Aerosol Microphysics Plans for GEOS-CHEM by rockandrolldreams

VIEWS: 11 PAGES: 15

									     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
     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
Aerosol activation is a
competition between




                               Number
solute and surface
tension effects


       4M w w 6ns M w
ln S           
       RT w D p  w D 3
                        p
                                             Particle Size
                                        Only largest particles
                                        activate to become
                     Solute             cloud droplets:
  Surface
                    (Raoult)            Dp>80 nm
  tension
                      term              S=0.2%
    term
Mechanistic vs. Empirical Models




                         Cloud Droplets (cm-3)
Number




         Particle Size                                       Boucher & Lohmann, 1995


Mechanistic: number
  of cloud drops                                     Sulfate Mass (mg m-3)
  depends on number                              Empirical: number of cloud
  of particles large                               drops correlated with
  enough to activate                               sulfate mass based on
                                                   observations
Kiehl et al. [2000]




                            Cloud Droplets (cm-3)
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]:                               Sulfate Mass (mg m-3)
   -0.80 W/m2
                                                    “It is argued that a less
IV: Boucher and                                         empirical and more
  Lohmann [1995]:                                       physically based
  -1.78 W/m2                                            approach is required…”
Two-Moment Sectional Algorithm
   Two moments of the size               M1 M2 ...
                                          N1 N2 ...
    distribution (mass and number)
    are tracked for each size bin.
   The average size of particles in a
    given section is not constant with
    time                                 mo 2mo …         Mass
   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
     Aerosol Microphysics
                                                                                mk 1                                             ~30,000 grid
                                                         Nk  
                                                                                                                            
    General Dynamic                                                         mk
                                                                                        nk ( m )dm
                                                                                                                                  cells
    Equation                                                                    mk 1
                                                         Mk                           mnk ( m )dm                              1 year
                                                                                mk
                                                                                                                                 Adaptive
                                                                                                                                  time steps

    Coagulation:                                                                     Condensation:
                                                                                                                                        3/ 2
                                  k 2
                                                                                                                            2/3 2 
                                                                                        M k t  t                                         nk m dm
dN k 1
     K k 1,k 1 N k 1   k 1  K k 1,i M i
                                                                                                                mk 1, f

                                                                                                                           m  3  
                    2

 dt  2                            i 1
                                                                                                                mk , f
                                                                                                                                   
     k  fk        k 1                                    I                                                              3/ 2
                   K k ,i M i mi  K k ,k N  N k
                                              2
                                              k           K        k ,i   Ni           mk , f
                                                                                                    2        2 
                                                                                                  mk ,/i 3   
       2 xk         i 1                                 i  k 1
                                                                                                             3 
     k 1  f k 1        k 2                   k 1
                        K k 1,i M i mi   k  K k ,i M i
                                                                                                 t  t

        2 xk 1
                                                                                                       CS t dt
                           i 1                   i 1                                           t
Aerosol Microphysics
   Aerosol Types
            / sea-salt implemented in GISS GCM
     Sulfate
     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
     insignificantcontribution 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
                   0

                 -100                             Modern Day: SO2
                 -200                             Modern Day: SO2/SO4
                                                  Preindustrial
                 -300
Pressure (mb)




                 -400

                 -500

                 -600

                 -700

                 -800

                 -900

                -1000
                        0   50           100             150            200
                                                      -3
                            CCN 0.2% Concentration (cm STP)
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

								
To top