aerosols. what is the mean diameter of the particles?" the answer to this question changes...
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What is the mean diameter of the particles?"
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)
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 DistributionsAerosol DistributionsAerosol DistributionsAerosol DistributionsNumber
cloud formation
Surface visibility
Volume mass
Mass & Number human health
Number cloud formation
Surface visibility
Volume mass
Mass & Number human health
Number distribution functionNumber distribution function
The number of particles with diameter between Dp and Dp + dDp in a cm3
fn(Dp) dDp (particles cm-3/m)
The total number of particles, N:
N = fn(Dp) dDp (particles cm-3 )
The number of particles with diameter between Dp and Dp + dDp in a cm3
fn(Dp) dDp (particles cm-3/m)
The total number of particles, N:
N = fn(Dp) dDp (particles cm-3 )
Surface Area Distribution Function
Surface Area Distribution Function
The surface area of particles in a size range per cm3 of air
fs(Dp)dDp = Dp2 fn(Dp ) (m2 m-1
cm-3
The total surface area of the particles, S, is given by the integral over all diameters:
S = fs(Dp) dDp = Dp2 fn (Dp) dDp (m2 cm-3)
The surface area of particles in a size range per cm3 of air
fs(Dp)dDp = Dp2 fn(Dp ) (m2 m-1
cm-3
The total surface area of the particles, S, is given by the integral over all diameters:
S = fs(Dp) dDp = Dp2 fn (Dp) dDp (m2 cm-3)
Volume Distribution Function
Volume Distribution Function
The Volume distribution function can be defined
fv (Dp) dDp = {/6} Dp3 fn (Dp) (m3 m-1 cm-3 )
So the total volume occupied can be written
V = fv(Dp) dDp = /6 Dp3 fn(Dp) dDp (m3 cm-
3)
The Volume distribution function can be defined
fv (Dp) dDp = {/6} Dp3 fn (Dp) (m3 m-1 cm-3 )
So the total volume occupied can be written
V = fv(Dp) dDp = /6 Dp3 fn(Dp) dDp (m3 cm-
3)
Log NormalLog 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.
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.
8
Aerosol particle size distribution
∞
=0
drdr
dNNtot
∞
⋅=0
24 drdr
dNrStot π
∞
=0
drdr
dNN tot
∞
⋅=0
3
3
4dr
dr
dNrVtot π
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
TYPICAL U.S. AEROSOL SIZE DISTRIBUTIONS
Freshurban
Agedurban
rural
remote
Warneck [1999]
SAMPLE AEROSOL SIZE DISTRIBUTION (MARINE AIR)
SAMPLE AEROSOL SIZE DISTRIBUTION (MARINE AIR)
Seasalt
Sulfate(natural)
Toronto (1997-99)Egbert (1994-99)
Abbotsford (1994-95)
Quaker City OH (1999)
Arendstville PA (1999)
Atlanta (1999)Yorkville (1999)Mexico City - Pedregal (1997)
Los Angeles (1995-96)
Fresno (1988-89)
Kern Wildlife Refuge (1988-89)
Sulfate
Nitrate
Ammonium
Black carbon
Organic carbon
Soil
Other
12.3 ug m-38.9 ug m-3
7.8 ug m-3
12.4 ug m-3
10.4 ug m-3
19.2 ug m-314.7 ug m-3
55.4 ug m-3
30.3 ug m-3
23.3 ug m-3
39.2 ug m-3
Washington DC (1996-99)
14.5 ug m-3
Colorado Plateau (1996-99)3.0 ug m-3
Mexico City - Netzahualcoyotl (1997)
24.6 ug m-3
Esther (1995-99)
St. Andrews (1994-97)5.3 ug m-3
4.6 ug m-3
COMPOSITION OF PM2.5 (NARSTO PM ASSESSMENT)
COMPOSITION OF PM2.5 (NARSTO PM ASSESSMENT)
Light ExtinctionLight Extinction
I/I = e(-bX)I/I = e(-bX)
I0 I
absorption
scattering
scattering
X
Intensity
Extinction Coefficientb (in a few more slides)
EPA REGIONAL HAZE RULE: FEDERAL CLASS I AREAS TO RETURN TO “NATURAL” VISIBILITY LEVELS BY 2064EPA REGIONAL HAZE RULE: FEDERAL CLASS I AREAS
TO RETURN TO “NATURAL” VISIBILITY LEVELS BY 2064
Acadia National Park
clean day moderately polluted day
http://www.hazecam.net/
…will require essentially total elimination of anthropogenic aerosols!
Atmospheric VisibilityAtmospheric Visibility (absorption & scattering)
Atmospheric VisibilityAtmospheric Visibility (absorption & scattering)
1.Residual2.Scattered
away3.Scattered
into4.Airlight
1.Residual2.Scattered
away3.Scattered
into4.Airlight
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)
Extinction Coefficient
VisibilityVisibility
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.
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 albedoSingle scattering albedo
is a measure of the fraction of aerosol extinction caused by scattering:
= bsp/(bsp + bap)
is a measure of the fraction of aerosol extinction caused by scattering:
= bsp/(bsp + bap)
Optical Properties of Small Particles
Optical Properties of Small Particlesm = n + ik
m = complex index of refractionn = scattering (real part)k = absorption (imaginary part)
m = n + ikm = complex index of refractionn = 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
Refractive indicies of aerosol particles at = 589 nm
m = n + ik
Substance n kWater 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 = Sm
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)
Scattering Cross Section
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, , 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
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
ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, 1995-2000
NARSTO PM Assessment, 2003
ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, 1995-2000
NARSTO PM Assessment, 2003
PM10 (particles > 10 m) PM2.5 (particles > 2.5 m)
Red circles indicate violations of national air quality standard:50 g m-3 for PM10 15 g m-3 for PM2.5
AEROSOL OBSERVATIONS FROM SPACE
AEROSOL OBSERVATIONS FROM SPACE
Biomass fire haze in central America (4/30/03)
Fire locationsin red
Modis.gsfc.nasa.gov
BLACK CARBON EMISSIONSBLACK CARBON EMISSIONS
Chin et al. [2000]
DIESEL
DOMESTICCOAL BURNING
BIOMASSBURNING
RADIATIVE FORCING OF CLIMATE 1750-PRESENT
RADIATIVE FORCING OF CLIMATE 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)
IPCC [2001]
ASIAN DUST INFLUENCE IN UNITED STATESDust observations from U.S. IMPROVE
network
ASIAN DUST INFLUENCE IN UNITED STATESDust observations from U.S. IMPROVE
networkApril 16, 2001Asian dust in western U.S.
April 22, 2001Asian dust in southeastern U.S.
GlenCanyon, AZ
Clear day April 16, 2001: Asian dust!
0 2 4 6 8g m-3
LONGITUDE
AL
TIT
UD
E (
km)
100E 150E 150W 100W
TRANSPACIFIC TRANSPORT OF ASIAN DUST PLUMES
TRANSPACIFIC TRANSPORT OF ASIAN DUST PLUMES
Subsidenceover western U.S.
Source region(inner Asia)
Asian plumes over Pacific
GEOS-CHEM Longitude cross-section at 40N, 16 April, 2001
0
5
10
ASIA UNITED STATES
T.D. Fairlie, Harvard
Aerosols in the Atmosphere: Abundance and sizeAerosols 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 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
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
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
Summary:Origins of Atmospheric Aerosols
Cloud Condensation Nuclei - CCN
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).
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).
38
Mean Aerosol Optical Depth over INDOEX region from Dec 2001 to May 2003 from MODIS (Ramanathan & Ramana, Environ.
Managers, Dec. 2003).
Mean Aerosol Optical Depth over INDOEX region from Dec 2001 to May 2003 from MODIS (Ramanathan & Ramana, Environ.
Managers, Dec. 2003).
+ RV Ronald Brown
INDOEX
41
Air Flow During INDOEX 1999Air Flow During INDOEX 1999
Longitude40E60E80E20S020N
100ELatitude
Cruise track of R. H. BrownNo of legSHmTNHcXNHmTNHcT-BNHalcT-BengNHcT-W
1
1232Longitude40E60E80E
20S020N
100ELatitude
Cruise track of R. H. BrownNo of legSHmTNHcXNHmTNHcT-BNHalcT-BengNHcT-W
1
1232
42
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.)
Summary of Aerosol Physics
Summary of Aerosol Physics
How big are atmospheric particles depends on which effect interests you. CCN – number (r < 0.1 m) Radiative transfer & health – surface area
(0.1 < r < 1.0 m) Biogeochemical cycles – mass (r > 0.5 m).
Composition varies with size. Single scattering albedo and visibility
How big are atmospheric particles depends on which effect interests you. CCN – number (r < 0.1 m) Radiative transfer & health – surface area
(0.1 < r < 1.0 m) Biogeochemical cycles – mass (r > 0.5 m).
Composition varies with size. Single scattering albedo and visibility
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