Aerosol Microphysics: Plans for GEOS-CHEM

Peter J. AdamsCarnegie Mellon University

Civil and Environmental Engineering

GEOS-CHEM Activities at CMU

Comprehensive aerosol microphysics scheme implemented in GISS GCM

For details, see following publications Adams, P. J., and J. H. Seinfeld, Predicting global aerosol

size distributions in general circulation models, J. Geophys. Res., 10.1029/2001JD001010, 2002.

Adams, P. J., and J. H. Seinfeld, Disproportionate impact of particulate emissions on global cloud condensation nuclei concentrations, Geophysical Research Letters, 10.1029/2002GL016303, 2003.

Same aerosol microphysics will be implemented in GEOS-CHEM – this process is just beginning

Aerosol Activation

Particle Size

Num

ber

Aerosol activation is acompetition betweensolute and surfacetension effects

3

64ln

pw

ws

pw

ww

D

Mn

DRT

MS

Only largest particlesactivate to becomecloud droplets: Dp>80 nmS=0.2%

Surface tension

term

Solute (Raoult)

term

Mechanistic vs. Empirical Models

Sulfate Mass (g m-3)C

loud

Dro

plet

s (c

m-3)

Boucher & Lohmann, 1995Particle Size

Num

ber

Mechanistic: number of cloud drops depends on number of particles large enough to activate

Empirical: number of cloud drops correlated with sulfate mass based on observations

Kiehl et al. [2000]

I: Martin et al. [1994]: -0.68 W/m2

II: Martin et al. with background CCN: -0.40 W/m2

III: Jones et al. [1994]: -0.80 W/m2

IV: Boucher and Lohmann [1995]: -1.78 W/m2

“It is argued that a less empirical and more physically based approach is required…”

Clo

ud D

ropl

ets

(cm

-3)Sulfate Mass (g m-3)

Two moments of the size

distribution (mass and number)

are tracked for each size bin. The average size of particles in a

given section is not constant with

time Two-moment method conserves both mass and number

precisely Prevents numerical diffusion present in single-moment

methods Excellent size resolution: 30 sections from .01 m to 10

m

Two-Moment Sectional Algorithm

mo 2mo … Mass

M1

N1

M2

N2

...

...

Aerosol Microphysics

Coagulation:

General Dynamic

Equation

1

1

)(

)(

k

k

k

k

m

m kk

m

m kk

dmmmnM

dmmnN

Condensation:

1

1,

2

1,1

1

11

1,

2,

1

1,

2

1,11

211,1

2

2

2

1

k

iiikk

k

iiiik

k

kk

I

kiiikkkkk

k

iiiik

k

kk

k

iiikkkkk

k

MKmMKx

f

NKNNKmMKx

f

MKNKdt

dN

tt

t

ikfk

m

m kk

dttSC

mm

dmmnmttMfk

fk

2/33/2

,,

2/33/2

3

2

3

2,1

,

~30,000 grid cells

1 year Adaptive

time steps

Aerosol Microphysics

Aerosol Types Sulfate / sea-salt implemented in GISS GCM Carbonaceous and dust in development

Processes Emissions Chemistry Microphysics Cloud processing Size-resolved dry / wet deposition

Cloud Condensation Nuclei

Uncertainties

Particulate Emissions Most sulfate aerosols results from gas-phase SO2

emissions Particulate sulfate: <5% of anthropogenic sulfur

emissions Nucleation of new aerosol particles

Important uncertainties in mechanism and rate Both processes contribute significant numbers of small

particles insignificant contribution to sulfate mass important contribution to aerosol number concentrations

and size distributions Must quantify sensitivity to these uncertainties

Sensitivity Scenarios

Base Case 1985 sulfur emissions all emissions as gas-phase SO2

nucleation based on critical concentration from binary (H2SO4-H2O) theory

Primary Emissions 3% of sulfur emissions as sulfate

Enhanced Nucleation critical H2SO4 concentration factor of 10 lower

Pre-industrial no anthropogenic emissions (but no sea salt)

Vertical Profiles

-1000

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

0 50 100 150 200CCN 0.2% Concentration (cm-3 STP)

Pre

ssu

re (

mb

)

Modern Day: SO2

Modern Day: SO2/SO4

Preindustrial

Impact of Particulate Emissions

Summary and Conclusions

Aerosol microphysical simulations developed in general circulation model

Results for sulfate / sea-salt are promising Allow mechanistic simulations of indirect

climate forcing Require better knowledge of sources of particle

number Indicate impact of primary particles on CCN

concentrations This microphysical simulation will be

incorporated into GEOS-CHEM

Future Work

Include other aerosol types Organic carbon Elemental carbon Mineral dust

Couple size-resolved aerosol model to GCM clouds Model evaluation

Real world meteorologies GEOS CHEM (assimilated winds) Nudged version of GISS model

Field campaigns Satellite

MODIS / MISR will provide more size and chemically resolved observations than previous satellite instruments