AerosolsAerosols

Dr. Martin LeachNovember 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 CommentsAerosols: 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)

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 EffectsAerosol Effects

Climate Weather Visibility Health Effects

Climate Weather Visibility Health Effects

Clouds?

Natural Sources and Estimates of Global Emissions of Atmospheric

Aerosols 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 1500

Sea Salt 1000-10000 1300

Botanical Debris 26-80 50

Volcanoes 4-10000 30

Forest Fires 3-150 20

Gas conversion 100-260 180

Photochem 40-200 60

Total 2200-24000 3100

Anthropogenic Sources of AerosolsAnthropogenic 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: 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.

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 AerosolNatural 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

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, 1991Mount Pinatubo, 1991

Urban AerosolUrban Aerosol

Dominated by anthropogenic sources Three Modes

– Nuclei Aitken– Accumulation Large– Coarse Giant

Dominated by anthropogenic sources Three Modes

– Nuclei Aitken– Accumulation Large– Coarse Giant

What is meant by the size of an aerosol? What does a size distribution mean?

ORIGIN OF THE ATMOSPHERIC AEROSOL

ORIGIN OF THE ATMOSPHERIC AEROSOL

Soil dustSea salt

Aerosol:Size range: 0.001 m (molecular cluster) to 100 m (small raindrop)

Environmental importance: health (respiration), visibility, radiative balance,cloud formation, heterogeneous reactions, delivery of nutrients…

AEROSOL NUCLEATIONAEROSOL NUCLEATION

# molecules 1 2 3 4

G

cluster sizeCritical cluster size

Surface tension effect

Thermo driving force

Atmospheric AerosolsAtmospheric AerosolsAtmospheric AerosolsAtmospheric Aerosols

Question?Question?

Considering the Urban Aerosol, where are most of the particles? Where is the most mass?

How many 0.01 m particles are necessary to have the same mass as one 1m particles?

Considering the Urban Aerosol, where are most of the particles? Where is the most mass?

How many 0.01 m particles are necessary to have the same mass as one 1m particles?

Urban Aerosol Size DistributionUrban Aerosol Size Distribution

Nuclei Mode (<0.1m)Nuclei Mode (<0.1m)

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

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)

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.

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)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.

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 AerosolDynamic Processes of Atmospheric Aerosol

Formation– Gas to particle conversion– Photochemical processes

Growth– Coagulation, condensation, evaporation

Removal– Settling– Deposition– Rainout, washout

Formation– Gas to particle conversion– Photochemical processes

Growth– Coagulation, condensation, evaporation

Removal– Settling– Deposition– Rainout, washout

Global Effects of AerosolsGlobal 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

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?"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 Distributions

Number 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 FunctionSurface 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 FunctionVolume 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.

27

Aerosol particle size distribution

0

drdr

dNNtot

0

24 drdr

dNrStot

0

drdr

dNN tot

0

3

3

4dr

dr

dNrVtot

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 2064

EPA 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

Seinfeld and Pandis, 1998

Atmospheric VisibilityAtmospheric Visibility (absorption & scattering)

Atmospheric VisibilityAtmospheric Visibility (absorption & scattering)

1.Residual

2.Scattered away

3.Scattered into

4.Airlight

1.Residual

2.Scattered away

3.Scattered into

4.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 ParticlesOptical Properties of Small Particles

m = n + ikm = complex index of refraction

n = scattering (real part)

k = absorption (imaginary part)

m = n + ikm = 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

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 SurrogateExtinction 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,

1995-2000NARSTO PM Assessment, 2003

ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES,

1995-2000NARSTO 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 network

April 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

Summary:Origins of Atmospheric AerosolsSummary: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

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 - CCNCloud 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).

56

INDOEX, 1999INDOEX, 1999INDOEX: Indian Ocean Experiment

57

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

58

INDOEXINDOEX

From Ramanathan 2001 0 to 3 km layer

59

NOAA R/V Ronald BrownNOAA R/V Ronald Brown

60

Air Flow During INDOEX 1999Air Flow During INDOEX 1999

Longitude

40E 6 0E 80E

20 S

0

20N

10 0E

Latit

ude

Cruise trac k of R. H. Brown

No of leg

1

1

2

3

2

Longitude

40E 6 0E 80E

20 S

0

20N

10 0E

Latit

ude

Cruise trac k of R. H. Brown

No of leg

1

1

2

3

2

61

                                                                     

            

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 PhysicsSummary 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