aerosol microphysics: plans for geos-chem peter j. adams carnegie mellon university civil and...
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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