Transcript Aerosols

Aerosols
Dr. Martin Leach
November 1, 2010
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.
Aerosols: General Comments
 Any solid, liquid (or mixture) in the atmosphere
 Sources
–
–
–
–
Natural
Anthropogenic (urban, construction, agriculture)
Primary (introduced directly into the atmosphere)
Secondary (formed in the attmosphere)
Aerosol Effects




Climate
Weather
Visibility
Health Effects
Clouds?
Natural Sources and Estimates of Global Emissions of Atmospheric
Aerosols
Source
Amount-range (Tg yr-1)
Amount -best
estimate (Tg yr-1)
Soil Dust
1000-3000
Sea Salt
1000-10000
Botanical Debris 26-80
1500
1300
50
Volcanoes
4-10000
30
Forest Fires
Gas conversion
Photochem
Total
3-150
100-260
40-200
2200-24000
20
180
60
3100
Anthropogenic Sources of Aerosols
Source
Amount Range
(Tg yr-1)
Best Estimate
Direct Emission
50-160
120
Gas to particle
260-460
330
Photochemistry
5-25
10
Total
320-640
460
Reference: W.C. Hinds, Aerosol Technology, 2nd
Edition, Wiley Interscience
Gas-to-particle conversion:
 Certain gas phase reactions result in formation of
low-vapor-pressure reaction products.
 Because of their low vapor pressure, they exist at
high supersaturations and can form particles.
Natural Background Aerosol
 Stratospheric
– Major volcanic activity injects sulfur dioxide (SO2) into
the stratosphere
– Gas to particle conversion, SO2 into sulfuric acid
(H2SO4)
 Tropospheric
– Vegetation, deserts and ocean
– Primarily in the lowest few km
Mount Pinatubo, 1991
Urban Aerosol
 Dominated by anthropogenic sources
 Three Modes
– Nuclei
– Accumulation
– Coarse
Aitken
Large
Giant
What is meant by the size of an aerosol?
What does a size distribution mean?
ORIGIN OF THE ATMOSPHERIC
AEROSOL
Aerosol: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
Question?
 Considering the Urban Aerosol, where are most of
the particles? Where is the most mass?
 How many 0.01 mm particles are necessary to
have the same mass as one 1mm particles?
Urban Aerosol Size Distribution
Nuclei Mode (<0.1mm)
 Consist of:
– Direct combustion particles emitted
– Particles formed by gas-to-particle conversion
 Usually found near sources of combustion (e.g.
highways!)
 Due to their high number concentration:
– Coagulate rapidly.
– End up in accumulation mode
– Relatively short lifetime
Aitken Particles
Accumulation Mode (0.1 μm < particle size <
2.5 μm)
 Includes combustion particles, smog particles, and
coagulated nuclei-mode particles.
(Smog particles are formed in the atmosphere by photochemical
reactions)
 Particles in this mode are small but they coagulate too
slowly to reach the coarse-particle mode.
– they have a relatively long lifetime in the atmosphere
– they account for most of the visibility effects of atmospheric
aerosols.
 The nuclei and accumulation modes together constitute
“fine” particles.
Large Particles
Coarse-particle mode (particle size > 2.5 μm)
 Consist of
– Windblown dust, large salt particles from sea spray,
– Mechanically generated anthropogenic particles such as
those from agriculture and surface mining.
 Due to their large size
– Readily settle out or impact on surface,
– Lifetime in the atmosphere is only a few hours.
Giant Particles
Dynamic Processes of Atmospheric Aerosol
 Formation
– Gas to particle conversion
– Photochemical processes
 Growth
– Coagulation, condensation, evaporation
 Removal
– Settling
– Deposition
– Rainout, washout
Global Effects of Aerosols
 Global Cooling
– Direct effect
– Indirect effect
 Ozone depletion
– Polar stratospheric clouds (PSC)
– Surfaces of PSC act to catalyze Cl compounds to
atomic Cl
QuickTime™ and a
decompressor
are needed to see this picture.
What is the mean diameter of the particles?"
 The answer to this question changes with your point of
view.
 What size particles carry the most mass?
(Biogeochemical cycles)
 What size particles cover the largest surface area?
(visibility)
 What is the size of the most abundant particles? (cloud
microphysics)
Aerosol Distributions
Number
 cloud formation
Surface
 visibility
Volume
 mass
Mass & Number
 human health
Number distribution function
 The number of particles with diameter between Dp and
Dp + dDp in a cm3
fn(Dp) dDp
(particles cm-3/mm)
 The total number of particles, N:
N =  fn(Dp) dDp
(particles cm-3 )
Surface Area Distribution Function
 The surface area of particles in a size range per
cm3 of air
fs(Dp)dDp = pDp2 fn(Dp )
(mm2 mm-1 cm-3 )
 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)
Volume Distribution Function
 The Volume distribution function can be defined
fv (Dp) dDp = {p/6} Dp3 fn (Dp)
(mm3 mm-1 cm-3 )
 So the total volume occupied can be written
V =  fv(Dp) dDp =  p/6 Dp3 fn(Dp) dDp (mm3 cm-3)
Log Normal
 Distributions based on log Dp can be defined
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 )
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.
Aerosol particle size distribution
N tot  

0
N tot  

0
dN
dr
dr
S tot  

0
Vtot  

0
dN
dr
dr
dN
4pr 
dr
dr
2
4 3 dN
pr 
dr
3
dr
27
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
TYPICAL U.S. AEROSOL SIZE
DISTRIBUTIONS
Fresh
urban
Aged
urban
rural
remote
Warneck [1999]
SAMPLE AEROSOL SIZE DISTRIBUTION (MARINE AIR)
Sea
salt
Sulfate
(natural)
COMPOSITION OF PM2.5
(NARSTO PM
ASSESSMENT)
Sulf ate
Es ther (1995- 99)
Egber t (1994- 99)
4.6 ug m-3
8.9 ug m-3
Nitrate
Toronto ( 1997- 99)
12.3 ug m-3
A mmonium
Blac k c ar bon
A bbots for d (1994- 95)
Or ganic c ar bon
7.8 ug m-3
Soil
Other
St. A ndrew s (1994- 97)
5.3 ug m-3
Fres no (1988-89)
39.2 ug m-3
Quaker City OH ( 1999)
12.4 ug m-3
Kern Wildlife Ref uge ( 1988- 89)
23.3 ug m-3
Los A ngeles (1995- 96)
30.3 ug m-3
Colorado Plateau ( 1996-99)
3.0 ug m-3
A r ends tv ille PA ( 1999)
10.4 ug m-3
Mex ic o City Netz ahualc oy otl (1997)
55.4 ug m-3
Mex ic o City - Pedr egal (1997)
24.6 ug m-3
Y orkv ille (1999)
14.7 ug m-3
Was hington DC ( 1996- 99)
14.5 ug m-3
A tlanta (1999)
19.2 ug m-3
Aerosols: Visibility
Washington, DC
Light Extinction
DX
I0
I
scattering
absorption
scattering
DI/I = e(-bDX)
Intensity
b (in a few more slides)
Extinction Coefficient
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/
Radiation and fine particles
Seinfeld and Pandis, 1998
Atmospheric Visibility
(absorption & scattering)
1.Residual
2.Scattered away
3.Scattered into
4.Airlight
Extinction Coefficient
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)
Visibility
 The ultimate limit in a very clean atmosphere is
Rayleigh scattering
 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 indicies 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
Scattering Cross Section
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 = 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
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]
RADIATIVE FORCING OF CLIMATE, 1750PRESENT
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)
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
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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
Summary: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
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 dimethylsulfide (see
Charlson et al., Nature, 326, 655-661).
INDOEX: Indian Ocean Experiment
INDOEX, 1999
56
INDOEX
+ RV Ronald Brown
Mean Aerosol Optical Depth over INDOEX region from Dec 2001 to May 2003
from MODIS (Ramanathan & Ramana, Environ. Managers, Dec. 2003).
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INDOEX
58
From Ramanathan 2001
0 to 3 km layer
NOAA R/V Ronald Brown
59
Air Flow During INDOEX 1999
NHcX
20N
2
-W
al
cT
ng
NH
e
B
c TNH
NHc T-B
3
La titude
0
NHm
2
T
1
SH
mT
20 S
Cru ise trac k o f
R. H. Bro wn
1
40E
6 0E
80E
No of leg
10 0E
Longitude
60
Field data showing the high variability of aerosol light absorption coefficient
with latitude and longitude, measured by NOAA/PMEL scientists aboard the
NOAA Research Vessel Ron Brown during the Aerosols 99 and INDOEX
(Indian Ocean Experiment) cruises. The aerosol light absorption coefficient is
presented in all figures in units of Mm-1. Measurements are made at a
wavelength of 550nm. (Courtesy of P.Quinn and T. Bates, NOAA/PMEL.)
61
Summary of Aerosol Physics
 How big are atmospheric particles depends on
which effect interests you.
– CCN – number (r < 0.1 mm)
– Radiative transfer & health – surface area (0.1 < r < 1.0
mm)
– Biogeochemical cycles – mass (r > 0.5 mm).
 Composition varies with size.
 Single scattering albedo and visibility