Mixing State of Aerosols: Excess Atmospheric Absorption Paradox

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Mixing State of AerosolsExcess Atmospheric
Absorption Paradox
Term Paper Presentation for EAS-6410

• Shekhar Chandra
• Graduate Student, EAS

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Introduction

• Excess Atmospheric Absorption Paradox?
• Mixing State of Aerosols?
• Externally Mixed
• Internally Mixed
• Layered (Core- Shell) Structure

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Mixing State of Aerosols
Different Cases of Mixing State Case-1
Case-2 Case-3 Case-4
Internally Mixed
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Optical Properties of Aerosols

• Absorption Coefficient
• Scattering Coefficient

• Extinction Coefficient (km-1)
• Optical Depth (unit less)
• Radiative Forcing (W/m-2)

• TOA Forcing (W/m-2)
• Surface Forcing (W/m-2)

TOA
Atmospheric Forcing (W/m-2)
Surface
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Radiative Impact of Aerosol on Climate
Direct Impact
Indirect Impact
Scattering and Absorption of Radiation
Alter Cloud Properties
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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)

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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).

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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
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(No Transcript)
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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 (July-August) 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

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Other Facts are

• Recent experiments over southern Arabian Sea have
  shown that the amount of BC mass fraction reduces
  from 11 during Jan-March 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

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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
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Methodology of mixing and simulations

• Ratio of core to shell size
  is constant

Shell thickness is
constant
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Tools Used

• Mie treatment of coated sphere for estimating
  optical properties
• SBDART for estimating radiative forcing

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SSA for different mixing cases of aerosols
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Optical depth contribution of different aerosol
types at 550 nm
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Over Arabian Sea in August-2003
28 BC aerosols are assumed to be forming
core-shell with sea-salt
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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