Mixing State of Aerosols:Excess Atmospheric Absorption Paradox

Shekhar Chandra

Graduate Student, EAS

Term Paper Presentation for EAS-6410

Introduction:

•Excess Atmospheric Absorption Paradox?

•Mixing State of Aerosols?

•Externally Mixed

•Internally Mixed

•Layered (Core- Shell) Structure

Mixing State of Aerosols

Different Cases ofMixing State:

Case-1; Case-2

Case-3; Case-4

Internally Mixed

Optical Properties of Aerosols

•Extinction Coefficient (km-1)

•Optical Depth (unit less)

•Radiative Forcing (W/m-2)

•Absorption Coefficient

•Scattering Coefficient

•TOA Forcing (W/m-2)

•Surface Forcing (W/m-2)

Atmospheric Forcing (W/m-2)

TOA

Surface

Radiative Impact of Aerosol on Climate

Direct Impact Indirect Impact

Scattering and Absorptionof Radiation

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)

•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).

Motivation

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 (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

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

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

Different cases of mixing of BC (black carbon) with other aerosol species

Methodology of mixing and simulations

•Ratio of core to shell size is constant

•

)/( shellcore RR

Shell thickness is constant )( coreshell RR

shellcoreshellBC RRM )(3

4 33

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

Optical depth contribution of different aerosol types at 550 nm

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Surface Radiation (Measured ) (W m-2)

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) Mean Diff. : + 93 W m-2

Mean Diff. : + 3 W m-2

28% BC aerosols are assumed to be forming core-shell with sea-salt

Over Arabian Sea in August-2003

Précis

•Mixing state of aerosols may have possible linkage with ‘excess’ atmospheric absorption issue and mismatch between models andobservations

• 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