VARIATION OF

SOL OPTICAL PROPERTIES

RADIATIVE IMPLICATIONS

Stephen E. Schwartz

Upton, Long Island, NY

rosol Metrology for Climate Workshop

Gaithersburg, MD

March 14 – 15, 2011

www.ecd.bnl.gov/steve
VARIATION OF

SOL OPTICAL PROPERTIES

RADIATIVE IMPLICATIONS

Stephen E. Schwartz

Upton, Long Island, NY

rosol Metrology for Climate Workshop

Gaithersburg, MD

March 14 – 15, 2011

www.ecd.bnl.gov/steve
OVERVIEW

Aerosol processes that must be understoodClasses of aerosol properties and processesRelation to climate changeAerosol optical propertiesAerosol radiative influencesSome implications for climate change
DIRECT RADIATIVE FORCING DUE TO ANTHROPOGENIC SULFA

Geophysics

 AerosolMicrophysics

 Column BurdenAtmospheric Chemistry

Aerosol Optical Depth

∆F FT A R f Q Y R T

SO SO SO SO = − −⋅ ⋅



 − −

−

- −

12

RH

MW

MW c s

SO

S

2 2 1 1 4

2 2 4

2 4

2

4

( )( ) ( ) βα

θ 2

∆

FR is the area-average shortwave radiative forcing due to the aerosol, W m-2

FT is the solar constant, W m-2

Ac is the fractional cloud cover

T is the fraction of incident light transmitted by the atmosphere above the aerosol

Rs is the albedo of the underlying surface

β is upward fraction of the radiation scattered by the aerosol,

αSO4

2− is the scattering efficiency of sulfate and associated cations at a reference low relative hum

ƒ(RH) accounts for the relative increase in scattering due to relative humidity

QSO2 is the source strength of anthropogenic SO

2 g S yr-1

YSO4

2− is the fractional yield of emitted SO

2 that reacts to produce sulfate aerosol

MW is the molecular weight

θSO4

2− is the sulfate lifetime in the atmosphere, yr

A is the area of the geographical region under consideration, m

2

Charlson, Schwartz, Hales, Cess, Coakley, Hansen 

& 
AEROSOL DIRECT SHORTWAVE FOR

Global Average for Nonabsorbing Aerosol

Cloud FraSurface RMean Ups

Aerosol O

AtmospheSolar Con

Change in 

∆F FT A R =− − − 1

2

0 1 1 2 2 ( )( ) c βτ

Mass Scat

Mass ConcLight Scatt

τα σ = = ∫ ∫ Cdz dz sp

Global Average for Absorbing Aerosol

∆F FT A R

R

R

=− − − −

−



−



12

11 1

2

1

1

0

2 2

2 ( )( ) ( ) ( )

c βτω

ω

βω

Single Scattering Alb
OSOL PROCESSES THAT MUST BE

TOOD AND REPRESENTED IN MODELS

water

uptake

rsor emissions

ulation

evaporation

new particle

formation

subcloudscavenging

aqueouschemistr

y

ry

dry

deposition

activation

diffusion

ensatio

n

oration

scavengin

g

primary emission

s

autoconversion 

ering

)

orption

Radiation transfer in clouds

Modified from Ghan and Schwartz, Bull. Amer. Meterol. Soc., 2007
APPROACH TO DETERMINE

AEROSOL FORCING

Numerical simulation of physical processes

Isomorphism of processes to computer code

Modeling aerosol processes requires understanding these processes,

developing and testing their numerical representations, and 

incorporating these representations in global scale models.

water

uptake

precursor emissions

coagulation

evaporation

new particle

formation

subcloud

scavenging

aqueous

chemistry

surface

chemistry

dry

deposition

activation

diffusion

condensation

evaporation

oxidation

scavenging

primary emissions

autoconversion light scattering

ƒ(RH)

and absorption

Radiation transfer in clouds
APPROACH TO MODELING AEROSOL RADIATIVE EFFECTS

Transport

Chemistry

Microphysics

Removal

Model Emissions

Size-distributed

Composition

Optical

Properties

Radiative

Effects

Optics

Model

Radiation

Transfer

Model
APPROACH TO MODELING AEROSOL RADIATIVE EFFECTS

Transport

Chemistry

Microphysics

Removal

Model Emissions

Size-distributed

Composition

Optical

Properties

Radiative

Effects

Optics

Model

Radiation

Transfer

Model
LASSES OF AEROSOL

ERTIES AND PROCESSES
LASSES OF AEROSOL

ERTIES AND PROCESSES

sential to distinguish . . .

 chemical and microphysical properties.

 optical properties. radiative influences. radiative forcing.
ROSOL CHEMICAL AND

OPHYSICAL PROPERTIES

ted chemical composition: dry, or at reference

midity (RH).

 on RH: hygroscopic growth

lid; homogeneous spheres? crystalline vs

 . . .

xternal mixture:

rnally mixed: All particles have same composition.

ndent internal mixture: Particles of a given size

me composition.

mixture: Particles of same size may exhibit different

tion.
GLE-PARTICLE OPTICAL

PROPERTIES

aginary components of index of refraction

us, vs spherically symmetric, vs inhomogeneous

ng theory allows calculation of angular dependent

for homogeneous or for radially dependent,

 symmetric refractive index (e.g. spherical shells).

al particles: Approximate methods for spheroids,

ical methods.

Mishchenko, Hovenier, Travis, Light Scattering by

cal Particles, Academic Press, 2000.
GLE-PARTICLE OPTICAL

PROPERTIES

aginary components of index of refraction

us, vs spherically symmetric, vs inhomogeneous

ng theory allows calculation of angular dependent

for homogeneous or for radially dependent,

 symmetric refractive index (e.g. spherical shells).

al particles: Approximate methods for spheroids,

ical methods.

Mishchenko, Hovenier, Travis, Light Scattering by

cal Particles, Academic Press, 2000.
SOL OPTICAL PROPERTIES

erosol optical properties for calculating radiative

are:

ng coeff 

tion coeff

} <=

>

{Extinction coeff Single scat albed

o

}

parameter: Average of cosine of scattering angle.

ce on wavelength.

ce on relative humidity.

t:

ction – Intensity of scattering as function of

g angle.

on – Spherical vs crystal, pertinent to source.
SOL OPTICAL PROPERTIES

erosol optical properties for calculating radiative

are:

ng coeff 

tion coeff

} <=

>

{Extinction coeff Single scat albed

o

}

parameter: Average of cosine of scattering angle.

ce on wavelength.

ce on relative humidity.

t:

ction – Intensity of scattering as function of

g angle.

on – Spherical vs crystal, pertinent to source.
SOL OPTICAL PROPERTIES

erosol optical properties for calculating radiative

are:

ng coeff 

tion coeff} <=> {Extinction coeff Single scat co-albedo

}

parameter: Average of cosine of scattering angle.

ce on wavelength.

ce on relative humidity.

t:

ction – Intensity of scattering as function of

g angle.

on – Spherical vs crystal, pertinent to source.
DIATIVE INFLUENCES

radiative flux due to of the presence of aerosol.

cification of the radiative situation: solar zenith

ace albedo, height of aerosol in atmosphere,

vs all-sky, . . .

ptical depth: 

∆ optical depth due to aerosol.

ormal solar irradiance.downwelling shortwave irradiance at surface.

ed downwelling shortwave irradiance at surface.

ng shortwave irradiance at top of atmosphere (TOA).

tantaneous and local; might be temporal and/or

rage.
RADIATIVE FORCING

fference; a change in radiative flux between two

onditions.

y the total aerosol: Flux with aerosol minus flux

aerosol.

genic aerosol forcing: Flux with anthropogenic

minus flux without.

tantaneous, local; might be temporal and/or spatial

nstantaneous, local; might be temporal and/or

rage; cloud-free or all-sky; might be surface or
TION TO CLIMATE CHANGE

ER THE INDUSTRIAL ERA
GLOBAL ENERGY BALANCE

nnual average energy fluxes in watts per square meter

Schwartz, 1996, modified from Ramanathan, 1987
GLOBAL ENERGY BALANCE

nnual average energy fluxes in watts per square meter

Schwartz, 1996, modified from Ramanathan, 1987
GLOBAL ENERGY BALANCE

nnual average energy fluxes in watts per square meter

Schwartz, 1996, modified from Ramanathan, 1987
GLOBAL ENERGY BALANCE

nnual average energy fluxes in watts per square meter

Schwartz, 1996, modified from Ramanathan, 1987
GLOBAL ENERGY BALANCE

nnual average energy fluxes in watts per square meter

Schwartz, 1996, modified from Ramanathan, 1987
CLIMATE FORCINGS OVER THE

INDUSTRIAL PERIOD

Extracted from IPCC AR4 (2007)

-2 -1 0 1 2 3

Forcing, W m-2

CO2 CH4

CFCs

N2O

Long Lived

Greenhouse Gases

Tropospheric

Aerosols

Direct

Effect

Cloud Albedo

Effect

Negative aerosol forcing substantially offsets GHG forcing.

Aerosol forcing is highly uncertain. 
CLIMATE FORCINGS OVER THE

INDUSTRIAL PERIOD

Extracted from IPCC AR4 (2007)

-2 -1 0 1 2 3

Forcing, W m-2

CO2 CH4

CFCs

N2O

Long Lived

Greenhouse Gases

Tropospheric

Aerosols

Direct

Effect

Cloud Albedo

Effect

Total Forcing

Total forcing includes other anthropogenic and natural (solar) forcings.

Forcing by tropospheric ozone, ~0.35 W m-2, is the greatest of these.

Uncertainty in aerosol forcing dominates uncertainty in total forcing. 
AEROSOL OPTICAL

PROPERTIES
HT SCATTERING EFFICIENCY

Dependence on particle radius

Ammonium Sulfate, 550 nm

0.1

1

Radius, μm

5

Data of Ouimette and Flagan, 1982
LIGHT SCATTERING CROSS SE

DEPENDENCE ON PARTICLE SIZE, COMP
CATTERING BY AEROSOL PARTICLES

e of angular distribution of scattering (phase function) on

particle size

-1

01

-1

0 1

r = 0.01 μm

-1

01

-1

0 1

r = 0.02 μm

-1

01

-1

0 1

r = 0.04 μm

-2

02

-2

0

2

r = 0.06 μm

-3

03

-3

0

3

r = 0.10 μm

-4

04

-4

0

4

r = 0.12 μm

-5

05

-5

0

5

r = 0.14 μm

-5

05

-5

0

5

r = 0.16 μm

Calculations by R. Wagener, BNL

s scatter increasingly in the forward direction.
EROSOL RADIATIVE

INFLUENCES
UPSCATTER FRACTION

Dependence on solar zenith angle, particle radius, and refractive index

= cos(SZA)
Global average, cloud-free sky - Scattering aerosol

120

100

80

60

40

20

0

-20

– (Forcing per OD), W m-2

3 4 5 6 7 8 9

0.1

2 3 4 5 6 7 8 9

1

Radius, μm

Global Average Forcing / Optical Depth

(Surface Reflectance = 0.15)

(n = 1.40)

ing per optical depth depends rather strongly on particle size.

ing accuracy 0.5 W m-2 requires optical depth accuracy 0.005 - 0.01

01 - 0.02 for 60% cloud cover).
Global average, cloud-free sky - Absorbing aerosol

120

100

80

60

40

20

0

-20

– (Forcing per OD), W m-2

3 4 5 6 7 8 9

0.1

2 3 4 5 6 7 8 9

1

Radius, μm

Global Average Forcing / Optical Depth

k = 0 

0.002 

0.004 

0.006

0.008

0.01

0.02

0.03

0.04

0.05

(Surface Reflectance = 0.15)

(n = 1.40 - ik)

ing per optical depth depends rather strongly on particle size –

d also rather strongly on aerosol absorption.
ESTIMATES OF AEROSOL DIRECT F

By linear model and by radiation transfer modeli

-30-20-10

0

0.0 0.1 0.2 0.3

Aerosol Optical Thickness at 550 nm

-60-40-20

0

Radiation Transfer Model

Linear  Model

Instantaneous Cloud-Free Forcing, W m-2

0.03 Global Sulfate Mid-continent U.S. 

Global average sulfate optical thickness is 0.03: 1 W m-2 cooli

In continental U. S. typical aerosol optical thickness is 0.1: 3 
OL OPTICAL DEPTH AT ARM SGP

of daily average 500 nm AOD in North Central Oklahoma

Michalsky, Denn, Flynn, Hodges, Kiedron, Koontz, Schlemmer, Schwartz, JGR, 2010

 LOWESS (locally weighted scatterplot smoothing) fit.
10

8

6

4

2

0

Concentration, μg m-3

2000 2001 2002 2003 2004 2005 2006 2007

Ammonium

 Chloride

 Nitrate

 Sulfate

 Sulfate, smoothed 

AEROSOL COMPOSITION AT ARM SGP

Seven years of daily average composition in North Central Oklahoma

Measurements of P. Quinn, NOAA, PMEL

Black curve is LOWESS (locally weighted scatterplot smoothing) fit.

Note summertime peak of sulfate.
AEROSOL OPTICAL DEPTH AND

COMPOSITION AT ARM SGP

Daily average optical depth and composition in North Central Oklahoma

10

8

6

4

2

0

Concentration, μg m-3

2000 2001 2002 2003 2004 2005 2006 2007

Ammonium

 Chloride

 Nitrate

 Sulfate

 Sulfate, smoothed 

Daily-average aerosol optical depth at 500 nm

Summertime sulfate peak coincides with maximum optical depth.
AEROSOL COMPOSITION AT ARM SGP

Six days of 30-minute average composition in North Central Oklahoma

7

6

5

4

3

2

1

0

12/3/10 00:00

12/4/10 00:00

12/5/10 00:00

12/6/10 00:00

12/7/10 00:00

12/8/10 00:00

UTC Time

 Ammonium

 Organic

 Sulfate

 Nitrate

 Chloride

Concentration, μg m-3

 Preliminary Data

• First data from new aerosol mass spectrometer installed at SGP.

• Continuous data; for particles with diameter ≤ 0.5 μm.

• Note high nitrate compared to sulfate; substantial organic component.
OSURE EXPERIMENTS

Cubic meter experiments”

asured optical properties with those modeled based

ed aerosol composition and size distribution.

Can be done locally. Requiretime series of composition,size distribution, and opticalproperties.

lumn closure experiments”

tical depth, direct beam irradiance, diffuse

 total and/or spectral, at surface or ƒ(z). Require

ation of aerosol optical properties vertically.
OPTICAL CLOSURE EXPERIMENT

aft measurements, south of Japan, ACE-Asia, 2001

Wang et al., JGR 02

alculated by Mie theory for measured composition; index of

om Bruggeman mixing rule.
OPTICAL CLOSURE EXPERIMENT

aft measurements, south of Japan, ACE-Asia, 2001

Wang et al., JGR 02

inction coefficient and wavelength dependence agreed with

 within fairly broad uncertainty limits.
MEASURED AND MODELED

SCATTERING COEFFICIENT

North Central Oklahoma, May 2003

Submicrometer aerosol, low RH

1:1

Modified from Andrews et al. JGR, 2006
MEASURED AND MODELED

ASYMMETRY PARAMETER

North Central Oklahoma, May 2003

ASP, Neph dry TDMA, 85% RH; Neph, Ambient

AERONET, ambient column

Andrews et al. JGR, 2006
AINTY IN AEROSOL DIRECT FORCING

om typical uncertainty in measurements of input variables

Top of

Atmosphere

Surface

optical depth

0.03 single scat albedo

0.05 asym param

0.05 UV-vis sfc albedo

0.05 SW IR sfc albedo

 Western Pacific Southern Gt Plains North Slope Alaska

McComiskey, Schwartz, Schmid, Guan, Lewis, Ricchiazzi,& Ogren, JGR, 2008

enote uncertainties in 24-hr average forcing at equinox

m uncertainties in the individual parameters.

otes resultant uncertainty in forcing.

are substantial in context of forcings over industrial period.
OME IMPLICATIONS

R CLIMATE CHANGE
PIRICAL DETERMINATION OF

RTH’S CLIMATE SENSITIVITY

Dependence on aerosol forcing

T

F N

− −

∆

obs

ot aer 

)

86420

Sensitivity, K/(W m-2)

-2.5 -2.0 -1.5 -1.0 -0.5

Aerosol Forcing, W m-2

3025201510

50

Climate Sensitivity ∆T2×, K

 ∆

Tobs = 0.8 K

Ftot - Faer = 2.9 W m-2N = 0.4 W m-2

IPCC AR4 sensitivity

estimate and uncertainty

IPCC AR4 aerosol forcing

estimate and uncertainty

ue mainly to LLGHGs.

 determined from rate of increase of ocean heat content.

d for uncertainty range of aerosol forcing from IPCC AR4.

reases strongly for aerosol forcing > 2 W m-2.
WABLE FUTURE CO

2 EMISSIONS

ce on climate sensitivity and acceptable increase in

temperature relative to preindustrial

4.03.53.02.52.01.51.0

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

CO

2 Doubling Temperature 

D

T

2 , K

0.6 0.8 1.0 1.2

Equilibrium Climate Sensitivity, K/(W m-2)

Incremental GHGs Present

¥

Widely acceptedmaximum TD

IPCC AR4

Best

estimate

 K,

y 

∆

T2

× is 3 K, no further emissions!
WABLE FUTURE CO

2 EMISSIONS

ce on climate sensitivity and acceptable increase in

temperature relative to preindustrial

4.03.53.02.52.01.51.0

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

CO

2 Doubling Temperature 

D

T

2 , K

0.6 0.8 1.0 1.2

Equilibrium Climate Sensitivity, K/(W m-2)

Incremental GHGs Present

¥

Widely acceptedmaximum TD

IPCC AR4

"Likely" range ~1

s

Best

estimate

 K,

y of future emissions depends on climate sensitivity.
WABLE FUTURE CO

2 EMISSIONS

ce on climate sensitivity and acceptable increase in

temperature relative to preindustrial

4.03.53.02.52.01.51.0

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

CO

2 Doubling Temperature 

D

T

2 , K

0.6 0.8 1.0 1.2

Equilibrium Climate Sensitivity, K/(W m-2)

Incremental GHGs Present

¥

Widely acceptedmaximum TD

IPCC AR4

"Likely" range ~1

s

Best

estimate

Further emissionsallowed 

 K and/or sensitivity 

∆

T2

× < 3 K, further emissions are

hout exceeding 

∆

Tmax.
WABLE FUTURE CO

2 EMISSIONS

ce on climate sensitivity and acceptable increase in

temperature relative to preindustrial

4.03.53.02.52.01.51.0

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

CO

2 Doubling Temperature 

D

T

2 , K

0.6 0.8 1.0 1.2

Equilibrium Climate Sensitivity, K/(W m-2)

Incremental GHGs Present

¥

Widely acceptedmaximum TD

IPCC AR4

"Likely" range ~1

s

Best

estimate

Further emissionsallowed 

Incremental GHGsalready exceedallowable limit 

 K and/or sensitivity 

∆

T2

× < 3 K, further emissions are

hout exceeding 

∆

Tmax.

 K and/or sensitivity 

∆

T2

× > 3 K, committed temperature

ady exceeds 

∆

Tmax.
WABLE FUTURE CO

2 EMISSIONS

ce on climate sensitivity and acceptable increase in

temperature relative to preindustrial

4.0

3.5

3.0

2.5

2.0

1.51.0

2.01.5 2.5 3.0 3.5 4.0 4.5 5.0

CO

2 Doubling Temperature 

D

T

2 , K

0.80.6 1.0 1.2

Equilibrium Climate Sensitivity, K/(W m-2)

–400

–300

 –200 

–100

 0 

 200

400

600

800

Pg C

¥

CO2 E

Widely acceptedmaximum TD

or amount by which present GHGs exceed

e threshold depend on climate sensitivity and

∆

Tmax.

ure emissions
WABLE FUTURE CO

2 EMISSIONS

ce on climate sensitivity and acceptable increase in

temperature relative to preindustrial

4.0

3.5

3.0

2.5

2.0

1.51.0

2.01.5 2.5 3.0 3.5 4.0 4.5 5.0

CO

2 Doubling Temperature 

D

T

2 , K

0.80.6 1.0 1.2

Equilibrium Climate Sensitivity, K/(W m-2)

–400

–300

 –200 

–100

 0 

 200

400

600

800

Pg C

 80

 60

 40

 20 

 0 

–10

–20

 –30 

 –40 

years

¥

CO2 E

tCO2

Widely acceptedmaximum TD

 K . . .

y 

∆

T2

× is 3 K, no more emissions.

y 

∆

T2

× is 2 K, ~ .

y 

∆

T2

× is 4.5 K, threshold is exceeded by ~30 years.

30 more years of emissions at present rate
 TEMPERATURE RESPONSE TO

MOVAL OF AEROSOL SOURCES

Experiment with ECHAM-5 GCM

6 yr

63% 100%

Constant GHGs and aerosols GHGs only; no aerosol

Modified from Brasseur and Roeckner, GRL, 2005

moved from atmosphere in days to weeks.

ture rapidly increases following removal of aerosol forcing.

for climate response to step-change in forcing is about 6 years.
 TEMPERATURE RESPONSE TO

MOVAL OF AEROSOL SOURCES

Experiment with ECHAM-5 GCM

6 yr

63% 100%

Constant GHGs and aerosols GHGs only; no aerosol

Modified from Brasseur and Roeckner, GRL, 2005

moved from atmosphere in days to weeks.

ture rapidly increases following removal of aerosol forcing.

for climate response to step-change in forcing is about 6 years.