SENSITIVITY TESTS WITH GEOS-Chem AEROSOL OPTICAL
PROPERTIES: THE ROLE OF BLACK CARBON
Gabriele Curci
CETEMPS Dept. Physical and Chemical Sciences
University of L’Aquila
6th International GEOS-Chem Meeting, 6-9 May 2013, Harvard, USA
τ(λ), ω(λ), g(λ), m(λ), dV/d log(r), …
STANDARD GEOS-Chem CALCULATION: EXTERNAL MIXING
Bulk aerosol (mass only) chemistry-transport model Need to assign RH-dependent size distributions and species densities
ln r
dN
/ d
ln r
rm
σg
Species 1
RH
Species 2
Species N . . .
If external mixing and spherical particles are
assumed:
Mie parameters averaged over size distributions may be pre-tabulated as a function of RH, and then used to calculate e.g. AOD at each time step and grid cell:
N
i
i
ieffi
iextM
RHr
RHQAOD
1 ,
,
)(
)(
4
3
INTERNAL MIXING OPTIONS
homogeneous mixing core/shell mixing
Species 1
Species 2
Species N
. . .
Species combine in a new size distribution at each time step and grid cell
Distribution average Mie parameters are not useful anymore
Radius-by-radius calculations using Mie parameters tabulated as a function of size and refractive index (CPU time x 10-20 w.r.t. external mixing)
SENSITIVITY TESTS ON MIXING AND BLACK CARBON
Name Description
EXT GEOS-Chem external mixing
CS Core-Shell Core with all insoluble species: BC, DUST, POC
BCPICORE Core-Shell Core with soluble BC only
BCHI Core-Shell Core same as CS, but BC refractive index from Bond and Bergstrom (2006)
AERONET AEROSOL OPTICAL DEPTH @ 443 nm
• General overestimation where anthropogenic and fire sources are large (NA, SA, EU, China) • Not bad over desert and sea
AERONET SINGLE SCATTERING ALBEDO
• Overestimated • Model display very low variability with respect to observations
AERONET SINGLE SCATTERING ALBEDO
• Very sensitive to mixing assumption • Core-Shell assumption enhance variability • BC-only core unrealistic
EXT CS BCPICORE BCHI
AERONET ASYMMETRY PARAMETER
• Overestimation where anthropogenic sources are large (NA, EU, China) • Consistent with Reff positive bias (not shown): if Reff is too large, g is expected to be overestimated
AERONET ASYMMETRY PARAMETER
• Core-Shell with large core seems to be the more realistic assumption
EXT CS BCPICORE BCHI
INPUT TO RADIATIVE TRANSFER MODEL (LIBRADTRAN)
• Mixing assumption may thus have a large effect on radiative forcing calculations …
FUTURE DEVELOPMENTS
• Complete debugging and release version with internal mixing option and AERONET-like output
• Extensive sensitivity tests on size distributions, refractive indices against AERONET
• Extension to sectional and modal aerosol models
• Definition of a general input format. Also useful to input idealized profiles
• Parallelization
• Online coupling with libRadTran?
• Online coupling with GEOS-Chem?
FlexAOD WORKING DIAGRAM
Size Distributions
Refractive Indices
Hygroscopic Growth Factors
Mixing Assumption
FlexAOD
Look-up Tables: • Mie Single Sphere • Mie Coated Sphere
Aerosol Optical Depth
Single Scattering Albedo
Asymmetry Parameter and Phase Function
Aggregated: • Effective radius • Volume Conc. • Refr. index
Satellites, Sunphotometers,
RTM, …
INPUT OUTPUT
USE
Aerosol Species Mass
RADIATION-AEROSOL OPTICAL SIMULATION
Species Densities