Transcript Aerosols

Aerosols
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
8
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
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
IPCC [2001]
1750-PRESENT
“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
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
37
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
39
From Ramanathan 2001
0 to 3 km layer
NOAA R/V Ronald Brown
40
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
41
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.)
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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