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Aerosol Microphysics: Plans for GEOS-CHEM

Aerosol Microphysics: Plans for GEOS-CHEM. Peter J. Adams Carnegie Mellon University Civil and Environmental Engineering. GEOS-CHEM Activities at CMU. Comprehensive aerosol microphysics scheme implemented in GISS GCM For details, see following publications

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Aerosol Microphysics: Plans for GEOS-CHEM

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  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., 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

  3. Number Particle Size Aerosol Activation 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 Mechanistic: number of cloud drops depends on number of particles large enough to activate Number Cloud Droplets (cm-3) Particle Size Boucher & Lohmann, 1995 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: -0.40 W/m2 III: Jones et al. [1994]: -0.80 W/m2 IV: Boucher and Lohmann [1995]: -1.78 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 M1 N1 M2 N2 ... ... • 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 • ~30,000 grid cells • 1 year • Adaptive time steps Coagulation: General Dynamic Equation Condensation:

  8. 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

  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

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