Transcript Mixing State of Aerosols: Excess Atmospheric Absorption

Term Paper Presentation for EAS-6410
Mixing State of Aerosols:
Excess Atmospheric Absorption Paradox
Shekhar Chandra
Graduate Student, EAS
Introduction:
•Excess Atmospheric Absorption Paradox?
•Mixing State of Aerosols?
•Externally Mixed
•Internally Mixed
•Layered (Core- Shell) Structure
Mixing State of Aerosols
Different Cases of
Mixing State:
Case-1; Case-2
Case-3; Case-4
Internally Mixed
Optical Properties of Aerosols
•Absorption Coefficient
•Extinction Coefficient (km-1)
•Scattering Coefficient
•Optical Depth (unit less)
•TOA Forcing (W/m-2)
•Radiative Forcing (W/m-2)
TOA
•Surface Forcing (W/m-2)
Atmospheric Forcing (W/m-2)
Surface
Radiative Impact of Aerosol on Climate
Direct Impact
Scattering and Absorption
of Radiation
Indirect Impact
Alter Cloud Properties
Motivation
•
Recent studies suggest that clear sky absorbs more short
wave model than predicted my radiative transfer models
(Arking et al., 1996; Halthore et al., 1998; Sato et al., 2003).
•
Overestimation of diffuse downward irradiance by 9-40%
at the surface in a cloud-free atmosphere while correctly
calculating direct normal solar irradiance
(Halthore et al., 1998).
•
Unidentified absorber in the atmosphere
(Halthore et al., 1998; Sato et al., 2003)
Motivation
•Some observers reported an agreement between models and
observations within instrumental uncertainties
(Cess et al., 1995; Kiehl et al., 1998; Satheesh et al., 1999).
•In précis some investigators report excellent agreement while
other report discrepancy between models and observations.
•Optical properties of aerosols may significantly differ in case
of internally mixed aerosols (Jacobson, M.Z.,2001).
Outline for Current Case Study
In this study, data set is size-segregated aerosol composition
along with observations from ground based radiometers over
Arabian Sea.
ARMEX-2003
Observed Facts from ARMEX
•Over tropical Indian Ocean, during Moist season (August)
estimated surface diffuse radiative fluxes exceed observations by
~92 Wm-2 . Same results showing excess absorption were reported
by Charlock et al., (2003)
•Investigators have reported that during the moist season (JulyAugust) average optical depth was as high as ~0.7. If we consider
case-I (BC shell and sulfate core) , corresponding reduction in
diffuse radiation will be around 200 Wm-2
•It was proposed by Sato et al., (2003) that estimations and
observations do agree well if we increase the BC by a factor ~2-3
Other Facts are:
•Recent experiments over southern Arabian Sea have shown that
the amount of BC mass fraction reduces from 11% during JanMarch to 0.5% in June (Babu, S.S., et al., 2004). Thus the
possibility of increasing BC by a factor ~2-3 to let observed and
modeled fluxes agree is an impossibility. In general:
•Optical Depth is going up
•SSA is going down (Absorption is more)
•Forcing is more
Different cases of mixing of BC (black carbon) with
other aerosol species
Case-I: BC in shell with Sulfate while all other species are
externally mixed
Case-II: BC in core with Sulfate while all other species are
externally mixed
Case-III: BC in shell with Sea-Salt while all other species
externally mixed
Case-IV: BC in core with Sea-Salt while all other species are
externally mixed
Case-V: All species are externally mixed
Methodology of mixing and simulations
•Ratio ( R core / Rshell ) of core to shell size is constant
• Shell thickness ( Rshell  Rcore ) is constant
M BC
4
3
3
  ( Rshell  Rcore )  shell
3
Tools Used
•Mie treatment of coated sphere for estimating optical properties
•SBDART for estimating radiative forcing
Ex
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M
ixe
BC
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BC
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BC
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BC
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Aerosol Optical Depth (500 nm)
AOD
0.7
0.6
0.90
0.5
0.85
0.4
0.3
0.80
0.2
0.75
0.1
0
0.70
SSA for different mixing cases of aerosols
Single Scattering Albedo
0.8
0.95
SSA
Optical depth contribution of different aerosol types at 550 nm
Surface Radiation (Estimated) (W m-2 )
Over Arabian Sea in August-2003
1000
Mean Diff. : + 93 W m -2
900
800
700
600
Mean Diff. : + 3 W m -2
500
400
400
500
600
700
800
900
1000
Surface Radiation (Measured ) (W m-2 )
28% BC aerosols are assumed to be forming core-shell with sea-salt
Précis
•Mixing state of aerosols may have possible linkage with ‘excess’
atmospheric absorption issue and mismatch between models and
observations
• Past estimates of climate forcing due to anthropogenic aerosols
represent the lower bound and actual values may be higher than
the current estimates
•IPCC has primarily focused on anthropogenic forcing but current
study suggests that when natural and anthropogenic aerosols
co-exist in core-shell form, one must talk about composite forcing