Aerosol Fundamentals
Material from James Smith and Steven Massie
Presented by Steven Massie
NCAR / ACD
February 22, 2011
massie@ucar.edu, jimsmith@ucar.edu
NCAR is sponsored by the National Science Foundation
“We have in this fine dust
[aerosols] a most beautiful
illustration of how the little things
in the world work great effects by
virtue of their numbers.”
John Aitken (1839-1919)
-John Aitken, 1880
Why should we care about aerosol?
Human health
Air quality
Global Cimate
Radiation, Chemistry, Rainfall
Aerosols are important from the molecular to the
global scale
Aerosols : solid and liquid particles suspended in the air
Size: nm to 100 microns (range of 105)
Lifetime: Troposphere (days to weeks) Stratosphere (year)
Primary aerosol: emitted directly into the air
Secondary aerosol: gas to particle conversion
Composition: sulfate, ammonium, nitrate, sodium,
trace metals, carbonaceous, crustal, water
Carbonaceous
elemental: emitted directly into the air (e.g. diesel soot)
organic: a) directly by sources (e.g combustion, plant leaf)
b) condensation of low volatile organic gases
“fine”
diameters D < 2.5 microns
sulfate, ammonium, organic carbon, elemental carbon
Nuclei mode 0.005 to 0.01 microns
condensation of vapors
Accumulation mode
0.1 to 2.5 microns coagulation
“coarse”
diameters D > 2.5 microns
natural dust (e.g. desert)
mechanical processes
crustal materials
biogenic (pollen, plant fragmets)
Aerosols come from a variety of sources,
and reside in the atmosphere for weeks
aerosols = particles suspended in a gas
Seinfeld and Pandis
Aerosols and human health
1952: the “London smog disaster”
Air pollution
O3, CO, NO2, SO2
aerosol
Aerosols are the principle component of what we
perceive as “smog”
Submicron aerosols are
primarily responsible for
visibility reduction.
Pasadena, CA, on a clear day (hills are 7 km away)
Environmental Protection Agency (EPA)
PM2.5 15 g /m3 (annual average)
PM10 150 g /m3 (24 hour)
regulations.
See national map of compliance at:
http://en.wikipedia.org/wiki/File:Pm25-24asuper.gif
Pasadena, CA, on a bad smog day
Developments in Asia
1000 cars / day are added
to the Beijing road system
China GDP and NO2
trends ~ 10 % / year
Massie
Aerosols and human health
Submicron aerosols can penetrate to
the deepest parts of the lung
whereupon they can affect the
pulmonary part of the respiratory
system. For this reason many,
including the EPA, consider them
dangerous air pollutants.
Deposition Fraction
Aerosol Deposition in Human Respiratory Tract
1
0.8
Total
Pulmonary
Nasal
Tracheo-bronchial
0.6
0.4
0.2
0
0.01
0.1
1
10
Particle Diameter (m)
100
nanoaerosols and human health
Nanoparticles have the ability to translocate from the lung to different parts of
the body such as the heart, liver, bone marrow and brain.
(Oberdörster, Env. Health Perspect., 2005)
Mt. Pinatubo June 1991
Impact of Pinatubo aerosols on average global
surface air temperature: ~0.5 °C!
June 12, 1991 eruption
Polar Stratospheric Cloud (PSC)
g, gas
s, particle
ClONO2 (g) + HCl (s) (PSC)  Cl2(g) + HNO3(s)
Cl2 + hv  2Cl
Leads to the “Ozone Hole” every Spring
Aerosols and climate: indirect effect
“Ship Tracks” off the
coast of Washington
• aerosols are the “seeds” upon
which water vapor condenses to
form a cloud (these are called
“cloud condensation nuclei, or
CCN).
• If people make more aerosols,
we make more cloud droplets,
but because there is a fixed
amount of water vapor in the air
these droplets will be smaller.
• smaller droplets scatter light
more efficiently!
• smaller cloud droplets may also
impact rain from these clouds.
• very difficult effect to observe
and model!
Aerosol “indirect effect” on climate
clean cloud (few particles):
large cloud droplets
• low albedo
• efficient precipitation
polluted cloud (many particles):
small cloud droplets
• high albedo
• suppressed precipitation
(very controversial)
Aerosols and climate
 Drives
Global Warming
Direct effect –
Light is scattered
and absorbed
IPCC, AR4
“Warm clouds” clouds with liquid droplets
“Mixed phase clouds” clouds with liquid and ice
Optical Depth
I = I0 exp [ -(scattering + absorption) (n L) ]
Beer’s Law
I0 original intensity of light that goes into the cell
I observed intensity of light after it travels through the cell
L, path length of cell (cm)
n, number density of particles (cm-3)
scattering scattering cross section (cm2)
absorption absorption cross section (cm2)
= (scattering + absorption) (n L) Optical depth (unitless)
a = (scattering ) / (scattering + absorption) Single scattering albedo
K = (scattering + absorption) n
Extinction coefficient (1 / km)
Size Distribution
<100 nm: ultrafine
“remote
continental
air”
Number
Density n
100 nm<dp<1 m:
accumulation
Surface
Area
(n π r2)
sub-2.5 m: fine
coarse
Volume
Density
(n 4/3 π r3)
Seinfeld and Pandis
Size Distribution
Log-normal particle size distribution
dN/dr =  (Ni / (2 ) 1/2 r ln) exp {- (ln r/ln r0)2/2ln2)
Units: number per cm-3 per microns
i=1,2
2 modes
Ni, total number of particles for mode i (cm-3)
r, radii (microns)
, mode width (dimensionless)
r0, modal radii (microns)
Optical Properties
ext () = 103  Qext (r,) r2 dN/dr dr
extinction (units km-1)
r particle radius (m)
dn / dr particle size distribution (# cm-3 m-1)
Qext (r ,  ) extinction efficiency (from Mie theory)
Qext = Qsca + Qabs
(sca=scattering, abs = absorption)
Q is a function of the complex index of refraction
i.e. composition
Size parameter x = 2  r / 
Optical depth contribution in path length ds is  () ds
Mie Scattering
Mie scattering
(spherical particles)
Each curve
is offset by 1
Sensitivity to m2
m2=0.001
0.01
Qext
0.03
0.10
m, complex index of
refraction
Increase absorption, m2
m = 1.33 + i m2
The fine details become
smoother
Note as particle size
becomes large,
Qext -> 2
Bohren and Huffman
General Results
Size parameter x = 2  r /  for  = wavelength
For very small particles x << 1 (Rayleigh scattering)
scattering  1 / 4
The sky is Blue
For medium size particles (aerosols), x ~1 (Mie
scattering)
scattering ~ 1/ 
The sky is grey
For big size particles (cloud droplets, cirrus) x >> 1
Qext -> 2 independent of  Clouds are white
aerosol optics
particles scatter most
efficiently in the part of
the size spectrum with
the longest atmospheric
lifetimes!
Global Sulfur Cycle
OCS
Image courtesy of Hugh Powell, Univ. of Durham, UK
Global Nitrogen Cycle
PSCs
Image courtesy of Hugh Powell, Univ. of Durham, UK
450 / 3100
= 15%
Seinfeld and Pandis
Annual mean PM2.5 concentration (2002) derived
from MODIS satellite instrument data
The chemical properties of atmospheric aerosols: North America
http://eosweb.larc.nasa.gov/PRODOCS/narsto/table_narsto.htm
l
Annual mean
PM2.5
concentrations
(NARSTO, 2004)
Black carbon primary emissions (estimates based on
satellite obs)
DIESEL
DOMESTIC
COAL BURNING
BIOMASS
BURNING
Mean sea
salt
aerosol
concentrations
Lower marine boundary layer (0-100 m)
Roaring
40’s
Max 50 S
wind
Alexander et al. [2005]
Global dust emissions (modeled)
1 US Penny
= 2.5 g
Fairlie et al. [2007]
Secondary aerosols: Those derived from the condensation of
atmospheric trace gases
ammonium sulfate aerosol
O
H
S
O
O
H
Seinfeld and Pandis
+
+
O
NH4
+
NH3
-
O
-
O
NH4
S
O
O
pKa= -3
IMPROVE network
(http://vista.cira.colostate.edu/improve/)
The formation of ammonium nitrate aerosol
mixed NH4+/SO42-/NO3- system
+
HO
O
N
-
NH4 O
+
NH3
O
N
O
O
pKa= -1.5
Seinfeld and Pandis
IMPROVE network
(http://vista.cira.colostate.edu/improve/)
Impact of aerosol acidity: Acid deposition (1985)
sulfate
ammonium
nitrate
National Acid Deposition Program (http://nadp.sws.uiuc.edu/)
Impact of aerosol acidity: Acid deposition (2005)
sulfate
ammonium
nitrate
From 1985 to 2005 …
•Significant increases in precipitation
ammonium concentrations at 64% of sites.
•Statistically significant decreases in sulfate
at 89% of sites.
•Ammonium now exceeds sulfate over
more than half of the continental U.S.
National Acid Deposition Program (http://nadp.sws.uiuc.edu/)
Carbonaceous aerosol: Secondary and primary
ORGANIC CARBON (OC)
ELEMENTAL CARBON (EC)
Fossil fuel
Biofuel
Biomass burning
Vegetation
GLOBAL
130 Tg
yr-1
Fossil fuel
Biofuel
Biomass burning
Vegetation
22 Tg yr-1
Fossil fuel
Biofuel
Biomass burning
Vegetation
UNITED
STATES
2.7 Tg yr-1
0.66 Tg yr-1
Secondary Organic Aerosol (SOA)
SOA accounts for large fraction of submicron
particulate mass
Mixture of hundreds of compounds
Aging of SOA is important (there’s a complicated
time history)
Take e.g. toluene, isoprene, .. + ozone participate in very
many gas phase reactions, produce products with high and low
vapor pressures (e.g oxalic, adipic acid). The low vapor
pressure products will go into particles.
See Alex Gunther’s and
Mary Barth’s lectures.
Jimenez, Science, 326, Dec 2009
Odum, Env Sci Tech, 30, 2580, 1996
Ervens, JGR, 109, 2004
Mexico City : organic species and nanoparticle growth
Size (nm)
TDCIMS
Organics 82% Sulfate 10 %
Nitrate 8%
Smith, Geophy Res Lett, 35, 2008
The one-slide story behind SOA formation
VOC -> a1P1 + a2P2
In a model use:
Caer = Y ΔVOC
for a specific VOC
Aerosol mass yield (Y) as a function of:
caer: the total organic aerosol mass concentration
ci°: the saturation mass concentration of product i in the aerosol
ai: molar yield of product i
Find a1 and a2 by least
Mi: molecular weight of product i
squares fit to lab data
pi°: equilibrium vapor pressure of project i
“two-product model” works pretty well
T: temperature
Y
Y
Caer
Odum et al., EST, 1996
Caer
Modeling Organic Aerosol : What are the challenges?
Partitioning
Semi-Volatile
Organic Vapors
Nucleation
Cloud Processing
Oxidation
by OH, O3, NO3
Deposition
Evaporation
upon dilution
VOCs
Isoprene
Monoterpenes
Sesquiterpenes
Forest
Surface / multiphase
reactions
POA
Aromatics
Alkanes
For MILARGO
application see:
Hodzic, ACP,2009
SOA is
underestimated
Direct
Emission
Traffic Industries
Secondary
Organic
Aerosol
Biomass
Burning
Biological
Debris
Alma Hodzic
Nanoparticle Formation
• Nanoparticles form in the atmosphere by condensation to stable clusters
formed by nucleation. They can also be emitted directly, e.g., by diesel
engines.
• So how are stable clusters formed in the atmosphere?
– The formation of stable clusters from low vapor pressure atmospheric
species is known as homogeneous nucleation.
McMurry, Smith et al. in Aerosol Measurement Techniques, 2009.
One important, and poorly understood, source of
Cloud Condensation Nuclei is new particle formation
BEACHON Manitou Forest Observatory
New Particle Formation event on Dec 10, 2008
diameter that can activate
into a cloud droplet at
0.2% supersaturation
Smith, unpublished
 Model estimates suggest
that new particle formation
can contribute up to 40% of
the CCN at the boundary
layer, and 90% in the
remote troposphere (Pierce
and Adams, ACP, 2007).
 New particle formation is
estimated to add as much
as a 8 times more particles
to the remote southern
ocean atmosphere than
anthropogenic primary
particles (Spracklen et al.,
ACP, 2006).
Which compounds lead to particles?
What are the barriers to particle formation?
Saturation vapor pressure = P i, sat
A high saturation vapor pressure of a substance
means it is volatile – molecules easily escape
from its liquid surface, and do not stick back
easily on the surface.
Saturation ratio Sr = Pi / Pi, sat
Pi, partial pressure
When Sr > 1 (supersaturation), particles will form.
What controls Sr?
Saturated Vapor Pressure
H2SO4
1.3 x 10-8 atm
Water
0.02 atm (20 C)
Ethanol (liquor)
0.05 atm
Channel #5
expensive
The energetics of particle formation
Nature follows the lowest energy path “Why pay more?”
If you form a particle, you add mass. What is the cost of doing this?
Gibbs Free Energy G
dG= -S dT + Vdp +  dni
Chemical Potential 
Increase of energy of a mass when a
single particle is added to the mass.
Units: energy/particle
S entropy,
V volume
T temperature
G = G (droplet) – G ( pure vapor)
Seinfeld and Pandis
Critical cluster energy barrier
subsaturated
supersaturated
R particle radius T temperature Sr saturation ratio ( Pi / Pi, sat )
V volume of liquid molecule  surface tension
Seinfeld and Pandis
Getting to the Critical Nucleus
Particles readily grow if they are past the G* nucleation barrier
But what is the chemical composition of the critical nucleus?
Recent work points to how
organic acids + sulfuric acid
leads to particle creation faster than by just sulfuric acid alone
Organic acid : e.g. contains carboxyl group –COOH, benzoic acid
Organic acid concentrations > sulfuric acid concentrations
Zhang, Science, vol 328, June 2010
Zhang, Science, vol 304, June 2004
Cloud droplet formation is heterogeneous
Observations: supersaturation in natural clouds rarely
exceeds a few percent
Consequence: cloud droplets in natural clouds do not form
by homogeneous nucleation of pure water
Instead they form on atmospheric aerosol particles (cloud
condensation nuclei or CCN): heterogeneous nucleation

The CCN must be large and wettable
ok
Cloud droplet formation: Can clouds form from the
homogeneous nucleation of water vapor?
For Pure Water, not easy !
Why pay this price ?
Saturation
ratio S
Critical radius r*
Number of
molecules
1.01
0.12 m
2.5 x 108
1.1
0.0126 m
2.8 x 105
2
1.73 nm
730
10
0.52 nm
20
e.g. for S=1.01, particles with
R < 0.012 will evaporate
The vapor pressure in equilibrium is larger over a droplet with radius r than
over a bulk/flat surface (Kelvin equation)
Supercooling of Pure Water Droplets
( Nucleation of ice is aided by particles)
10 m
Pruppacher and Klett
Kelvin Equation
Vapor pressure - it’s higher over a curved
surface than over a flat surface
Ps = Ps’ exp ( 2  V / K T r )
Ps > Ps’
Ps’ saturation (equilibrium) vapor pressure over
a flat surface
Ps saturation vapor pressure over the particle
of size r, volume V in liquid phase, and surface
tension  (energy/area of the liquid/gas
interface)
Heterogeneous nucleation of clouds: Raoult’s
law must be considered
o
H 2O , SAT
P
water saturation vapor pressure
over pure liquid water surface
PH 2O,SAT  x
o
H 2O H 2O , SAT
P
water saturation vapor pressure
over aqueous solution of water
mixing ratio xH2O
Raoult’s Law
Consider nA moles of water
nB moles of salt
XA = mole fraction of water = nA / (nA + nB)
P’ Pressure of water over a flat surface
P’pure Pressure of pure water over flat surface
P’ = XA P’pure
Köhler theory (Kelvin effect + Raoult effect)
S = supersaturation ratio = Psat (r) / Psat (flat surface)
b
a
b
 a 
S  1  1  3   1   3
r r
 r  r 
1. term: “curvature term” (Kelvin)
2. term: “solution term” (Raoult)
where a 
2 lv vl
3 i ms M w
and b 
k BT
4l M s
rcrit = (3b/a)1/2 ,
Scrit = 1 + (4 a3/[27 b])1/2
e.g. w = water, s=solute
Köhler Curve
Seinfeld, Air Pollution book
Supersaturation and internal aqueous chemistry
Seinfeld, Air Pollution book
Aerosol and Asia
Black carbon absorbs radiation (leads to heating)
If present in the lower troposphere:
Stabilizes temperature profiles
Inhibits convection
To what extent will future fresh water resources
in Asia be dependent upon aerosol trends?
AOD = Aerosol
Optical Depth
Massie
60
Columbia University Database
Massie
61
Aerosol - Several Sources !
Forest Fires
Massie, O. Torres, S. Smith, JGR, 2004
62
Increase in aerosol aod over India
% / decade
Population increased by 46% , aerosol by 20% 1980 - 2000
Massie, Torres, Smith, JGR, 2004
63
Example of CALIPSO lidar vertical feature map
30
Clear Sky
Clouds
Z (km)
Aerosol
Rock
0
58
Latitude
10
Aerosol over India
Drought Frequency over India
Future 
ABC = Asian Brown Cloud
Ramanathan, PNAS, April 12, v102, p5326-5333, 2005
Elevated Heat Pump Hypothesis – William Lau (NASA Goddard)
Effects of black carbon and desert dust over the Tibetan Plateau
Warm
E
qe
W
warm
Warmer
E
qe
Normal monsoon
Water cycle (mid-May to
mid-June)
Warm
EHP-accelerated
Monsoon water cycle
(mid-May –June)
W
Cool
EHP postulates:
a) Warming and moistening of the upper troposphere over the Tibetan Plateau
b) An advance of the rainy season in northern India/Napal region in May-June
c) The increased convection spreads from the foothills of the Himalayas
to central India, resulting in an intensification of the Indian monsoon. in June-July
d) Subsequent reduction of monsoon rain in central India in July-August
e) Enhanced snowmelt and rapid retreat of mountain glacier
References
Seinfeld and Pandis, Atmospheric Chemistry
and Physics, 1998.
Rogers and Yau, A Short Course in Cloud Physics,
1989.
Bohren and Huffman, Absorption and Scattering
of Light by Small Particles, 1983.
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