Uncertainties and Frontiers in 

Aerosol Research

Dorothy Koch

Department of Energy

Climate and Environmental Sciences Division

Aerosol Metrology for Climate Workshop March 14, 2011 NIST, MD
Outline

• Introduction: aerosol-climate effects

• 7 “hottest” research areas

• Agency activities
Kiehl (2007) 

showed that AR4 

climate models 

have smaller 

forcing if climate 

more sensitive...

Anthropogenic aerosols have cooled 

the climate, by some amount...

Amount of warming for doubled CO2
Model sensitivity

And most of the 

forcing 

difference is 

from aerosols
Aerosol uncertainty: about 1 Wm-2
Aerosol Climate Effects

1. “Direct” effects: scatter, absorb incoming 

solar radiation. Sulfate, nitrate, organic 

carbon scatter. Black carbon (BC) also 

absorbs. 

4. BC-snow-albedo effect: BC on snow reduces snow 

albedo, promotes melting

2. Cloud micro-physical effects: Aerosol pollution 

increases cloud droplet number, clouds brighter and 

longer-lived.

3. Cloud macro-physical effects: cloud response to 

change in atmospheric thermal structure
Aerosol research approach

Emission Source 

(Mass, Composition, Size)

Test: In situ 

(number, mass, scatter/absorb, 

composition, mixing state)

Remote 

(scatter/absorb)

(Climate)

Model

(Transport, Removal, Mixing, Chemistry, Climate)

Climate Effects

Resource: AeroCom, an international model 

intercomparison activity, since 2003

http://dataipsl.ipsl.jussieu.fr/AEROCOM/
Aerosols from pollution

Combustion of fossil fuel (coal, oil, diesel, 

gasoline), domestic wood burning, forest fires
Natural sources of aerosols

desert dust, sea salt, volcanoes, oceanic/terrestrial 

biological sources, natural fires
Transport

Power

Industry

Biomass 

burning

Residential

Human activity Perspective Aerosol Sources (rather than species)
Transport

Each sector, each 

source, generates 

its own cocktail of 

chemical species. 

Practically, the 

impact of e.g. 

sulfate or black 

carbon on climate 

is meaningless

•Koch et al., JGR, 2007.

•Unger et al., Proc. Natl. Acad. Sci., 2010. 

Aerosol Sector/Source Perspective
“Natural” aerosols: to what extent 

has human activity affected these?

desert dust, oceanic/terrestrial biological sources, 

natural fires, sea salt

#1 What is natural?
Vegetation changes

Pongratz et al., GBC, 2007

#1
Biomass burning 

Emissions for AR5: 

Substantial high-latitude burning

Land clearing implies burning??

#1
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

– What is natural: how have dust, burning, bio-sources 

changes?
Aerosol microphysics: 

size distribution, mixing state, morphology, shape

BC

(Dalzell & Sarofim, 1969)

diameter

Supersaturation_cr

Riemer et 

al., 2009

1. Size 

– optimal size for absorption

– large particles better cloud/ice nuclei

2. Mixing state

– BC absorption increases when mixed with non- absorbing material (Jacobson, 2000)

– Affects particle hygroscopicity, ice nucleation 

potential

3. Shape

– Important for direct effect, ice nucleation 

potential

Models now simulate #1, 2 (nucleation, 

condensation, coagulation) but only beginning to 

compare with field measurements

#2

Volume absorption cross section

diameter
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, 

modeled
Remote vs in-situ measures of aerosols

Emissionsmodel, compare model to obs

✔Model ≅ Obs for in-situ (mass of most stuff)

✖ Model < Obs for aerosol optical depth (AOD), 

absorption aerosol optical depth (AAOD), direct 

radiative effects

Yu et al., ACP, 2006 TOA surface

#3 Closure
Closure: Remote vs in-situ measures of aerosols

Emissionsmodel

✔Model ≅ Obs for in-situ mass 

✖ Model < Obs for absorption 

aerosol optical depth (AAOD)

BC surface concentration, AAOD, 

ratio of model/observed, 15 models 

(Koch et al., ACP, 2009)

Kondo et al., JGR, in press

Biomass burning differences in BC

Asian

North American

#3
Closure: Remote vs in-situ measures of aerosols

Potential solutions (Myhre, Science, 2009):

1. Harmonize model diagnostics to observations 

(“simulators” for e.g. clear-sky, satellite overpass) 

2. Aerosol microphysics in models (increases absorption) 

3. Source deficiencies (biomass burning, Asia)

4. Missing organic aerosols

#3
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, 

modeled

3. Closure (remote vs in-situ)
• Pollution/burning: mostly primary (organic) particles

• Biological: mostly secondary

• Less than 1⁄2 of organics have been identified

Heald et al, GRL, 2010

Comparing forward 

model with MISR 

satellite retrieval 

model needed about 

3x OA source used in 

model

#4 Organic aerosols
Organic aerosols

• Vary in hygroscopicity

• Vary in volatility

• Vary in absorption (“brown carbon”)

• Potentially big impact on aerosol-cloud effects: 

particle number and hygroscopicity (Liu and Wang, 

Env Res Lett, 2010: -1.3±0.4 Wm-2)

• Some global models now have (simple) SOA 

chemistry, beginning to compare with observations

Arola et al., ACP, 2011

#4
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, 

modeled

3. Closure (remote vs in-situ)

4. Organic aerosol sources, load, optical properties, 

hygroscopicity
#5 Aerosol-cloud effects

1. Liquid cloud microphysics: Enhancement of cloud 

condensation nuclei (CCN) and droplet number (“indirect 

effect”) 

2. Ice cloud microphysics

3. “Semi-direct effects”

4. Convective cloud dynamics

Lohmann et al., ACP, 2010
1. Aerosols enhance liquid clouds (cool)

Aerosol activation to a 

cloud droplet: 

depends on its size

and composition

(hygroscopicity)

Refractory particles 

(soot) correlate with CCN

Clarke et al., Science, 2010

#5
2. Aerosols alter ice clouds 

Ice clouds (cirrus) are warming. 

Aerosol pollution impact may be to increase or 

decrease ice number 

Soot might act as Ice Nuclei (IN):

1. Laboratory evidence is weak

2. Field evidence suggests role: enhanced soot in 

ice particles

Particle size/shape/morphology matters: Big, large 

aspect ratio is good IN (dust)

#5
3. Intensification of deep convection

Clean: larger 

drops rain 

sooner

Polluted: 

smaller 

droplets, 

precip

delay, 

clouds 

deepen 

and 

intensify

(Rosenfeld et al., 

Science, 2008)

#5
4. Absorbing aerosols (AA) alter thermal structure of 

atmosphere

Effect depends on:

Absorption optical depth

Altitude of aerosol

Dynamical and hydrological conditions

Coordinated (field + cloud-scale model + global scale model) studies 

needed

Koch and Del Genio, ACP, 2010

#5
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, modeled

3. Closure (remote vs in-situ)

4. Organic aerosol load, optical properties, 

hygroscopicity

5. a. Warm cloud: validation of aerosol microphysical 

models

b. Ice clouds: role of soot-cloud effects in situ

c. Thermal atmospheric changes: coordinated field- model study
Black carbon mitigation to cool climate: 

BC suspended in atmosphere, deposited on snow are 

warming. Because of short lifetime, climate response to 

reduction is rapid

• IGAC: “Bounding the Role of Black Carbon on 

Climate” (Bond, Fahey, Forster, Doherty, et al.)

• UNEP: “Impacts and mitigation potentials of 

Ozone and Black Carbon” (Shindell, 

Ramanathan, Raes et al.)

• EPA: Report on the climate effects of BC

• Arctic council: task force on how to reduce 

global BC production 

#6
BC-climate uncertainties

• BC is never “pure”, it comes with organic carbon 

(biofuels), SO2 (fossil fuels). Must consider 

impact of sources, not just the BC. (e.g. diesel)

• BC-OC (soot) plays an important CCN (cloudseeding) role which is cooling (e.g. Chen et al., 

2010; Bauer et al., 2010; Koch et al., 2011)

• BC absorption may enhance low-level clouds in 

some regions (e.g. Koch and Del Genio, 2010)

• BC impacts on ice-clouds poorly understood

#6
BC

-climate uncertainties

#6
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, modeled

3. Closure (remote vs in-situ)

4. Organic aerosol load, optical properties, hygroscopicity

5. a. Warm cloud: validation of aerosol microphysical models

b. Ice clouds: role of soot-cloud effects in situ

c. Thermal atmospheric changes: coordinated field-model 

study

6. BC mitigation potentials (how warming is BC + co-emitted 

species?)
Aerosol pollution: complex, uncertain history

Lamarque et al., 

ACP, 2010

Assumed for 

IPCC AR5

However these are very heterogeneous in space-time

1850 2000

#7 Historical climate effects
For example, black carbon emissions

N. America SE Asia

Europe

Bond, et al, Glob. Biogeochem. Cyc., 21, 

GB2018, doi:10.1029/2006GB002840, 2007.

#7
Greenland ice core records 

McConnell et al., 2007

Indicator of North American pollution changes

Coal

Oil, Gas

#7
Historical aerosol-climate effects

Wild et al., Science, 2005

• Dimming/brightening (models 

underestimate)

• Southward shift of ITCZ due to more 

cooling from aerosols in NH (e.g. 

Takemura et al. 2005; Rotstayn et al. 

2000; Zhang et al. 2007; Koch et al., 

2011; Chang et al., 2011), contributed 

to drought over West Africa

• Hydrologic slowing (precipitation 

suppression) even from absorbing 

aerosols (Ming et al., GRL, 2010)

Koch et al., J Clim, in press

Change in precip over 20th century

Surface radiation #7
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, modeled

3. Closure (remote vs in-situ)

4. Organic aerosol load, optical properties, hygroscopicity

5. a. Warm cloud: validation of aerosol microphysical models

b. Ice clouds: role of soot-cloud effects in situ

c. Thermal atmospheric changes: coordinated field-model 

study

6. BC mitigation potentials (how warming is BC + co-emitted 

species?)

7. Detangling aerosol historical impacts on climate
•Laboratory, field, modeling activities

•Milagro field campaign(2006): 

Evolution of pollution from Mexico 

City

•BC-on-snow (Warren, Grenfell, et al)

•Ice core records of aerosols (e.g. 

McConnell, Ross, et al)

Yukon River

Baker Lake

Kugluktuk

Petermann

GITS

NASA-SE

Dye-2 Saddle

South D

Petermann ELA Summit

NASA-E

Thule

SGW-NE

Nar'y an Mar

Noril'sk Vorkuta

Khatanga

Dikson

Tiksi

Pevek

Anady r

Chersky

Uelen

Yakutsk

Magadan

 60° N 

 70° N  80° N 

 90°

 N 

Ice Core records in Greenland, Arctic, Antarctic
• Monitoring sites: http://www.esrl.noaa.gov/gmd/aero/net

Nephelometer, Particle Soot Absorption Photometer, Condensation 

Nuclei Counter

(optical properties, CCN, chemistry, f(RH), Angstrom exponent, size 

distribution)

• Field studies with aircraft and ships

• GFDL model 
E.g. Aircraft: Schwarz et al., GRL, 2010; Koch et al., ACP, 2009

Also in ice cores McConnell et al., 2007

BC measurements: SP2 Particle Soot Photometer

Obs black

Models 

color

North America

Pacific Ocean

Obs color

Models 

black/gray
ARM sites, campaigns

ARM provides long-term measure of cloud and 

aerosol properties 

(http://www.arm.gov/instruments)

Aerosol observing system: aerosol absorption, 

concentration, scattering, backscattered 

radiation, CCN, hygroscopic growth, inorganic 

composition, number concentration, size 

distribution

Cimel sunphotometer, Aerosol profiles (aircraft), 

Radiometers, Lidar, Spectrometer

PNNL:

Mirage modal 

aerosol 

microphysical 

scheme, in WRF 

and CAM5 

models

Aerosol 

microphysical 

module testbed

Organic aerosol 

research
• AERONET network of sun 

photometers (15+ years of AOD, 

Angstrom parameter retrievals)

http://aeronet.gsfc.nasa.gov/new_web/aerosols.html

• MODIS, MISR (AOD)

• Calipso (infrared radiometer - vertical distribution)

• Field campaigns

• Models: GEOS-5, GISS
Concentration, Deposition 

Monitoring Networks

IMPROVE: US air concentrations 

for sulfate, nitrate, dust, organic, 

elemental carbon

NADP: sulfur and nitrate 

deposition since 1980

EMEP: European air 

concentrations, deposition for 

sulfate, nitrate since 1980

EANET: Asian acid deposition, 

since 1998

Koch et al., J Clim, in press

dash: model
Aerosol uncertainties: about 1 Wm-2

1. Aerosol sources, histories

- What is natural: how have dust, burning, bio-sources 

changes?

2. Aerosol size, mixing state, shape; Observed, modeled

3. Closure (remote vs in-situ)

4. Organic aerosol load, optical properties, hygroscopicity

5. a. Warm cloud: validation of aerosol microphysical models

b. Ice clouds: role of soot-cloud effects in situ

c. Thermal atmospheric changes: coordinated field-model 

study

6. BC mitigation potentials (how warming is BC + co-emitted 

species?)

7. Detangling aerosol historical impacts on climate