AerosolsAerosols

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)

Aerosols: VisibilityAerosols: Visibility

Washington, DC

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!

Radiation and fine particlesRadiation and fine particles

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 OPTICAL DEPTH (GLOBAL MODEL)

AEROSOL OPTICAL DEPTH (GLOBAL MODEL)

Annual mean

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).

37

INDOEX, 1999INDOEX, 1999

INDOEX: Indian Ocean Experiment

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

39

INDOEXINDOEX

From Ramanathan 2001 0 to 3 km layer

40

NOAA R/V Ronald BrownNOAA R/V Ronald Brown

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