LECTURE 13
Atmospheric Aerosols
AOSC 434
AIR POLLUTION
RUSSELL R. DICKERSON
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.
EPA REGIONAL HAZE RULE: FEDERAL CLASS I AREAS
TO RETURN TO “NATURAL” VISIBILITY LEVELS BY 2064
…will require essentially total elimination of anthropogenic aerosols!
•
clean day
moderately polluted day
Acadia National Park
http://www.hazecam.net/
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 < 100 μm.
Particulate is an adjective, in spite of what EPA tries to say.
Aerosol is a suspension of particles in a gas.
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.
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=
 f (D ) dD
n
p
p
(particles cm-3 )
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)
Likewise the volume distribution function and the total volume:
fv (Dp) dDp = {p/6} Dp3 fn (Dp)
V =  fv(Dp) dDp
=
(mm3 mm-1 cm-3 )
 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.
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,
VOC’s, 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.
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…
AEROSOL NUCLEATION
# molecules 1
2
3
4
DG
Surface
tension
effect
Thermo
driving
force
Critical
cluster size
cluster size
Atmospheric Aerosols
Aerosol Distributions
Number
• cloud formation
Surface
• visibility
Volume
• mass
Mass & Number
• human health
TYPICAL U.S. AEROSOL SIZE DISTRIBUTIONS
Fresh
urban
Aged
urban
rural
remote
Warneck [1999]
SAMPLE AEROSOL SIZE DISTRIBUTION (MARINE AIR)
Sea
salt
Sulfate
(natural)
From USEPA (Neil Frank)
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
Radiation and fine particles
Atmospheric Visibility
(absorption & scattering)
1.Residual
2.Scattered away
3.Scattered into
4.Airlight
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 < 10% of 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)
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.
Refractive indices of aerosol
particles at  = 589 nm
Substance
m = n + ik
n
k
Water
1.333
10-8
Ice
1.309
10-8
NaCl
1.544
0
H2SO4
1.426
0
NH4HSO4
1.473
0
(NH4)2SO4
1.521
0
SiO2
1.55
0
Black Carbon (soot)
1.96
0.66
Mineral dust
~1.53
~0.006
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)
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 = 4.0 x 10-5 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
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.
Extinction Coefficient as a PM2.5 Surrogate
PM2.5 = 7.6 mg/m3
PM2.5 = 21.7 mg/m3
PM2.5 = 65.3 mg/m3
Glacier National Park images are adapted from Malm, An Introduction to
Visibility (1999) http://webcam.srs.fs.fed.us/intropdf.htm
ANNUAL MEAN PARTICULATE MATTER (PM)
CONCENTRATIONS AT U.S. SITES, 1995-2000
NARSTO 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
AEROSOL OPTICAL DEPTH (GLOBAL MODEL)
Annual mean
AEROSOL OBSERVATIONS FROM SPACE
Biomass fire haze in central America (4/30/03)
Fire locations
in red
Modis.gsfc.nasa.gov
BLACK CARBON EMISSIONS
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING
Chin et al. [2000]
ASIAN DUST INFLUENCE IN UNITED STATES
Dust observations from U.S. IMPROVE network
April 16, 2001
Asian dust in western U.S.
0
2
April 22, 2001
Asian dust in southeastern U.S.
4
mg m-3
6
8
Glen
Canyon,
AZ
Clear day
April 16, 2001: Asian dust!
TRANSPACIFIC TRANSPORT OF ASIAN DUST PLUMES
GEOS-CHEM Longitude cross-section at 40N, 16 April, 2001
Source region
(inner Asia)
Asian plumes
over Pacific
Subsidence
over western U.S.
ALTITUDE (km)
10
5
0
100E
ASIA
150E
150W
LONGITUDE
100W
UNITED STATES
T.D. Fairlie, Harvard
The future is hard to predict.
What really happened?
National Emission Trends
Courtesy of Dr. Hao He
http://www.epa.gov/ttn/chief/trends/index.html
Major NOx controls
Implemented after 2000
43
NASA Aura OMI Shows Air Quality is Improving
• OMI nitrogen dioxide data
indicate a 30-40% decrease in
the pollutant’s levels from 2005
to 2011.
• NO2 levels have dropped
through the implementation of
emission control devices on
coal-burning power plants and
more fuel-efficient cars.
• NASA AQAST members are
working with state air quality
agencies to demonstrate the
effectiveness of their efforts to
improve air quality and to find
novel uses of satellite data for
air quality applications.
The bad news: PM2.5 only fell modestly.
SO2
PM2.5
Why? Sulfate
was dominant
and has a longer
lifetime than SO2.
SOA becoming
dominant; it also
has a lifetime of
~10 d.
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
Aerosol Size Naming Convention
Usually divided into three size groups ( D - diameter)
1.
Aitken Nuclei 
2.
Large Nuclei 
2 µm < D 2.5 µm
(also called the accumulation mode)
3. Giant Nuclei 
2 x 10-3 µm < D 2 µm
D > 2.5 µm
Other Naming Convention
•
Nucleation mode  D ≤ 0.1 µm
•
Accumulation or coagulation mode  0.1 µm < D ~ 1 µm
Thought to be most important in natural cloud formation
•
Coarse Particle Mode  D ~ 1 µm
Origins of Atmospheric Aerosols
1.
Condensation and sublimation of of vapors and the formation of
smokes in natural and man-made combustion.
2.
Reactions between trace gases in the atmosphere through the
action of heat, radiation, or humidity.
3.
The mechanical disruption and dispersal of matter at the earth’s
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
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).
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
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 < D < 1 µm, the main component is thought to be sulfate,
which may be present as sulfuric acid, ammonium sulfate, or from
phytoplankton produced dimethyl sulfide (see Charlson et al.,
Nature, 326, 655-661).
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 < C < 300 cm-3; 0.3 < k < 1
Continental air:
300 < C < 3000 cm-3; 0.2 < k < 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.
How aerosols affect the radiative properties of clouds.By nucleating a larger number of
smaller cloud drops, aerosols affect cloud radiative forcing in various ways.
D Rosenfeld et al. Science 2014;343:379-380
Published by AAAS
Summary
1.
2.
3.
4.
5.
6.
7.
Aerosols are the most visible of air pollution classes.
The PM2.5 (annual average) standard to protect health is 12 mg/m3.
Number, surface and volume (mass) all play a role.
Log-normal distributions.
Single scattering albedo w = bsp/(bsp + bap)
Composition reflects sources and sinks.
Trends have been downward over North America.