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Aerosol Makeup

• Typical substances formed in large quantities by
  condensation following combustion include ashes,
  soot, tar products, oils as well as sulfuric acid
  and sulfates. These particles are primarily
  within the range of Aitken nuclei.
• Mechanical disintegration, by wind and water, of
  rocks and soil produces particles with diameters
  0.2 µm. These fall primarily in the large
  nuclei range.
• According to Jaenicke (Science, 308 p. 73, 2005)
  about 25 of the number of particles with
  diameter greater than 0.2 µm are biogenic.
  (remains to be verified).

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Aerosol Makeup - continued

• Chemical reactions between nitrogen, oxygen,
  water vapor and various trace gases (e.g., sulfur
  dioxide, chlorine, ammonia, ozone, and oxides of
  nitrogen) primarily produce particles in the
  Aitken and Large range.
• Examples
• Formation of ammonium chloride from NH3 and HCl
• Oxidation of SO2 to H2SO4
• Reaction of sulfur dioxide, ammonia, and water to
  produce ammonium sulfate particles.
• Production of higher oxides of nitrogen through
  the action of heat, ozone or ultraviolet radiation

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Cloud Condensation Nuclei - CCN

• Comprises a small fraction of the total aerosol
  population
• Sea salt is the predominant constituent of CCN
  with D 1µm
• For 0.1 µm thought to be sulfate, which may be present as
  sulfuric acid, ammonium sulfate, or from
  phytoplankton produced dimethylsulfide (see
  Charlson et al., Nature, 326, 655-661).

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Activity Spectrum

• Let Nc be the number of particles per unit
  volume that are activated to become cloud
  droplets.
• Data from cloud chamber measurements are often
  parameterized as
• Nc C (S-1)k
• where C and k are parameters that depend on air
  mass type.
• Rogers gives
• Maritime air 30 1
• Continental air 300 2
• Thus, for the same saturation ratio, one would
  expect to find small numbers of CCN per unit
  volume in maritime air and large numbers per unit
  volume in continental air.