1. Aerosol Microphysics: Plans for GEOS-CHEM
Peter J. Adams
Carnegie Mellon University
Civil and Environmental Engineering

2. 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., /2001JD001010, 2002.
Adams, P. J., and J. H. Seinfeld, Disproportionate impact of particulate emissions on global cloud condensation nuclei concentrations, Geophysical Research Letters, /2002GL016303, 2003.
Same aerosol microphysics will be implemented in GEOS-CHEM – this process is just beginning

3. Aerosol Activation Aerosol activation is a competition between
Particle Size
Number
Aerosol activation is a
competition between
solute and surface
tension effects
Only largest particles
activate to become
cloud droplets:
Dp>80 nm
S=0.2%
Solute (Raoult) term
Surface tension term

4. Mechanistic vs. Empirical Models
Number
Cloud Droplets (cm-3)
Particle Size
Boucher & Lohmann, 1995
Mechanistic: number of cloud drops depends on number of particles large enough to activate
Sulfate Mass (mg m-3)
Empirical: number of cloud drops correlated with sulfate mass based on observations

5. Kiehl et al. [2000] I: Martin et al. [1994]: -0.68 W/m2
II: Martin et al. with background CCN: W/m2
III: Jones et al. [1994]: W/m2
IV: Boucher and Lohmann [1995]: W/m2
Cloud Droplets (cm-3)
Sulfate Mass (mg m-3)
“It is argued that a less empirical and more physically based approach is required…”

6. Two-Moment Sectional Algorithm
...
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 mm to 10 mm
mo 2mo … Mass

7. Aerosol Microphysics General Dynamic Equation Coagulation:
~30,000 grid cells
1 year
Adaptive time steps
General Dynamic
Equation
Coagulation:
Condensation:

8. Aerosol Microphysics Aerosol Types Processes
Sulfate / sea-salt implemented in GISS GCM
Carbonaceous and dust in development
Processes
Emissions
Chemistry
Microphysics
Cloud processing
Size-resolved dry / wet deposition

9. Cloud Condensation Nuclei

10. 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

11. 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)

12. Vertical Profiles

13. Impact of Particulate Emissions

14. 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

15. 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